Browsing by Author "Naik, Gururaj"
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Item 1T-TaS2 for Nonlinear Applications in Optical Neural Networks(2020-04-17) Wang, Yuning; Naik, GururajArtificial neural networks (ANNs) are a set of algorithms which can realize many different functions, such as patterns recognition, classification and prediction. ANNs has become a well-worthy field of study in multiple industries over the past years. Much effort has been contributed to this area and it turns back with significant values in terms of commerce as well as research. However, nowadays most of the ANNs are completed based on electronic hardware, which have some natural limitations such as computation speed and energy cost. Optical neural networks (ONNs) use photons, instead of electrons to transport and compute information. ONNs can easily realize linear computation just by the interference effect of lights and directly use optical nonlinear behavior to realize the nonlinear computation. Besides, ONNs own faster speed and lower energy cost compared with electronic ANNs. However, most of the ONNs are still in the developing stage and the nonlinear activation function has not been applied to real word application. Although there exist some optical nonlinear phenomena, currently the nonliearities are either too small or too slow. Besides, it is very hard to change the optical properties at room temperature with small stimulus, which limits the application of nonlinear applications. Since most of the existing devices requests strict restrictions, researchers keep exploring new materials for better efficiency and lower cost. Here, we propose 1T-TaS2, a strongly correlated material for nonlinear behavior. The optical properties of this material are very sensitive to the external stimulus, and the response speed of this material is very fast, reaching to nano seconds, which make these materials promising in nonlinear applications in ONNs. 1T-TaS2 is one kind of the Transition Metal Dichalcogenides (TMDs). TMDs are one of the strongly correlated materials involving abundant phases, such as distorted phase, metallic phase, insulating phase, charge density wave (CDW) phase, superconducting phase and topological phase. Due to these different phases, TMDs exhibit possibilities for optical tunabilities. More attractively, 1T-TaS2 shows nearly commensurate charge density wave (NCCDWs) at room temperature, which means that its nonequilibrium states can be tuned under stimulus in a relatively easy way. In this thesis, we show that the refractive index of 1T-TaS2 can be tuned under white light excitation with intensity from 2.5 mW/cm2 to 250 mW/cm2. We also find that this tunability comes from the out-of-plane, not the in-plane, stacking difference of the material. By implementing this tunability, we design three nanophotonic nonlinear devices: reflection-tunable, transmission-tunable and angle-tunable nonlinear device. By changing the parameters of the device structures dynamically, we collect, analyze and summarize the simulation results to finalize the implementation of the most efficient devices that are meeting the expectations. And we find that by giving a specific structure, the optical limiter can offer more than 10% reflection or transmission difference under different incident light intensity. The grating device could have a significant difference on the diffraction angles with a value of 6 degree under different incident light intensities.Item Detectivity vs. Sensitivity: Non-Hermitian Metasurface Sensor(2023-10-06) Li, Zhichao; Naik, GururajDetectivity is the most important performance metric of any sensor. Detectivity is limited by the sensitivity of a sensor when the probe significantly overwhelms the detector noise. But, when the probe is limited in its power budget, as in sensors on mobile platforms, high sensitivity does not guarantee high detectivity. Here, we demonstrate the trade-off between the detectivity and sensitivity of a plasmonic sensor. Plasmonic sensors are popular for their high sensitivity arising from large local field enhancements. However, their Q-factors are bad resulting in low detectivity for limited power-budget situations. This problem can be alleviated by using an array of plasmonic structures forming a quasi bound state-in-the-continuum (BIC). The plasmonic BIC sensors show higher detectivity but smaller sensitivity. We identify the physics behind this trade-off and experimentally demonstrate antimouse-IgG sensing in a gold nano-disk array-based plasmonic BIC sensor. Moreover, we induce a non-Hermitian system to control the Q factor by changing the non-radiative loss with little effect on the near field. Using this non-Hermitian structure we can break the trade-off effectItem Super-Mossian and Super-Anisotropic Materials for Nanophotonics(2023-11-16) Wang, Xielin; Naik, Gururaj; Lou, JunNanophotonics using high-index dielectrics has opened pathways to engineer and control light at the nanoscale with better confinement and practically no absorption loss. However, the absence of a complete library of high-index dielectric materials hinders the understanding of the full potential for dielectric nanophotonics and constrains us to a very limited range of materials. Generally, as the refractive index goes up, materials become lossy, exhibiting a trade-off between the absorption edge and the sub-bandgap refractive index of a semiconductor, popularly known as the Moss rule, which seems to set an upper limit on the refractive index of a dielectric for a given operating wavelength. Here, we develop the recipe to break this index upper bound, looking for super-Mossian materials. We demonstrate super-Mossian nanophotonics for a larger Q-factor and better phase control based on bulk Molybdenum disulfide. We use Rigorous Coupled Wave Analysis (RCWA) to calculate the distribution of different diffraction orders and the results agree well with experiment. Aligned carbon nanotubes (CNTs) make a promising platform for thermal radiation applications due to their broadband IR hyperbolic dispersion and their ability to withstand high temperatures. However, their temperature-dependent optical properties remain to be explored. Previously, the thermal stability of CNTs has been studied in helium and hydrogen atmospheres and vacuum, yet ambient air is yet to be explored. Here, we study optical properties of aligned CNTs at high temperatures in ambient air. We show that these films improve thermal stability when coated with a thin layer of dielectric and exhibit broadband IR hyperbolic dispersion at elevated temperatures.