2024 |
Kishor Kumar Mandal, Anuj Kumar Singh, Brijesh Kumar, Amit P Shah, Rishabh Vij, Amrita Majumder, Janhavi Jayawant Khunte, Venu Gopal Achanta, Anshuman Kumar Emission engineering in monolithically integrated silicon nitride microring resonators Journal Article ACS Materials Letters, 6 , pp. 1831–1840, 2024. @article{arXiv:2401.04963, title = {Emission engineering in monolithically integrated silicon nitride microring resonators}, author = {Kishor Kumar Mandal, Anuj Kumar Singh, Brijesh Kumar, Amit P Shah, Rishabh Vij, Amrita Majumder, Janhavi Jayawant Khunte, Venu Gopal Achanta, Anshuman Kumar}, url = {https://doi.org/10.1021/acsmaterialslett.4c00105}, doi = {10.1021/acsmaterialslett.4c00105}, year = {2024}, date = {2024-04-05}, journal = {ACS Materials Letters}, volume = {6}, pages = {1831--1840}, abstract = {Monolithic integration of solid-state color centers with photonic elements of the same material is a promising approach to overcome the constraints of fabrication complexity and coupling losses in traditional hybrid integration approaches. A wide band-gap, low-loss silicon nitride (SiN) platform is a mature technology, having CMOS compatibility, widely used in hybrid integrated photonics and optoelectronics. However, it has been shown that certain growth conditions enable the SiN material to host color centers, whose origin is currently under investigation. In this work, we have engineered a novel technique for the efficient coupling of these intrinsic emitters into the whispering gallery modes (WGMs) of the SiN microring cavity -- which has not been explored previously. We have engineered a subwavelength-sized notch into the rim of the SiN microring structure, to optimize the collection efficiency of the cavity-coupled enhanced photoluminescence (PL) spectra at room temperature. The platform presented in this work will enable the development of monolithic integration of color centers with nanophotonic elements for application to quantum photonic technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Monolithic integration of solid-state color centers with photonic elements of the same material is a promising approach to overcome the constraints of fabrication complexity and coupling losses in traditional hybrid integration approaches. A wide band-gap, low-loss silicon nitride (SiN) platform is a mature technology, having CMOS compatibility, widely used in hybrid integrated photonics and optoelectronics. However, it has been shown that certain growth conditions enable the SiN material to host color centers, whose origin is currently under investigation. In this work, we have engineered a novel technique for the efficient coupling of these intrinsic emitters into the whispering gallery modes (WGMs) of the SiN microring cavity -- which has not been explored previously. We have engineered a subwavelength-sized notch into the rim of the SiN microring structure, to optimize the collection efficiency of the cavity-coupled enhanced photoluminescence (PL) spectra at room temperature. The platform presented in this work will enable the development of monolithic integration of color centers with nanophotonic elements for application to quantum photonic technologies.  |
Anuj Kumar Singh, Utkarsh, Pablo Tieben, Kishor Kumar Mandal, Brijesh Kumar, Rishabh Vij, Amrita Majumder, Ikshvaku Shyam, Shagun Kumar, Kenji Watanabe, Takashi Taniguchi, Venu Gopal Achanta, Andreas Schell, Anshuman Kumar Interplay of plasmonics and strain for hexagonal Boron Nitride emission engineering Journal Article arXiv preprint, arXiv:2401.11428 , 2024. @article{arXiv:2401.11428, title = {Interplay of plasmonics and strain for hexagonal Boron Nitride emission engineering}, author = {Anuj Kumar Singh, Utkarsh, Pablo Tieben, Kishor Kumar Mandal, Brijesh Kumar, Rishabh Vij, Amrita Majumder, Ikshvaku Shyam, Shagun Kumar, Kenji Watanabe, Takashi Taniguchi, Venu Gopal Achanta, Andreas Schell, Anshuman Kumar}, url = {https://doi.org/10.48550/arXiv.2401.11428}, doi = {10.48550/arXiv.2401.11428}, year = {2024}, date = {2024-01-21}, journal = {arXiv preprint}, volume = {arXiv:2401.11428}, abstract = {In the realm of quantum information and sensing, there has been substantial interest in the single-photon emission associated with defects in hexagonal boron nitride (hBN). With the goal of producing deterministic emission centers, in this work, we present a platform for engineering emission in hBN integrated with gold truncated nanocone structures. Our findings highlights that, the activation of emission is due to the truncated gold nanocones. Furthermore, we measure the quantum characteristics of this emission and find that while our system demonstrates support for single-photon emission, the origin of this emission remains ambiguous. Specifically, it is unclear whether the emission arises from defects generated by the induced strain or from alternative defect mechanisms. This uncertainty stems from the fluorescence properties inherent to gold, complicating our definitive attribution of the quantum emission source. To provide a rigorous theoretical foundation, we elucidate the effects of strain via the Kirchhoff-Love theory. Additionally, the enhancements observed due to plasmonic effects are comprehensively explained through the resolution of Maxwell's equations. This study will be useful for the development of deterministic and tunable single photonic sources in two dimensional materials and their integration with plasmonic platforms.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In the realm of quantum information and sensing, there has been substantial interest in the single-photon emission associated with defects in hexagonal boron nitride (hBN). With the goal of producing deterministic emission centers, in this work, we present a platform for engineering emission in hBN integrated with gold truncated nanocone structures. Our findings highlights that, the activation of emission is due to the truncated gold nanocones. Furthermore, we measure the quantum characteristics of this emission and find that while our system demonstrates support for single-photon emission, the origin of this emission remains ambiguous. Specifically, it is unclear whether the emission arises from defects generated by the induced strain or from alternative defect mechanisms. This uncertainty stems from the fluorescence properties inherent to gold, complicating our definitive attribution of the quantum emission source. To provide a rigorous theoretical foundation, we elucidate the effects of strain via the Kirchhoff-Love theory. Additionally, the enhancements observed due to plasmonic effects are comprehensively explained through the resolution of Maxwell's equations. This study will be useful for the development of deterministic and tunable single photonic sources in two dimensional materials and their integration with plasmonic platforms.  |
Hongwei Wang*, Anshuman Kumar*, Siyuan Dai, Xiao Lin, Zubin Jacob, Sang-Hyun Oh, Vinod Menon, Evgenii Narimanov, Young Duck Kim, Jian-Ping Wang, Phaedon Avouris, Luis Martin Moreno, Joshua Caldwell, Tony Low Planar hyperbolic polaritons in 2D van der Waals materials Journal Article Nature Comunications, 15 (69), 2024. @article{natcom2023_1, title = {Planar hyperbolic polaritons in 2D van der Waals materials}, author = {Hongwei Wang*, Anshuman Kumar*, Siyuan Dai, Xiao Lin, Zubin Jacob, Sang-Hyun Oh, Vinod Menon, Evgenii Narimanov, Young Duck Kim, Jian-Ping Wang, Phaedon Avouris, Luis Martin Moreno, Joshua Caldwell, Tony Low }, url = {https://doi.org/10.1038/s41467-023-43992-8}, doi = {10.1038/s41467-023-43992-8}, year = {2024}, date = {2024-01-02}, journal = {Nature Comunications}, volume = {15}, number = {69}, abstract = {Anisotropic planar polaritons - hybrid electromagnetic modes mediated by phonons, plasmons, or excitons - in biaxial two-dimensional (2D) van der Waals crystals have attracted significant attention due to their fundamental physics and potential nanophotonic applications. In this Perspective, we review the properties of planar hyperbolic polaritons and the variety of methods that can be used to experimentally tune them. We argue that such natural, planar hyperbolic media should be fairly common in biaxial and uniaxial 2D and 1D van der Waals crystals, and identify the untapped opportunities they could enable for functional (i.e. ferromagnetic, ferroelectric, and piezoelectric) polaritons. Lastly, we provide our perspectives on the technological applications of such planar hyperbolic polaritons.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Anisotropic planar polaritons - hybrid electromagnetic modes mediated by phonons, plasmons, or excitons - in biaxial two-dimensional (2D) van der Waals crystals have attracted significant attention due to their fundamental physics and potential nanophotonic applications. In this Perspective, we review the properties of planar hyperbolic polaritons and the variety of methods that can be used to experimentally tune them. We argue that such natural, planar hyperbolic media should be fairly common in biaxial and uniaxial 2D and 1D van der Waals crystals, and identify the untapped opportunities they could enable for functional (i.e. ferromagnetic, ferroelectric, and piezoelectric) polaritons. Lastly, we provide our perspectives on the technological applications of such planar hyperbolic polaritons.  |
2023 |
Abhay Anand V S, Mihir Kumar Sahoo, Faiha Mujeeb, Abin Varghese, Subhabrata Dhar, Saurabh Lodha, Anshuman Kumar Novel Nano-Electroplating-Based Plasmonic Platform for Giant Emission Enhancement in Monolayer Semiconductors Journal Article ACS Applied Materials & Interfaces, 15 (49), pp. 57783-57790, 2023. @article{arXiv:2306.13507, title = {Novel Nano-Electroplating-Based Plasmonic Platform for Giant Emission Enhancement in Monolayer Semiconductors}, author = {Abhay Anand V S, Mihir Kumar Sahoo, Faiha Mujeeb, Abin Varghese, Subhabrata Dhar, Saurabh Lodha, Anshuman Kumar}, url = {https://doi.org/10.1021/acsami.3c11564}, doi = {10.1021/acsami.3c11564}, year = {2023}, date = {2023-12-04}, journal = {ACS Applied Materials & Interfaces}, volume = {15}, number = {49}, pages = {57783-57790}, abstract = {Two dimensional semiconductors have attracted considerable attention owing to their exceptional electronic and optical characteristics. However, their practical application has been hindered by the limited light absorption resulting from their atomically thin thickness and low quantum yield. A highly effective approach to manipulate optical properties and address these limitations is integrating subwavelength plasmonic nanostructures with these monolayers. In this study, we employed electron beam lithography and electroplating technique to fabricate a gold nanodisc (AuND) array capable of enhancing the photoluminescence (PL) of monolayer MoS2 giantly. Monolayer MoS2 placed on the top of the AuND array yields up to 150-fold PL enhancement compared to that on a gold film. We explain our experimental findings based on electromagnetic simulations.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two dimensional semiconductors have attracted considerable attention owing to their exceptional electronic and optical characteristics. However, their practical application has been hindered by the limited light absorption resulting from their atomically thin thickness and low quantum yield. A highly effective approach to manipulate optical properties and address these limitations is integrating subwavelength plasmonic nanostructures with these monolayers. In this study, we employed electron beam lithography and electroplating technique to fabricate a gold nanodisc (AuND) array capable of enhancing the photoluminescence (PL) of monolayer MoS2 giantly. Monolayer MoS2 placed on the top of the AuND array yields up to 150-fold PL enhancement compared to that on a gold film. We explain our experimental findings based on electromagnetic simulations.  |
Parul Sharma; Brijesh Kumar; Nihar Ranjan Sahoo; Anshuman Kumar Exceptional point sensing via energy loss profile in a non-Hermitian system Journal Article arXiv preprint, arXiv:2310.10111 , 2023. @article{arXiv:2310.10111, title = { Exceptional point sensing via energy loss profile in a non-Hermitian system}, author = {Parul Sharma and Brijesh Kumar and Nihar Ranjan Sahoo and Anshuman Kumar}, url = {https://doi.org/10.48550/arXiv.2310.10111}, doi = {10.48550/arXiv.2310.10111}, year = {2023}, date = {2023-10-16}, journal = {arXiv preprint}, volume = {arXiv:2310.10111}, abstract = {The heightened sensitivity observed in non-Hermitian systems at exceptional points (EPs) has garnered significant attention. Typical EP sensor implementations rely on precise measurements of spectra and importantly, for real time sensing measurements, the EP condition ceases to hold as the perturbation increases over time, thereby preventing the use of high sensitivity at the EP point. In this work, we present an new approach to EP sensing which goes beyond these two traditional constraints. Firstly, instead of measuring the spectra, our scheme of EP based sensing is based on the observation of decay length of the optical mode in finite size gratings, which is validated via coupled mode theory as well as full wave electrodynamic simulations. Secondly, for larger perturbation strengths, the EP is spectrally shifted instead of being destroyed -- this spectral shift of the EP is calibrated and using this look-up table, we propose continuous real time detection by varying the excitation laser wavelength. As a proof of principle of our technique, we present an application to the sensing of coronavirus particles, which shows unprecedented limit of detection. These findings will contribute to the expanding field of exceptional point based sensing technologies for real time applications beyond spectral measurements.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The heightened sensitivity observed in non-Hermitian systems at exceptional points (EPs) has garnered significant attention. Typical EP sensor implementations rely on precise measurements of spectra and importantly, for real time sensing measurements, the EP condition ceases to hold as the perturbation increases over time, thereby preventing the use of high sensitivity at the EP point. In this work, we present an new approach to EP sensing which goes beyond these two traditional constraints. Firstly, instead of measuring the spectra, our scheme of EP based sensing is based on the observation of decay length of the optical mode in finite size gratings, which is validated via coupled mode theory as well as full wave electrodynamic simulations. Secondly, for larger perturbation strengths, the EP is spectrally shifted instead of being destroyed -- this spectral shift of the EP is calibrated and using this look-up table, we propose continuous real time detection by varying the excitation laser wavelength. As a proof of principle of our technique, we present an application to the sensing of coronavirus particles, which shows unprecedented limit of detection. These findings will contribute to the expanding field of exceptional point based sensing technologies for real time applications beyond spectral measurements.  |
Nihar Ranjan Sahoo, Brijesh Kumar, S.S. Jatin Prasath, Aneesh Bapat, Parul Sharma; Anshuman Kumar Polaritons in photonic hypercrystals of van der Waals materials Journal Article arXiv preprint, arXiv:2309.17146 , 2023. @article{arXiv:2309.17146, title = {Polaritons in photonic hypercrystals of van der Waals materials}, author = {Nihar Ranjan Sahoo, Brijesh Kumar, S.S. Jatin Prasath, Aneesh Bapat, Parul Sharma and Anshuman Kumar}, url = {https://doi.org/10.48550/arXiv.2309.17146}, doi = {10.48550/arXiv.2309.17146}, year = {2023}, date = {2023-10-02}, journal = {arXiv preprint}, volume = {arXiv:2309.17146}, abstract = {In-plane Hyperbolic Phonon polaritons (HPhPs) are quasiparticles formed via coupling of photons and optical phonons in in-plane hyperbolic materials and offer unique applications in sensing, thermal emitters and high resolution imaging. However, the large momentum mismatch between photons and these in-plane HPhPs has restricted their technological potential as most experimental demonstration rely on sophisticated and expensive near field detection schemes. In this work, using the example of α−MoO3, we demonstrate that by constructing photonic hypercrystals of this material, one can not only excite these in-plane HPhPs in the far field but also tune the far field response via twisting the hypercrystal lattice with respect the lattice of α−MoO3. Our findings will pave the way for the development of practical in-plane HPhP devices as well as provide access to new fundamental physics of such materials via conventional and well developed far field measurement techniques.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In-plane Hyperbolic Phonon polaritons (HPhPs) are quasiparticles formed via coupling of photons and optical phonons in in-plane hyperbolic materials and offer unique applications in sensing, thermal emitters and high resolution imaging. However, the large momentum mismatch between photons and these in-plane HPhPs has restricted their technological potential as most experimental demonstration rely on sophisticated and expensive near field detection schemes. In this work, using the example of α−MoO3, we demonstrate that by constructing photonic hypercrystals of this material, one can not only excite these in-plane HPhPs in the far field but also tune the far field response via twisting the hypercrystal lattice with respect the lattice of α−MoO3. Our findings will pave the way for the development of practical in-plane HPhP devices as well as provide access to new fundamental physics of such materials via conventional and well developed far field measurement techniques.  |
Brijesh Kumar, Anuj Kumar Singh, Kishor K Mandal, Parul Sharma, Nihar Ranjan Sahoo, Anshuman Kumar A universal and stable metasurface for photonic quasi bound state in continuum coupled with two dimensional semiconductors Journal Article Journal of Physics D: Applied Physics, 56 (42), pp. 425105, 2023. @article{arXiv:2212.05951, title = {A universal and stable metasurface for photonic quasi bound state in continuum coupled with two dimensional semiconductors}, author = {Brijesh Kumar, Anuj Kumar Singh, Kishor K Mandal, Parul Sharma, Nihar Ranjan Sahoo, Anshuman Kumar}, url = {https://dx.doi.org/10.1088/1361-6463/ace78b}, doi = {10.1088/1361-6463/ace78b}, year = {2023}, date = {2023-07-27}, journal = {Journal of Physics D: Applied Physics}, volume = {56}, number = {42}, pages = {425105}, abstract = {The strong coupling of excitons to optical cavity modes is of immense importance when understanding the fundamental physics of quantum electrodynamics at the nanoscale as well as for practical applications in quantum information technologies. There have been several attempts at achieving strong coupling between excitons in two-dimensional semiconductors, such as transition metal dichalcogenides (TMDCs) and photonic quasi-bound states in the continuum (BICs). We identify two gaps in the platforms for achieving strong coupling between TMDC excitons and photonic quasi-BICs: firstly, in the studies so far, different cavity architectures have been employed for coupling to different TMDCs. This would mean that typically, the fabrication process flow for the cavities will need to be modified as one moves from one TMDC to the other, which can limit the technological progress in the field. Secondly, there has been no discussion of the impact of fabrication imperfections in the studies on the strong coupling of these subsystems so far. In this work, we address these two questions by optimizing a cavity with the same architecture, which can couple to the four typical TMDCs (MoS2, WS2, MoSe2, WSe2) and perform a detailed investigation on the fabrication tolerance of the associated photonic quasi-BICs and their impact on strong coupling.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The strong coupling of excitons to optical cavity modes is of immense importance when understanding the fundamental physics of quantum electrodynamics at the nanoscale as well as for practical applications in quantum information technologies. There have been several attempts at achieving strong coupling between excitons in two-dimensional semiconductors, such as transition metal dichalcogenides (TMDCs) and photonic quasi-bound states in the continuum (BICs). We identify two gaps in the platforms for achieving strong coupling between TMDC excitons and photonic quasi-BICs: firstly, in the studies so far, different cavity architectures have been employed for coupling to different TMDCs. This would mean that typically, the fabrication process flow for the cavities will need to be modified as one moves from one TMDC to the other, which can limit the technological progress in the field. Secondly, there has been no discussion of the impact of fabrication imperfections in the studies on the strong coupling of these subsystems so far. In this work, we address these two questions by optimizing a cavity with the same architecture, which can couple to the four typical TMDCs (MoS2, WS2, MoSe2, WSe2) and perform a detailed investigation on the fabrication tolerance of the associated photonic quasi-BICs and their impact on strong coupling.  |
Anuj Kumar Singh, Kishor K Mandal, Yashika Gupta, Abhay Anand VS, Lekshmi Eswaramoorthy, Brijesh Kumar, Abhinav Kala, Saurabh Dixit, Venu Gopal Achanta, Anshuman Kumar Low-Cost Plasmonic Platform for Photon-Emission Engineering of Two-Dimensional Semiconductors Journal Article Physical Review Applied, 19 (4), pp. 044012, 2023. @article{PhysRevApplied.19.044012, title = {Low-Cost Plasmonic Platform for Photon-Emission Engineering of Two-Dimensional Semiconductors}, author = {Anuj Kumar Singh, Kishor K Mandal, Yashika Gupta, Abhay Anand VS, Lekshmi Eswaramoorthy, Brijesh Kumar, Abhinav Kala, Saurabh Dixit, Venu Gopal Achanta, Anshuman Kumar}, url = {https://doi.org/10.1103/PhysRevApplied.19.044012}, doi = {10.1103/PhysRevApplied.19.044012}, year = {2023}, date = {2023-04-05}, journal = {Physical Review Applied}, volume = {19}, number = {4}, pages = {044012}, abstract = {Although the field of 2D materials has democratized materials science by making high quality samples accessible cheaply, due to the atomically thin nature of these systems, an integration with nanostructures is almost always required to obtain a significant optical response. Traditionally, these nanostructures are fabricated via electron beam lithography or focused ion beam milling, which are expensive and large area fabrication can be further time consuming. In order to overcome this problem, we report the integration of 2D semiconductors on a cost-effective and large area fabricated nanocone platform. We show that the plasmon modes of our nanocone structures lead to photoluminescence (PL) enhancement of monolayer WSe2 by about eight to ten times compared to the non-plasmonic case, consistent with finite-difference time-domain simulations. Excitation power-dependent measurements reveal that our nanocone platform enables a versatile route to engineering the relative exciton trion contributions to the emission.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Although the field of 2D materials has democratized materials science by making high quality samples accessible cheaply, due to the atomically thin nature of these systems, an integration with nanostructures is almost always required to obtain a significant optical response. Traditionally, these nanostructures are fabricated via electron beam lithography or focused ion beam milling, which are expensive and large area fabrication can be further time consuming. In order to overcome this problem, we report the integration of 2D semiconductors on a cost-effective and large area fabricated nanocone platform. We show that the plasmon modes of our nanocone structures lead to photoluminescence (PL) enhancement of monolayer WSe2 by about eight to ten times compared to the non-plasmonic case, consistent with finite-difference time-domain simulations. Excitation power-dependent measurements reveal that our nanocone platform enables a versatile route to engineering the relative exciton trion contributions to the emission.  |
Kishor Kumar Mandal; Yashika Gupta; Brijesh Kumar; Mandar Sohoni; Achanta Venu Gopal; Anshuman Kumar A photonic integrated chip platform for interlayer exciton valley routing Journal Article Journal of Applied Physics, 133 (12), pp. 123104, 2023. @article{doi:10.1063/5.0139880, title = {A photonic integrated chip platform for interlayer exciton valley routing}, author = {Kishor Kumar Mandal and Yashika Gupta and Brijesh Kumar and Mandar Sohoni and Achanta Venu Gopal and Anshuman Kumar}, url = {https://doi.org/10.1063/5.0139880}, doi = {10.1063/5.0139880}, year = {2023}, date = {2023-03-30}, journal = {Journal of Applied Physics}, volume = {133}, number = {12}, pages = {123104}, abstract = {Interlayer excitons in two-dimensional semiconductor heterostructures show suppressed electron–hole overlap resulting in longer radiative lifetimes as compared to intralayer excitons. Such tightly bound interlayer excitons are relevant for important optoelectronic applications, including light storage and quantum communication. Their optical accessibility is, however, limited due to their out-of-plane transition dipole moment. In this work, we design a complementary metal–oxide–semiconductor-compatible photonic integrated chip platform for enhanced near-field coupling of these interlayer excitons with the whispering gallery modes of a microresonator, exploiting the high confinement of light in a small modal volume and high-quality factor of the system. Our platform allows for highly selective emission routing via engineering an asymmetric light transmission that facilitates efficient readout and channeling of the excitonic valley state from such systems. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Interlayer excitons in two-dimensional semiconductor heterostructures show suppressed electron–hole overlap resulting in longer radiative lifetimes as compared to intralayer excitons. Such tightly bound interlayer excitons are relevant for important optoelectronic applications, including light storage and quantum communication. Their optical accessibility is, however, limited due to their out-of-plane transition dipole moment. In this work, we design a complementary metal–oxide–semiconductor-compatible photonic integrated chip platform for enhanced near-field coupling of these interlayer excitons with the whispering gallery modes of a microresonator, exploiting the high confinement of light in a small modal volume and high-quality factor of the system. Our platform allows for highly selective emission routing via engineering an asymmetric light transmission that facilitates efficient readout and channeling of the excitonic valley state from such systems.  |
Mihir Kumar Sahoo; Abhay Anand VS; Anshuman Kumar Electroplating based engineering of plasmonic nanorod metamaterials for biosensing applications Journal Article Nanotechnology, 34 (19), pp. 195301, 2023. @article{arXiv:2211.10361, title = {Electroplating based engineering of plasmonic nanorod metamaterials for biosensing applications}, author = {Mihir Kumar Sahoo and Abhay Anand VS and Anshuman Kumar}, url = {https://doi.org/10.1088/1361-6528/acb948}, doi = {10.1088/1361-6528/acb948}, year = {2023}, date = {2023-02-24}, journal = {Nanotechnology}, volume = {34}, number = {19}, pages = {195301}, abstract = {Sensing lower molecular weight in a diluted solution using a label-free biosensor is challenging and requires a miniaturized plasmonic structure, e.g., a vertical Au nanorod (AuNR) array based metamaterials. The sensitivity of a sensor mainly depends on transducer properties and hence for instance, the AuNR array geometry requires optimization. Physical vapour deposition methods (e.g., sputtering and e-beam evaporation) require a vacuum environment to deposit Au, which is costly, time-consuming, and thickness-limited. On the other hand, chemical deposition, i.e., electroplating deposit higher thickness in less time and at lower cost, becomes an alternative method for Au deposition. In this work, we present a detailed optimization for electroplating based fabrication of these metamaterials. We find that slightly acidic (6.0 < pH < 7.0) gold sulfite solution supports immersion deposition, which should be minimized to avoid uncontrolled Au deposition. Immersion deposition leads to plate-like (for smaller radius AuNR) or capped-like, i.e., mushroom (for higher radius AuNR) structure formation. The electroplating time and DC supply are the tuning parameters that decide the geometry of the vertically aligned AuNR array in area-dependent electroplating deposition. This work will have implications for developing plasmonic metamaterial based sensors.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Sensing lower molecular weight in a diluted solution using a label-free biosensor is challenging and requires a miniaturized plasmonic structure, e.g., a vertical Au nanorod (AuNR) array based metamaterials. The sensitivity of a sensor mainly depends on transducer properties and hence for instance, the AuNR array geometry requires optimization. Physical vapour deposition methods (e.g., sputtering and e-beam evaporation) require a vacuum environment to deposit Au, which is costly, time-consuming, and thickness-limited. On the other hand, chemical deposition, i.e., electroplating deposit higher thickness in less time and at lower cost, becomes an alternative method for Au deposition. In this work, we present a detailed optimization for electroplating based fabrication of these metamaterials. We find that slightly acidic (6.0 < pH < 7.0) gold sulfite solution supports immersion deposition, which should be minimized to avoid uncontrolled Au deposition. Immersion deposition leads to plate-like (for smaller radius AuNR) or capped-like, i.e., mushroom (for higher radius AuNR) structure formation. The electroplating time and DC supply are the tuning parameters that decide the geometry of the vertically aligned AuNR array in area-dependent electroplating deposition. This work will have implications for developing plasmonic metamaterial based sensors.  |
2022 |
Yashika Gupta; Anuj K. Singh; Abhay Anand V. S.; Anshuman Kumar Plasmonic nanocavity enhanced luminescence for in-plane and out-of-plane excitons in BA2PbI4 Ruddlesden Popper perovskite thin film Journal Article MRS Advances, 2022. @article{Yashika_MRS, title = {Plasmonic nanocavity enhanced luminescence for in-plane and out-of-plane excitons in BA2PbI4 Ruddlesden Popper perovskite thin film}, author = {Yashika Gupta and Anuj K. Singh and Abhay Anand V. S. and Anshuman Kumar}, url = {https://doi.org/10.1557/s43580-022-00272-9}, doi = {10.1557/s43580-022-00272-9}, year = {2022}, date = {2022-04-06}, journal = {MRS Advances}, abstract = {In this work, we propose an easy-to-fabricate plasmonic cavity design to enhance luminescence from BA2PbI4 Ruddlesden Popper thin films, which otherwise suffer from a very low quantum yield due to strong exciton-phonon interactions in the lattice. We designed a nanoparticle on mirror plasmonic cavity using 3D-finite-difference time-domain simulations, with perovskite film sandwiched between the two metallic structures yielding a 7-fold local enhancement in photoluminescence for intralayer excitons emitting at 520 nm. The designed cavity also aids the detection of interlayer excitons at 540 nm, for which a huge PL enhancement of more than 1500 times is easily obtained.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this work, we propose an easy-to-fabricate plasmonic cavity design to enhance luminescence from BA2PbI4 Ruddlesden Popper thin films, which otherwise suffer from a very low quantum yield due to strong exciton-phonon interactions in the lattice. We designed a nanoparticle on mirror plasmonic cavity using 3D-finite-difference time-domain simulations, with perovskite film sandwiched between the two metallic structures yielding a 7-fold local enhancement in photoluminescence for intralayer excitons emitting at 520 nm. The designed cavity also aids the detection of interlayer excitons at 540 nm, for which a huge PL enhancement of more than 1500 times is easily obtained.  |
Aneesh Bapat; Saurabh Dixit; Yashika Gupta; Tony Low; Anshuman Kumar Gate tunable light-matter interaction in natural biaxial hyperbolic van der Waals heterostructures Journal Article Nanophotonics, 2022. @article{arXiv:2110.07526, title = {Gate tunable light-matter interaction in natural biaxial hyperbolic van der Waals heterostructures}, author = {Aneesh Bapat and Saurabh Dixit and Yashika Gupta and Tony Low and Anshuman Kumar}, url = {https://doi.org/10.1515/nanoph-2022-0034}, doi = {10.1515/nanoph-2022-0034}, year = {2022}, date = {2022-04-04}, journal = {Nanophotonics}, abstract = {The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, has opened up new avenues for mid-IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α-MoO3 for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid-infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography-free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub-wavelength thick naturally biaxial hyperbolic material.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, has opened up new avenues for mid-IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α-MoO3 for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid-infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography-free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub-wavelength thick naturally biaxial hyperbolic material.  |
Lekshmi Eswaramoorthy; Sudha Mokkapati; Anshuman Kumar Engineering Purcell factor anisotropy for dark and bright excitons in two dimensional semiconductors Journal Article Journal of Physics D: Applied Physics, 2022. @article{arXiv:2108.10680, title = {Engineering Purcell factor anisotropy for dark and bright excitons in two dimensional semiconductors}, author = {Lekshmi Eswaramoorthy and Sudha Mokkapati and Anshuman Kumar}, url = {https://doi.org/10.1088/1361-6463/ac570e}, doi = {10.1088/1361-6463/ac570e}, year = {2022}, date = {2022-02-21}, journal = {Journal of Physics D: Applied Physics}, abstract = {Tightly bound dark excitons in atomically thin semiconductors can be used for various optoelectronic applications including light storage and quantum communication. Their optical accessibility is however limited due to their out-of-plane transition dipole moment. We thus propose to strengthen the coupling of dark excitons in two dimensional materials with out-of-plane resonant modes of a cavity at room temperature, by engineering the anisotropy in the Purcell factor. A silica micro-disk characterised by high confinement of light in small modal volume, high Q-factor and free spectral range is used to couple to the excitons in monolayer transition metal dichalcogenides. We show numerically that the tapering of sidewalls of the micro-disk is an extremely versatile route for achieving the selective coupling of whispering gallery modes to light emitted from out-of-plane dipoles to the detriment of that from in-plane ones for four representative monolayer transition metal dichalcogenides.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Tightly bound dark excitons in atomically thin semiconductors can be used for various optoelectronic applications including light storage and quantum communication. Their optical accessibility is however limited due to their out-of-plane transition dipole moment. We thus propose to strengthen the coupling of dark excitons in two dimensional materials with out-of-plane resonant modes of a cavity at room temperature, by engineering the anisotropy in the Purcell factor. A silica micro-disk characterised by high confinement of light in small modal volume, high Q-factor and free spectral range is used to couple to the excitons in monolayer transition metal dichalcogenides. We show numerically that the tapering of sidewalls of the micro-disk is an extremely versatile route for achieving the selective coupling of whispering gallery modes to light emitted from out-of-plane dipoles to the detriment of that from in-plane ones for four representative monolayer transition metal dichalcogenides.  |
Yashika Gupta, Sudharm Rathore, Aparna Singh, Anshuman Kumar Tailoring the mechanical response of Ruddlesden Popper lead halide perovskites Journal Article Journal of Alloys and Compounds, 901 , pp. 163575, 2022, ISSN: 0925-8388. @article{GUPTA2022163575, title = {Tailoring the mechanical response of Ruddlesden Popper lead halide perovskites}, author = {Yashika Gupta, Sudharm Rathore, Aparna Singh, Anshuman Kumar}, url = {https://www.sciencedirect.com/science/article/pii/S0925838821049859}, doi = {10.1016/j.jallcom.2021.163575}, issn = {0925-8388}, year = {2022}, date = {2022-01-05}, journal = {Journal of Alloys and Compounds}, volume = {901}, pages = {163575}, abstract = {Two dimensional (2D) Ruddlesden Popper perovskites have been extensively studied for their exceptional optical and electronic characteristics while only a few studies have shed light on their mechanical properties. The existing literature mainly discusses the mechanical strength of single crystal perovskites, however a study of structure tunability of 2D perovskite thin films is still missing. In this study, we report the effect of number of inorganic layers ‘n’ on elastic modulus of 2D, quasi-2D perovskites and 3D perovskite thin films using nanoindentation technique. Our studies indicate the role of orientation of the inorganic layers in perovskite films in tailoring their mechanical response. The experimental results have been substantiated using first principle density functional theory (DFT) calculations. We also report other important mechanical parameters namely, shear modulus, bulk modulus, Poisson’s ratio, Pugh’s ratio, Vickers hardness, yield strength and the universal elastic anisotropic index using DFT simulations. Anisotropy is observed in the elastic modulus of the materials under study and has been discussed in detail in the manuscript. Understanding the mechanical behaviour of 2D Ruddlesden Popper perovskites thin film in comparison with conventional 3D perovskite offers intriguing insights into the atomic layer dependent properties and paves the path for next generation mechanically durable and novel devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two dimensional (2D) Ruddlesden Popper perovskites have been extensively studied for their exceptional optical and electronic characteristics while only a few studies have shed light on their mechanical properties. The existing literature mainly discusses the mechanical strength of single crystal perovskites, however a study of structure tunability of 2D perovskite thin films is still missing. In this study, we report the effect of number of inorganic layers ‘n’ on elastic modulus of 2D, quasi-2D perovskites and 3D perovskite thin films using nanoindentation technique. Our studies indicate the role of orientation of the inorganic layers in perovskite films in tailoring their mechanical response. The experimental results have been substantiated using first principle density functional theory (DFT) calculations. We also report other important mechanical parameters namely, shear modulus, bulk modulus, Poisson’s ratio, Pugh’s ratio, Vickers hardness, yield strength and the universal elastic anisotropic index using DFT simulations. Anisotropy is observed in the elastic modulus of the materials under study and has been discussed in detail in the manuscript. Understanding the mechanical behaviour of 2D Ruddlesden Popper perovskites thin film in comparison with conventional 3D perovskite offers intriguing insights into the atomic layer dependent properties and paves the path for next generation mechanically durable and novel devices.  |
2021 |
Yongjun Lee, Johnathas D’arf Severo Forte, Andrey Chaves, Anshuman Kumar, Trang Thu Tran, Youngbum Kim, Shrawan Roy, Takashi Taniguchi, Kenji Watanabe, Alexey Chernikov, Joon I. Jang, Tony Low; Jeongyong Kim Boosting quantum yields in 2D semiconductors via proximal metal plates Journal Article Nature Communications, 12 (1), pp. 7095, 2021. @article{arXiv:2012.15114, title = {Boosting quantum yields in 2D semiconductors via proximal metal plates}, author = {Yongjun Lee, Johnathas D’arf Severo Forte, Andrey Chaves, Anshuman Kumar, Trang Thu Tran, Youngbum Kim, Shrawan Roy, Takashi Taniguchi, Kenji Watanabe, Alexey Chernikov, Joon I. Jang, Tony Low and Jeongyong Kim }, url = {https://www.nature.com/articles/s41467-021-27418-x}, doi = {10.1038/s41467-021-27418-x}, year = {2021}, date = {2021-12-07}, journal = {Nature Communications}, volume = {12}, number = {1}, pages = {7095}, abstract = {Monolayer transition metal dichalcogenides (1L-TMDs) have tremendous potential as atomically thin, direct bandgap semiconductors that can be used as convenient building blocks for quantum photonic devices. However, the short exciton lifetime due to the defect traps and the strong exciton-exciton interaction in TMDs has significantly limited the efficiency of exciton emission from this class of materials. Here, we show that exciton-exciton interaction in 1L-WS2 can be effectively screened using an ultra-flat Au film substrate separated by multilayers of hexagonal boron nitride. Under this geometry, induced dipolar exciton-exciton interaction becomes quadrupole-quadrupole interaction because of effective image dipoles formed within the metal. The suppressed exciton-exciton interaction leads to a significantly improved quantum yield by an order of magnitude, which is also accompanied by a reduction in the exciton-exciton annihilation (EEA) rate, as confirmed by time-resolved optical measurements. A theoretical model accounting for the screening of the dipole-dipole interaction is in a good agreement with the dependence of EEA on exciton densities. Our results suggest that fundamental EEA processes in the TMD can be engineered through proximal metallic screening, which represents a practical approach towards high-efficiency 2D light emitters.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Monolayer transition metal dichalcogenides (1L-TMDs) have tremendous potential as atomically thin, direct bandgap semiconductors that can be used as convenient building blocks for quantum photonic devices. However, the short exciton lifetime due to the defect traps and the strong exciton-exciton interaction in TMDs has significantly limited the efficiency of exciton emission from this class of materials. Here, we show that exciton-exciton interaction in 1L-WS2 can be effectively screened using an ultra-flat Au film substrate separated by multilayers of hexagonal boron nitride. Under this geometry, induced dipolar exciton-exciton interaction becomes quadrupole-quadrupole interaction because of effective image dipoles formed within the metal. The suppressed exciton-exciton interaction leads to a significantly improved quantum yield by an order of magnitude, which is also accompanied by a reduction in the exciton-exciton annihilation (EEA) rate, as confirmed by time-resolved optical measurements. A theoretical model accounting for the screening of the dipole-dipole interaction is in a good agreement with the dependence of EEA on exciton densities. Our results suggest that fundamental EEA processes in the TMD can be engineered through proximal metallic screening, which represents a practical approach towards high-efficiency 2D light emitters.  |
Nihar Ranjan Sahoo; Saurabh Dixit; Anuj Kumar Singh; Sang Hoon Nam; Nicholas X. Fang; Anshuman Kumar High temperature mid-IR polarizer via natural in-plane hyperbolic van der Waals crystals Journal Article Advanced Optical Materials, pp. 2101919, 2021. @article{arXiv:2108.08510, title = {High temperature mid-IR polarizer via natural in-plane hyperbolic van der Waals crystals}, author = {Nihar Ranjan Sahoo and Saurabh Dixit and Anuj Kumar Singh and Sang Hoon Nam and Nicholas X. Fang and Anshuman Kumar}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202101919}, doi = {10.1002/adom.202101919}, year = {2021}, date = {2021-12-07}, journal = {Advanced Optical Materials}, pages = {2101919}, abstract = {Integration of conventional mid to long-wavelength infrared polarizers with chip-scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its non-lithographic fabrication, ease of integration with chip-scale platforms, and room temperature operation. In the present work, mid-IR optical response of the sub-wavelength thin films of α-MoO3 is investigated for application towards high temperature mid-IR transmission and reflection type thin film polarizer. To our knowledge, this is the first report of above room temperature mid-IR optical response of α-MoO3 to determine the thermal stability of the proposed device. We find that our α-MoO3 based polarizer retains high extinction ratio with peak value exceeding 10 dB, up to a temperature of 140∘C. We explain our experimental findings by natural in-plane hyperbolic anisotropy of α-MoO3 in the mid-IR, high temperature X-ray diffraction and Raman spectroscopic measurements. This work opens up new avenues for naturally in-plane hyperbolic van der Waals thin-films to realize sub-wavelength IR optical components without lithographic constraints.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Integration of conventional mid to long-wavelength infrared polarizers with chip-scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its non-lithographic fabrication, ease of integration with chip-scale platforms, and room temperature operation. In the present work, mid-IR optical response of the sub-wavelength thin films of α-MoO3 is investigated for application towards high temperature mid-IR transmission and reflection type thin film polarizer. To our knowledge, this is the first report of above room temperature mid-IR optical response of α-MoO3 to determine the thermal stability of the proposed device. We find that our α-MoO3 based polarizer retains high extinction ratio with peak value exceeding 10 dB, up to a temperature of 140∘C. We explain our experimental findings by natural in-plane hyperbolic anisotropy of α-MoO3 in the mid-IR, high temperature X-ray diffraction and Raman spectroscopic measurements. This work opens up new avenues for naturally in-plane hyperbolic van der Waals thin-films to realize sub-wavelength IR optical components without lithographic constraints.  |
Sudharm Rathore; Guifang Han; Anshuman Kumar; Wei Lin Leong; Aparna Singh Elastic modulus tailoring in CH3NH3PbI3 perovskite system by the introduction of two dimensionality using (5-AVA)2PbI4 Journal Article Solar Energy, 224 , pp. 27-34, 2021, ISSN: 0038-092X. @article{AVA202127, title = {Elastic modulus tailoring in CH3NH3PbI3 perovskite system by the introduction of two dimensionality using (5-AVA)2PbI4}, author = {Sudharm Rathore and Guifang Han and Anshuman Kumar and Wei Lin Leong and Aparna Singh}, url = {https://www.sciencedirect.com/science/article/pii/S0038092X21003911}, doi = {10.1016/j.solener.2021.05.027}, issn = {0038-092X}, year = {2021}, date = {2021-06-04}, journal = {Solar Energy}, volume = {224}, pages = {27-34}, abstract = {The stiffness of the perovskite active layer plays a critical role in influencing the flexibility of a solar cell. This is one of the important parameters which must be considered while deciding the architecture of the flexible device. Till now, there have been limited studies on the mechanical properties of 2D and 3D perovskite thin films. In this study, we report the very first time, the elastic modulus of the most commonly used 3D perovskite i.e. methylammonium lead iodide (MaPbI3), the pure 2D perovskite (5-AVA)2PbI4 which is based on 5-aminovaleric acid (5-AVA) cation as well as the 2D-3D mixed perovskites thin films through nanoindentation technique. The elastic modulus for the 3D perovskite has been found to be around 16.5 GPa, while it decreases to 6.3 GPa for pure 2D perovskite. The experimental results have also been corroborated by density functional theory calculations done for the 2D and 3D perovskite. The 2D perovskite shows a much lower modulus than the 3D perovskite and can be used within a 2D-3D mixture for improving the mechanical flexibility of the active layer. Also, it is well established that 2D perovskites are much more stable against moisture as compared to their 3D counterparts due to the presence of hydrophobic organic alkyl ammonium cation. Thus, the device containing an active layer comprising of a mixture of 3D and 2D perovskite can be used to improve the environmental stability of the overall device in addition to achieving mechanical durability.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The stiffness of the perovskite active layer plays a critical role in influencing the flexibility of a solar cell. This is one of the important parameters which must be considered while deciding the architecture of the flexible device. Till now, there have been limited studies on the mechanical properties of 2D and 3D perovskite thin films. In this study, we report the very first time, the elastic modulus of the most commonly used 3D perovskite i.e. methylammonium lead iodide (MaPbI3), the pure 2D perovskite (5-AVA)2PbI4 which is based on 5-aminovaleric acid (5-AVA) cation as well as the 2D-3D mixed perovskites thin films through nanoindentation technique. The elastic modulus for the 3D perovskite has been found to be around 16.5 GPa, while it decreases to 6.3 GPa for pure 2D perovskite. The experimental results have also been corroborated by density functional theory calculations done for the 2D and 3D perovskite. The 2D perovskite shows a much lower modulus than the 3D perovskite and can be used within a 2D-3D mixture for improving the mechanical flexibility of the active layer. Also, it is well established that 2D perovskites are much more stable against moisture as compared to their 3D counterparts due to the presence of hydrophobic organic alkyl ammonium cation. Thus, the device containing an active layer comprising of a mixture of 3D and 2D perovskite can be used to improve the environmental stability of the overall device in addition to achieving mechanical durability.  |
Saurabh Dixit, Nihar Ranjan Sahoo, Abhishek Mall, Anshuman Kumar Mid infrared polarization engineering via sub-wavelength biaxial hyperbolic van der Waals crystals Journal Article Scientific Reports, 11 (1), pp. 6612, 2021. @article{arXiv:2007.11265, title = {Mid infrared polarization engineering via sub-wavelength biaxial hyperbolic van der Waals crystals}, author = {Saurabh Dixit, Nihar Ranjan Sahoo, Abhishek Mall, Anshuman Kumar}, url = {https://www.nature.com/articles/s41598-021-86056-x}, doi = {10.1038/s41598-021-86056-x}, year = {2021}, date = {2021-03-23}, journal = {Scientific Reports}, volume = {11}, number = {1}, pages = {6612}, abstract = {Mid-infrared (IR) spectral region is of immense importance for astronomy, medical diagnosis, security and imaging due to the existence of the vibrational modes of many important molecules in this spectral range. Therefore, there is a particular interest in miniaturization and integration of IR optical components. To this end, 2D van der Waals (vdW) crystals have shown great potential owing to their ease of integration with other optoelectronic platforms and room temperature operation. Recently, 2D vdW crystals of α-MoO3 and α-V2O5 have been shown to possess the unique phenomenon of natural in-plane biaxial hyperbolicity in the mid-infrared frequency regime at room temperature. Here, we report a unique application of this in-plane hyperbolicity for designing highly efficient, lithography free and extremely subwavelength mid-IR photonic devices for polarization engineering. In particular, we show the possibility of a significant reduction in the device footprint while maintaining an enormous extinction ratio from α-MoO3 and α-V2O5 based mid-IR polarizers. Furthermore, we investigate the application of sub-wavelength thin films of these vdW crystals towards engineering the polarization state of incident mid-IR light via precise control of polarization rotation, ellipticity and relative phase. We explain our results using natural in-plane hyperbolic anisotropy of α-MoO3 and α-V2O5 via both analytical and full-wave electromagnetic simulations. This work provides a lithography free alternative for miniaturized mid-infrared photonic devices using the hyperbolic anisotropy of α-MoO3 and α-V2O5.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Mid-infrared (IR) spectral region is of immense importance for astronomy, medical diagnosis, security and imaging due to the existence of the vibrational modes of many important molecules in this spectral range. Therefore, there is a particular interest in miniaturization and integration of IR optical components. To this end, 2D van der Waals (vdW) crystals have shown great potential owing to their ease of integration with other optoelectronic platforms and room temperature operation. Recently, 2D vdW crystals of α-MoO3 and α-V2O5 have been shown to possess the unique phenomenon of natural in-plane biaxial hyperbolicity in the mid-infrared frequency regime at room temperature. Here, we report a unique application of this in-plane hyperbolicity for designing highly efficient, lithography free and extremely subwavelength mid-IR photonic devices for polarization engineering. In particular, we show the possibility of a significant reduction in the device footprint while maintaining an enormous extinction ratio from α-MoO3 and α-V2O5 based mid-IR polarizers. Furthermore, we investigate the application of sub-wavelength thin films of these vdW crystals towards engineering the polarization state of incident mid-IR light via precise control of polarization rotation, ellipticity and relative phase. We explain our results using natural in-plane hyperbolic anisotropy of α-MoO3 and α-V2O5 via both analytical and full-wave electromagnetic simulations. This work provides a lithography free alternative for miniaturized mid-infrared photonic devices using the hyperbolic anisotropy of α-MoO3 and α-V2O5.  |
2020 |
Abhishek Mall, Abhijeet Patil, Amit Sethi, Anshuman Kumar A Cyclical Deep Learning Based Framework For Simultaneous Inverse and Forward design of Nanophotonic Metasurfaces Journal Article Scientific Reports, 10 (1), pp. 19427, 2020. @article{arXiv:2005.12796, title = {A Cyclical Deep Learning Based Framework For Simultaneous Inverse and Forward design of Nanophotonic Metasurfaces}, author = {Abhishek Mall, Abhijeet Patil, Amit Sethi, Anshuman Kumar}, url = {https://www.nature.com/articles/s41598-020-76400-y}, doi = {10.1038/s41598-020-76400-y}, year = {2020}, date = {2020-11-10}, journal = {Scientific Reports}, volume = {10}, number = {1}, pages = {19427}, abstract = {The conventional approach to nanophotonic metasurface design and optimization for a targeted electromagnetic response involves exploring large geometry and material spaces. This is a highly iterative process based on trial and error, which is computationally costly and time consuming. Moreover, the non-uniqueness of structural designs and high non-linearity between electromagnetic response and design makes this problem challenging. To model this unintuitive relationship between electromagnetic response and metasurface structural design as a probability distribution in the design space, we introduce a framework for inverse design of nanophotonic metasurfaces based on cyclical deep learning (DL). The proposed framework performs inverse design and optimization mechanism for the generation of meta-atoms and meta-molecules as metasurface units based on DL models and genetic algorithm. The framework includes consecutive DL models that emulate both numerical electromagnetic simulation and iterative processes of optimization, and generate optimized structural designs while simultaneously performing forward and inverse design tasks. A selection and evaluation of generated structural designs is performed by the genetic algorithm to construct a desired optical response and design space that mimics real world responses. Importantly, our cyclical generation framework also explores the space of new metasurface topologies. As an example application of the utility of our proposed architecture, we demonstrate the inverse design of gap-plasmon based half-wave plate metasurface for user-defined optical response. Our proposed technique can be easily generalized for designing nanophtonic metasurfaces for a wide range of targeted optical response.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The conventional approach to nanophotonic metasurface design and optimization for a targeted electromagnetic response involves exploring large geometry and material spaces. This is a highly iterative process based on trial and error, which is computationally costly and time consuming. Moreover, the non-uniqueness of structural designs and high non-linearity between electromagnetic response and design makes this problem challenging. To model this unintuitive relationship between electromagnetic response and metasurface structural design as a probability distribution in the design space, we introduce a framework for inverse design of nanophotonic metasurfaces based on cyclical deep learning (DL). The proposed framework performs inverse design and optimization mechanism for the generation of meta-atoms and meta-molecules as metasurface units based on DL models and genetic algorithm. The framework includes consecutive DL models that emulate both numerical electromagnetic simulation and iterative processes of optimization, and generate optimized structural designs while simultaneously performing forward and inverse design tasks. A selection and evaluation of generated structural designs is performed by the genetic algorithm to construct a desired optical response and design space that mimics real world responses. Importantly, our cyclical generation framework also explores the space of new metasurface topologies. As an example application of the utility of our proposed architecture, we demonstrate the inverse design of gap-plasmon based half-wave plate metasurface for user-defined optical response. Our proposed technique can be easily generalized for designing nanophtonic metasurfaces for a wide range of targeted optical response.  |
Mandar Sohoni, Pankaj K. Jha, Muralidhar Nalabothula, Anshuman Kumar Interlayer Exciton Valleytronics in Bilayer Heterostructures Interfaced with a Metasurface Journal Article Applied Physics Letters, 117 (12), pp. 121101, 2020. @article{arXiv:1912.12080, title = {Interlayer Exciton Valleytronics in Bilayer Heterostructures Interfaced with a Metasurface}, author = {Mandar Sohoni, Pankaj K. Jha, Muralidhar Nalabothula, Anshuman Kumar}, url = {https://doi.org/10.1063/5.0015087}, doi = {10.1063/5.0015087}, year = {2020}, date = {2020-09-21}, journal = {Applied Physics Letters}, volume = {117}, number = {12}, pages = {121101}, abstract = {The recent proposal of using an anisotropic vacuum for generating valley coherence in transition metal dichalcogenide (TMDC) monolayers has expanded the potential of such valley degrees of freedom for applications in valleytronics. In this work, we open up a completely new regime, inaccessible with monolayer TMDCs, of spontaneously generated valley coherence in interlayer excitons in commensurate TMDC bilayer heterostructures. Using the peculiar out of plane polarization of interlayer excitons in conjunction with an in-plane anisotropic electromagnetic vacuum, we show that a much larger region of the Bloch sphere can be accessible in these heterostructures. We show the accessible phases of these excitons given this in-plane anisotropic electromagnetic vacuum. Our analysis of spontaneous coherence for interlayer excitons may pave the way for engineering an array of interacting quantum emitters in Moiré heterostructures.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The recent proposal of using an anisotropic vacuum for generating valley coherence in transition metal dichalcogenide (TMDC) monolayers has expanded the potential of such valley degrees of freedom for applications in valleytronics. In this work, we open up a completely new regime, inaccessible with monolayer TMDCs, of spontaneously generated valley coherence in interlayer excitons in commensurate TMDC bilayer heterostructures. Using the peculiar out of plane polarization of interlayer excitons in conjunction with an in-plane anisotropic electromagnetic vacuum, we show that a much larger region of the Bloch sphere can be accessible in these heterostructures. We show the accessible phases of these excitons given this in-plane anisotropic electromagnetic vacuum. Our analysis of spontaneous coherence for interlayer excitons may pave the way for engineering an array of interacting quantum emitters in Moiré heterostructures.  |
Feliciano Giustino; Manuel Bibes; Jin Hong Lee; Felix Trier; Roser Valentí; Stephen M. Winter; Young-Woo Son; Louis Taillefer; Christoph Heil; Adriana I. Figueroa; Bernard Plaçais; QuanSheng Wu; Oleg V Yazyev; Erik P A M Bakkers; Jesper Nygård; Pol Forn-Díaz; Silvano de Franceschi; Luis E. F. Foa Torres; James McIver; Anshuman Kumar; Tony Low; Regina Galceran; Sergio O. Valenzuela; Marius Vasile Costache; Aurélien Manchon; Eun-Ah Kim; Gabriel Ravanhani Schleder; Adalberto Fazzio; Stephan Roche The 2020 Quantum Materials Roadmap Journal Article Journal of Physics: Materials, 2020. @article{quantum_Materials_Roadmap, title = {The 2020 Quantum Materials Roadmap}, author = {Feliciano Giustino and Manuel Bibes and Jin Hong Lee and Felix Trier and Roser Valentí and Stephen M. Winter and Young-Woo Son and Louis Taillefer and Christoph Heil and Adriana I. Figueroa and Bernard Plaçais and QuanSheng Wu and Oleg V Yazyev and Erik P A M Bakkers and Jesper Nygård and Pol Forn-Díaz and Silvano de Franceschi and Luis E. F. Foa Torres and James McIver and Anshuman Kumar and Tony Low and Regina Galceran and Sergio O. Valenzuela and Marius Vasile Costache and Aurélien Manchon and Eun-Ah Kim and Gabriel Ravanhani Schleder and Adalberto Fazzio and Stephan Roche}, url = {https://doi.org/10.1088/2515-7639/abb74e}, doi = {10.1088/2515-7639/abb74e}, year = {2020}, date = {2020-09-10}, journal = {Journal of Physics: Materials}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
Abhishek Mall, Abhijeet Patil, Dipesh Tamboli, Amit Sethi, Anshuman Kumar Fast Design of Plasmonic Metasurfaces Enabled by Deep Learning Journal Article Journal of Physics D: Applied Physics, 53 (49), pp. 49LT01, 2020. @article{arXiv:2003.12402, title = {Fast Design of Plasmonic Metasurfaces Enabled by Deep Learning}, author = {Abhishek Mall, Abhijeet Patil, Dipesh Tamboli, Amit Sethi, Anshuman Kumar}, url = {https://doi.org/10.1088/1361-6463/abb33c}, doi = {10.1088/1361-6463/abb33c}, year = {2020}, date = {2020-08-27}, journal = {Journal of Physics D: Applied Physics}, volume = {53}, number = {49}, pages = {49LT01}, abstract = {Metasurfaces is an emerging field that enables the manipulation of light by an ultrathin structure composed of subwavelength antennae and fulfills an important requirement for miniaturized optical elements. Finding a new design for a metasurface or optimizing an existing design for a desired functionality is a computationally expensive and time consuming process as it is based on an iterative process of trial and error. We propose a deep learning (DL) architecture dubbed bidirectional autoencoder for nanophotonic metasurface design via a template search methodology. In contrast with the earlier approaches based on DL, our methodology addresses optimization in the space of multiple metasurface topologies instead of just one, in order to tackle the one to many mapping problem of inverse design. We demonstrate the creation of a Geometry and Parameter Space Library (GPSL) of metasurface designs with their corresponding optical response using our DL model. This GPSL acts as a universal design and response space for the optimization. As an example application, we use our methodology to design a multi-band gap-plasmon based half-wave plate metasurface. Through this example, we demonstrate the power of our technique in addressing the nonuniqueness problem of common inverse design approaches by showing that our network converges aptly to multiple metasurface topologies for the desired optical response. Our proposed technique would enable fast and accurate design and optimization of various kinds of metasurfaces with different functionalities.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Metasurfaces is an emerging field that enables the manipulation of light by an ultrathin structure composed of subwavelength antennae and fulfills an important requirement for miniaturized optical elements. Finding a new design for a metasurface or optimizing an existing design for a desired functionality is a computationally expensive and time consuming process as it is based on an iterative process of trial and error. We propose a deep learning (DL) architecture dubbed bidirectional autoencoder for nanophotonic metasurface design via a template search methodology. In contrast with the earlier approaches based on DL, our methodology addresses optimization in the space of multiple metasurface topologies instead of just one, in order to tackle the one to many mapping problem of inverse design. We demonstrate the creation of a Geometry and Parameter Space Library (GPSL) of metasurface designs with their corresponding optical response using our DL model. This GPSL acts as a universal design and response space for the optimization. As an example application, we use our methodology to design a multi-band gap-plasmon based half-wave plate metasurface. Through this example, we demonstrate the power of our technique in addressing the nonuniqueness problem of common inverse design approaches by showing that our network converges aptly to multiple metasurface topologies for the desired optical response. Our proposed technique would enable fast and accurate design and optimization of various kinds of metasurfaces with different functionalities.  |
Muralidhar Nalabothula, Pankaj Jha, Tony Low, Anshuman Kumar Engineering valley quantum interference in anisotropic van der Waals heterostructures Journal Article Phys. Rev. B, 102 (4), pp. 045416, 2020. @article{PhysRevB.102.045416, title = {Engineering valley quantum interference in anisotropic van der Waals heterostructures}, author = {Muralidhar Nalabothula, Pankaj Jha, Tony Low, Anshuman Kumar}, url = {https://link.aps.org/doi/10.1103/PhysRevB.102.045416}, doi = {10.1103/PhysRevB.102.045416}, year = {2020}, date = {2020-07-15}, journal = {Phys. Rev. B}, volume = {102}, number = {4}, pages = {045416}, abstract = {In this paper, we present a novel route to manipulate the spontaneous valley coherence in two-dimensional valleytronic materials interfaced with other layered materials hosting anisotropic polaritonic modes. We propose two implementations — one, using anisotropic plasmons in phosphorene and another with hyperbolic phonon polaritons in MoO3. In particular, we show the electrostatic tunability of the spontaneous valley coherence achieving robust valley coherence values in the near-infrared wavelengths at room temperature. The tunability of this valley coherence shown in these heterostructures would enable the realization of active valleytronic quantum circuitry.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this paper, we present a novel route to manipulate the spontaneous valley coherence in two-dimensional valleytronic materials interfaced with other layered materials hosting anisotropic polaritonic modes. We propose two implementations — one, using anisotropic plasmons in phosphorene and another with hyperbolic phonon polaritons in MoO3. In particular, we show the electrostatic tunability of the spontaneous valley coherence achieving robust valley coherence values in the near-infrared wavelengths at room temperature. The tunability of this valley coherence shown in these heterostructures would enable the realization of active valleytronic quantum circuitry.  |
2019 |
Edo van Veen, Andrei Nemilentsau, Anshuman Kumar, Rafael Roldan, Mikhail I. Katsnelson, Tony Low; Shengjun Yuan Tuning Two-Dimensional Hyperbolic Plasmons in Black Phosphorus Journal Article Phys. Rev. Applied, 12 (014011), 2019. @article{BP_hyperbolic, title = {Tuning Two-Dimensional Hyperbolic Plasmons in Black Phosphorus}, author = {Edo van Veen, Andrei Nemilentsau, Anshuman Kumar, Rafael Roldan, Mikhail I. Katsnelson, Tony Low and Shengjun Yuan}, url = {https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.12.014011}, doi = {10.1103/PhysRevApplied.12.014011}, year = {2019}, date = {2019-07-08}, journal = {Phys. Rev. Applied}, volume = {12}, number = {014011}, abstract = {Black phosphorus presents a very anisotropic crystal structure, making it a potential candidate for hyperbolic plasmonics, characterized by a permittivity tensor where one of the principal components is metallic and the other dielectric. Here we demonstrate that atomically thin black phosphorus can be engineered to be a hyperbolic material operating in a broad range of the electromagnetic spectrum from the entire visible spectrum to ultraviolet. With the introduction of an optical gain, a new hyperbolic region emerges in the infrared. The character of this hyperbolic plasmon depends on the interplay between gain and loss along the two crystalline directions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Black phosphorus presents a very anisotropic crystal structure, making it a potential candidate for hyperbolic plasmonics, characterized by a permittivity tensor where one of the principal components is metallic and the other dielectric. Here we demonstrate that atomically thin black phosphorus can be engineered to be a hyperbolic material operating in a broad range of the electromagnetic spectrum from the entire visible spectrum to ultraviolet. With the introduction of an optical gain, a new hyperbolic region emerges in the infrared. The character of this hyperbolic plasmon depends on the interplay between gain and loss along the two crystalline directions.  |
2018 |
Yong-Liang Zhang; Raymond P. H. Wu; Anshuman Kumar; Tieyan Si; Kin Hung Fung Nonsymmorphic symmetry-protected topological modes in plasmonic nanoribbon lattices Journal Article Phys. Rev. B, 97 (144203), 2018. @article{PRB_nonsym, title = {Nonsymmorphic symmetry-protected topological modes in plasmonic nanoribbon lattices}, author = {Yong-Liang Zhang and Raymond P. H. Wu and Anshuman Kumar and Tieyan Si and Kin Hung Fung}, doi = {10.1103/PhysRevB.97.144203}, year = {2018}, date = {2018-04-20}, journal = {Phys. Rev. B}, volume = {97}, number = {144203}, abstract = {Using a dynamic eigenresponse theory, we study the topological edge plasmon modes in dispersive plasmonic lattices constructed by unit cells of multiple nanoribbons. In dipole approximation, the bulk-edge correspondence in the lattices made of dimerized unit cell and one of its square-root daughter with nonsymmorphic symmetry are demonstrated. Calculations with consideration of dynamic long-range effects and retardation are compared to those given by nearest-neighbor approximations. It is shown that nonsymmorphic symmetry opens up two symmetric gaps where versatile topological edge plasmon modes are found. Unprecedented spectral shifts of the edge states with respect to the zero modes due to long-range coupling are found. The proposed ribbon structure is favorable to electrical gating and thus could serve as an on-chip platform for electrically controllable subwavelength edge states at optical wavelengths. Our eigenresponse approach provides a powerful tool for the radiative topological mode analysis in strongly coupled plasmonic lattices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using a dynamic eigenresponse theory, we study the topological edge plasmon modes in dispersive plasmonic lattices constructed by unit cells of multiple nanoribbons. In dipole approximation, the bulk-edge correspondence in the lattices made of dimerized unit cell and one of its square-root daughter with nonsymmorphic symmetry are demonstrated. Calculations with consideration of dynamic long-range effects and retardation are compared to those given by nearest-neighbor approximations. It is shown that nonsymmorphic symmetry opens up two symmetric gaps where versatile topological edge plasmon modes are found. Unprecedented spectral shifts of the edge states with respect to the zero modes due to long-range coupling are found. The proposed ribbon structure is favorable to electrical gating and thus could serve as an on-chip platform for electrically controllable subwavelength edge states at optical wavelengths. Our eigenresponse approach provides a powerful tool for the radiative topological mode analysis in strongly coupled plasmonic lattices.  |
Cl'ement Javerzac-Galy*; Anshuman Kumar*; Ryan D. Schilling; Nicolas Piro; Sina Khorasani; Matteo Barbone; Ilya Goykhman; Jacob B. Khurgin; Andrea C. Ferrari; Tobias J Kippenberg Excitonic emission of monolayer semiconductors near-field coupled to high-Q microresonators Journal Article Nano Letters, 18 (5), pp. 3138–3146, 2018. @article{NL_EPFL, title = {Excitonic emission of monolayer semiconductors near-field coupled to high-Q microresonators}, author = {Cl{'e}ment Javerzac-Galy* and Anshuman Kumar* and Ryan D. Schilling and Nicolas Piro and Sina Khorasani and Matteo Barbone and Ilya Goykhman and Jacob B. Khurgin and Andrea C. Ferrari and Tobias J Kippenberg}, doi = {10.1021/acs.nanolett.8b00749}, year = {2018}, date = {2018-04-06}, journal = {Nano Letters}, volume = {18}, number = {5}, pages = {3138--3146}, abstract = {We present quantum yield measurements of single layer WSe2 (1L-WSe2) integrated with high-Q (Q > 106) optical microdisk cavities, using an efficient (η > 90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe2 in the evanescent cavity field. This preserves the microresonator high intrinsic quality factor (Q > 106) below the bandgap of 1L-WSe2. The cavity quantum yield is QYc ≈ 10^–3, consistent with operation in the broad emitter regime (i.e., the emission lifetime of 1L-WSe2 is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present quantum yield measurements of single layer WSe2 (1L-WSe2) integrated with high-Q (Q > 106) optical microdisk cavities, using an efficient (η > 90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe2 in the evanescent cavity field. This preserves the microresonator high intrinsic quality factor (Q > 106) below the bandgap of 1L-WSe2. The cavity quantum yield is QYc ≈ 10^–3, consistent with operation in the broad emitter regime (i.e., the emission lifetime of 1L-WSe2 is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation.  |
2017 |
Cl'ement Javerzac-Galy; Nicolas Piro; Ryan D. Schilling; Anshuman Kumar; Matteo Barbone; Ilya Goykhman; Andrea C. Ferrari; Tobias J. Kippenberg High-Q optical microresonators functionalized with two-dimensional material Conference Conference on Lasers and Electro-Optics, Optical Society of America Optical Society of America, 2017. @conference{JAVERZAC-GALY:17, title = {High-Q optical microresonators functionalized with two-dimensional material}, author = {Cl{'e}ment Javerzac-Galy and Nicolas Piro and Ryan D. Schilling and Anshuman Kumar and Matteo Barbone and Ilya Goykhman and Andrea C. Ferrari and Tobias J. Kippenberg}, url = {http://www.osapublishing.org/abstract.cfm?URI=CLEO_SI-2017-STh1I.2}, doi = {10.1364/CLEO_SI.2017.STh1I.2}, year = {2017}, date = {2017-01-01}, booktitle = {Conference on Lasers and Electro-Optics}, pages = {STh1I.2}, publisher = {Optical Society of America}, organization = {Optical Society of America}, abstract = {We report the functionalization of high-Q silica microdisks with WSe2 and their optical characterization. Background-free cavity enhanced photoluminescence and photoluminescence saturation are observed at room temperature. We show precise measurements of the quantum yield of WSe2.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } We report the functionalization of high-Q silica microdisks with WSe2 and their optical characterization. Background-free cavity enhanced photoluminescence and photoluminescence saturation are observed at room temperature. We show precise measurements of the quantum yield of WSe2.  |
Tony Low; Andrey Chaves; Joshua D. Caldwell; Anshuman Kumar; Nicholas X. Fang; Phaedon Avouris; Tony F. Heinz; Francisco Guinea; Luis Martin-Moreno; Frank Koppens Polaritons in layered two-dimensional materials Journal Article Nature Materials, 16 (2), pp. 182–194, 2017, ISSN: 1476-1122. @article{low_polaritons_2017, title = {Polaritons in layered two-dimensional materials}, author = {Tony Low and Andrey Chaves and Joshua D. Caldwell and Anshuman Kumar and Nicholas X. Fang and Phaedon Avouris and Tony F. Heinz and Francisco Guinea and Luis Martin-Moreno and Frank Koppens}, url = {http://www.nature.com/nmat/journal/v16/n2/full/nmat4792.html}, doi = {10.1038/nmat4792}, issn = {1476-1122}, year = {2017}, date = {2017-01-01}, journal = {Nature Materials}, volume = {16}, number = {2}, pages = {182–194}, abstract = {In recent years, enhanced light–matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials. In graphene, electrically tunable and highly confined plasmon-polaritons were predicted and observed, opening up opportunities for optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal boron nitride, low-loss infrared-active phonon-polaritons exhibit hyperbolic behaviour for some frequencies, allowing for ray-like propagation exhibiting high quality factors and hyperlensing effects. In transition metal dichalcogenides, reduced screening in the 2D limit leads to optically prominent excitons with large binding energy, with these polaritonic modes having been recently observed with scanning near-field optical microscopy. Here, we review recent progress in state-of-the-art experiments, and survey the vast library of polaritonic modes in 2D materials, their optical spectral properties, figures of merit and application space. Taken together, the emerging field of 2D material polaritonics and their hybrids provide enticing avenues for manipulating light–matter interactions across the visible, infrared to terahertz spectral ranges, with new optical control beyond what can be achieved using traditional bulk materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In recent years, enhanced light–matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials. In graphene, electrically tunable and highly confined plasmon-polaritons were predicted and observed, opening up opportunities for optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal boron nitride, low-loss infrared-active phonon-polaritons exhibit hyperbolic behaviour for some frequencies, allowing for ray-like propagation exhibiting high quality factors and hyperlensing effects. In transition metal dichalcogenides, reduced screening in the 2D limit leads to optically prominent excitons with large binding energy, with these polaritonic modes having been recently observed with scanning near-field optical microscopy. Here, we review recent progress in state-of-the-art experiments, and survey the vast library of polaritonic modes in 2D materials, their optical spectral properties, figures of merit and application space. Taken together, the emerging field of 2D material polaritonics and their hybrids provide enticing avenues for manipulating light–matter interactions across the visible, infrared to terahertz spectral ranges, with new optical control beyond what can be achieved using traditional bulk materials.  |
2016 |
Anshuman Kumar; Andrei Nemilentsau; Kin Hung Fung; George Hanson; Nicholas X. Fang; Tony Low Chiral plasmon in gapped Dirac systems Journal Article Phys. Rev. B, 93 , pp. 041413, 2016. @article{PhysRevB.93.041413, title = {Chiral plasmon in gapped Dirac systems}, author = {Anshuman Kumar and Andrei Nemilentsau and Kin Hung Fung and George Hanson and Nicholas X. Fang and Tony Low}, url = {http://link.aps.org/doi/10.1103/PhysRevB.93.041413}, doi = {10.1103/PhysRevB.93.041413}, year = {2016}, date = {2016-01-01}, journal = {Phys. Rev. B}, volume = {93}, pages = {041413}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
Himanshu Tyagi; Ajay Kushwaha; Anshuman Kumar; Mohammed Aslam A Facile pH Controlled Citrate-Based Reduction Method for Gold Nanoparticle Synthesis at Room Temperature Journal Article Nanoscale Research Letters, 11 (1), pp. 1–11, 2016, ISSN: 1556-276X. @article{Tyagi2016, title = {A Facile pH Controlled Citrate-Based Reduction Method for Gold Nanoparticle Synthesis at Room Temperature}, author = {Himanshu Tyagi and Ajay Kushwaha and Anshuman Kumar and Mohammed Aslam}, url = {http://dx.doi.org/10.1186/s11671-016-1576-5}, doi = {10.1186/s11671-016-1576-5}, issn = {1556-276X}, year = {2016}, date = {2016-01-01}, journal = {Nanoscale Research Letters}, volume = {11}, number = {1}, pages = {1–11}, abstract = {The synthesis of gold nanoparticles using citrate reduction process has been revisited. A simplified room temperature approach to standard Turkevich synthesis is employed to obtain fairly monodisperse gold nanoparticles. The role of initial pH alongside the concentration ratio of reactants is explored for the size control of Au nanoparticles. The particle size distribution has been investigated using UV-vis spectroscopy and transmission electron microscope (TEM). At optimal pH of 5, gold nanoparticles obtained are highly monodisperse and spherical in shape and have narrower size distribution (sharp surface plasmon at 520~nm). For other pH conditions, particles are non-uniform and polydisperse, showing a red-shift in plasmon peak due to aggregation and large particle size distribution. The room temperature approach results in highly stable textquotelefttextquoteleftcolloidaltextquoterighttextquoteright suspension of gold nanoparticles. The stability test through absorption spectroscopy indicates no sign of aggregation for a month. The rate of reduction of auric ionic species by citrate ions is determined via UV absorbance studies. The size of nanoparticles under various conditions is thus predicted using a theoretical model that incorporates nucleation, growth, and aggregation processes. The faster rate of reduction yields better size distribution for optimized pH and reactant concentrations. The model involves solving population balance equation for continuously evolving particle size distribution by discretization techniques. The particle sizes estimated from the simulations (13 to 25~nm) are close to the experimental ones (10 to 32~nm) and corroborate the similarity of reaction processes at 300 and 373~K (classical Turkevich reaction). Thus, substitution of experimentally measured rate of disappearance of auric ionic species into theoretical model enables us to capture the unusual experimental observations.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The synthesis of gold nanoparticles using citrate reduction process has been revisited. A simplified room temperature approach to standard Turkevich synthesis is employed to obtain fairly monodisperse gold nanoparticles. The role of initial pH alongside the concentration ratio of reactants is explored for the size control of Au nanoparticles. The particle size distribution has been investigated using UV-vis spectroscopy and transmission electron microscope (TEM). At optimal pH of 5, gold nanoparticles obtained are highly monodisperse and spherical in shape and have narrower size distribution (sharp surface plasmon at 520~nm). For other pH conditions, particles are non-uniform and polydisperse, showing a red-shift in plasmon peak due to aggregation and large particle size distribution. The room temperature approach results in highly stable textquotelefttextquoteleftcolloidaltextquoterighttextquoteright suspension of gold nanoparticles. The stability test through absorption spectroscopy indicates no sign of aggregation for a month. The rate of reduction of auric ionic species by citrate ions is determined via UV absorbance studies. The size of nanoparticles under various conditions is thus predicted using a theoretical model that incorporates nucleation, growth, and aggregation processes. The faster rate of reduction yields better size distribution for optimized pH and reactant concentrations. The model involves solving population balance equation for continuously evolving particle size distribution by discretization techniques. The particle sizes estimated from the simulations (13 to 25~nm) are close to the experimental ones (10 to 32~nm) and corroborate the similarity of reaction processes at 300 and 373~K (classical Turkevich reaction). Thus, substitution of experimentally measured rate of disappearance of auric ionic species into theoretical model enables us to capture the unusual experimental observations.  |
2015 |
Navid Nemati; Anshuman Kumar; Denis Lafarge; Nicholas X. Fang Nonlocal description of sound propagation through an array of Helmholtz resonators Journal Article Comptes Rendus M'ecanique, 343 (12), pp. 656 - 669, 2015, ISSN: 1631-0721, (Acoustic metamaterials and phononic crystals). @article{Nemati2015656, title = {Nonlocal description of sound propagation through an array of Helmholtz resonators}, author = {Navid Nemati and Anshuman Kumar and Denis Lafarge and Nicholas X. Fang}, url = {http://www.sciencedirect.com/science/article/pii/S1631072115000558}, doi = {http://dx.doi.org/10.1016/j.crme.2015.05.001}, issn = {1631-0721}, year = {2015}, date = {2015-01-01}, journal = {Comptes Rendus M'ecanique}, volume = {343}, number = {12}, pages = {656 - 669}, note = {Acoustic metamaterials and phononic crystals}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
Anshuman Kumar; Tony Low; Kin Hung Fung; Phaedon Avouris; Nicholas X. Fang Tunable Light–Matter Interaction and the Role of Hyperbolicity in Graphene–hBN System Journal Article Nano Letters, 15 (5), pp. 3172-3180, 2015. @article{doi:10.1021/acs.nanolett.5b01191, title = {Tunable Light–Matter Interaction and the Role of Hyperbolicity in Graphene–hBN System}, author = {Anshuman Kumar and Tony Low and Kin Hung Fung and Phaedon Avouris and Nicholas X. Fang}, url = {http://dx.doi.org/10.1021/acs.nanolett.5b01191}, doi = {10.1021/acs.nanolett.5b01191}, year = {2015}, date = {2015-01-01}, journal = {Nano Letters}, volume = {15}, number = {5}, pages = {3172-3180}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
2014 |
Anshuman Kumar; Kin Hung Fung; M. T. Homer Reid; Nicholas X. Fang Transformation optics scheme for two-dimensional materials Journal Article Opt. Lett., 39 (7), pp. 2113–2116, 2014. @article{Kumar_TO, title = {Transformation optics scheme for two-dimensional materials}, author = {Anshuman Kumar and Kin Hung Fung and M. T. Homer Reid and Nicholas X. Fang}, url = {http://ol.osa.org/abstract.cfm?URI=ol-39-7-2113}, doi = {10.1364/OL.39.002113}, year = {2014}, date = {2014-04-01}, journal = {Opt. Lett.}, volume = {39}, number = {7}, pages = {2113–2116}, publisher = {OSA}, abstract = {Two-dimensional optical materials, such as graphene, can be characterized by surface conductivity. So far, the transformation optics schemes have focused on three-dimensional properties such as permittivity ϵ and permeability μ. In this Letter, we use a scheme for transforming surface currents to highlight that the surface conductivity transforms in a way different from ϵ and μ. We use this surface conductivity transformation to demonstrate an example problem of reducing the scattering of the plasmon mode from sharp protrusions in graphene.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional optical materials, such as graphene, can be characterized by surface conductivity. So far, the transformation optics schemes have focused on three-dimensional properties such as permittivity ϵ and permeability μ. In this Letter, we use a scheme for transforming surface currents to highlight that the surface conductivity transforms in a way different from ϵ and μ. We use this surface conductivity transformation to demonstrate an example problem of reducing the scattering of the plasmon mode from sharp protrusions in graphene.  |
Anshuman Kumar; Kin Hung Fung; M. T. Homer Reid; Nicholas X. Fang Photon emission rate engineering using graphene nanodisc cavities Journal Article Opt. Express, 22 (6), pp. 6400–6415, 2014. @article{Kumar:14, title = {Photon emission rate engineering using graphene nanodisc cavities}, author = {Anshuman Kumar and Kin Hung Fung and M. T. Homer Reid and Nicholas X. Fang}, url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-22-6-6400}, doi = {10.1364/OE.22.006400}, year = {2014}, date = {2014-03-01}, journal = {Opt. Express}, volume = {22}, number = {6}, pages = {6400–6415}, publisher = {OSA}, abstract = {In this work, we present a systematic study of the plasmon modes in a system of vertically stacked pair of graphene discs. Quasistatic approximation is used to model the eigenmodes of the system. Eigen-response theory is employed to explain the spatial dependence of the coupling between the plasmon modes and a quantum emitter. These results show a good match between the semi-analytical calculation and full-wave simulations. Secondly, we have shown that it is possible to engineer the decay rates of a quantum emitter placed inside and near this cavity, using Fermi level tuning, via gate voltages and variation of emitter location and polarization. We highlighted that by coupling to the bright plasmon mode, the radiative efficiency of the emitter can be enhanced compared to the single graphene disc case, whereas the dark plasmon mode suppresses the radiative efficiency.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this work, we present a systematic study of the plasmon modes in a system of vertically stacked pair of graphene discs. Quasistatic approximation is used to model the eigenmodes of the system. Eigen-response theory is employed to explain the spatial dependence of the coupling between the plasmon modes and a quantum emitter. These results show a good match between the semi-analytical calculation and full-wave simulations. Secondly, we have shown that it is possible to engineer the decay rates of a quantum emitter placed inside and near this cavity, using Fermi level tuning, via gate voltages and variation of emitter location and polarization. We highlighted that by coupling to the bright plasmon mode, the radiative efficiency of the emitter can be enhanced compared to the single graphene disc case, whereas the dark plasmon mode suppresses the radiative efficiency.  |
2013 |
N. Boechler; J. K. Eliason; Anshuman Kumar; A. A. Maznev; K. A. Nelson; Nicholas X. Fang Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves Journal Article Phys. Rev. Lett., 111 , pp. 036103, 2013. @article{PhysRevLett.111.036103, title = {Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves}, author = {N. Boechler and J. K. Eliason and Anshuman Kumar and A. A. Maznev and K. A. Nelson and Nicholas X. Fang}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.111.036103}, doi = {10.1103/PhysRevLett.111.036103}, year = {2013}, date = {2013-07-01}, journal = {Phys. Rev. Lett.}, volume = {111}, pages = {036103}, publisher = {American Physical Society}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
Dafei Jin; Anshuman Kumar; Kin Hung Fung; Jun Xu; Nicholas X. Fang Terahertz plasmonics in ferroelectric-gated graphene Journal Article Applied Physics Letters, 102 (20), pp. -, 2013. @article{:/content/aip/journal/apl/102/20/10.1063/1.4807762, title = {Terahertz plasmonics in ferroelectric-gated graphene}, author = {Dafei Jin and Anshuman Kumar and Kin Hung Fung and Jun Xu and Nicholas X. Fang}, url = {http://scitation.aip.org/content/aip/journal/apl/102/20/10.1063/1.4807762}, doi = {http://dx.doi.org/10.1063/1.4807762}, year = {2013}, date = {2013-01-01}, journal = {Applied Physics Letters}, volume = {102}, number = {20}, pages = {-}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
2011 |
Anshuman Kumar; Matt Klug; Jin-hong Choi; Jin Wang; Nicholas X. Fang Blueprint of A Defect Tolerant Waveguide Isolator based on Unidirectional Surface Waves Conference Frontiers in Optics 2011/Laser Science XXVII, Optical Society of America Optical Society of America, 2011. @conference{Kumar:11, title = {Blueprint of A Defect Tolerant Waveguide Isolator based on Unidirectional Surface Waves}, author = {Anshuman Kumar and Matt Klug and Jin-hong Choi and Jin Wang and Nicholas X. Fang}, url = {http://www.opticsinfobase.org/abstract.cfm?URI=FiO-2011-FMG7}, doi = {10.1364/FIO.2011.FMG7}, year = {2011}, date = {2011-01-01}, booktitle = {Frontiers in Optics 2011/Laser Science XXVII}, pages = {FMG7}, publisher = {Optical Society of America}, organization = {Optical Society of America}, abstract = {We propose a scheme for defect tolerant broadband waveguide isolator in microwave & optical frequencies. By restricting bulk mode in the system, we obtain high values of isolation ratio, verified by finite element simulations in the microwave range.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } We propose a scheme for defect tolerant broadband waveguide isolator in microwave & optical frequencies. By restricting bulk mode in the system, we obtain high values of isolation ratio, verified by finite element simulations in the microwave range. |
Anshuman Kumar; Ramesh R. Navan; Ajay Kushwaha; M. Aslam; V. Ramgopal Rao Performance Enhancement of p-type Organic Thin Film Transistors Using Zinc Oxide Nanostructures Journal Article International Journal of Nanoscience, 10 (04n05), pp. 761-764, 2011. @article{doi:10.1142/S0219581X11008800, title = {Performance Enhancement of p-type Organic Thin Film Transistors Using Zinc Oxide Nanostructures}, author = {Anshuman Kumar and Ramesh R. Navan and Ajay Kushwaha and M. Aslam and V. Ramgopal Rao}, url = {http://www.worldscientific.com/doi/abs/10.1142/S0219581X11008800}, doi = {10.1142/S0219581X11008800}, year = {2011}, date = {2011-01-01}, journal = {International Journal of Nanoscience}, volume = {10}, number = {04n05}, pages = {761-764}, keywords = {}, pubstate = {published}, tppubtype = {article} }  |
Himanshu Tyagi; Ajay Kushwaha; Anshuman Kumar; M. Aslam pH-dependent Synthesis of Stabilized Gold Nanoparticles Using Ascorbic Acid Journal Article International Journal of Nanoscience, 10 (04n05), pp. 857-860, 2011. @article{doi:10.1142/S0219581X11009301, title = {pH-dependent Synthesis of Stabilized Gold Nanoparticles Using Ascorbic Acid}, author = {Himanshu Tyagi and Ajay Kushwaha and Anshuman Kumar and M. Aslam}, url = {http://www.worldscientific.com/doi/abs/10.1142/S0219581X11009301}, doi = {10.1142/S0219581X11009301}, year = {2011}, date = {2011-01-01}, journal = {International Journal of Nanoscience}, volume = {10}, number = {04n05}, pages = {857-860}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Laboratory of Optics of Quantum Materials
LOQM@IITB
