Browsing by Author "Naik, Gururaj V."
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Item 1T-TaS2: A new tunable optical materials platform for nanophotonics application(2022-03-08) Li, Weijian; Naik, Gururaj V.Many body solid-state systems have attracted increasing interest due to their diverse quantum phases and novel physical properties. Such unusual properties allow people to overcome the limitations caused by materials in many nanophotonics applications such as sensing, imaging, virtual reality, optical computing, etc. To date, many new material platforms have been proposed, improving the performance of nanophotonics devices. In this dissertation, I will demonstrate the limitation of the development of nanophotonics, and show the possible revolution raised from a two-dimensional strongly correlated material, 1T-TaS2, one of the many-body solid-state systems which exhibit quantum charge-density-wave (CDW) phases over a large temperature range. First, I will discuss the physics understanding of strongly correlated materials and charge density wave quantum phase. Next, I will present the optical characterization of 1T-TaS2 in its CDW phases under external stimuli, including light, temperature, and in-plane bias. This material exhibits a unitary order change of refractive index under white light illumination, and MHz switching speed at room temperature. Furthermore, I will propose a physics model to understand the mechanism of such tunability. Nevertheless, the tunable optical properties of 1T-TaS2 can be implemented in tunable nanophotonics applications. I will show the theoretical demonstration of some tunable nanophotonic devices by using the 1T-TaS2, and the experimental results of the tunable meta-grating and meta-color-filter. Additionally, I will present the theoretical realization of the correlation behavior of percolation systems by using the renormalization theory.Item Alternative Materials for Harnessing Symmetry and Topology in Thermal Light Sources for Thermophotovoltaics(2020-11-30) Doiron, Chloe; Naik, Gururaj V.Selective thermal emitters emit thermal radiation in a narrow frequency range enabling applications in sensing, waste heat energy conversion, and radiative cooling. Waste heat energy recovery through thermophotovoltaics requires high performance selective thermal emitters. To date, the achieved conversion efficiency values fall well below thermodynamic limits. The primary factors limiting device performance arise from material limitations of commonly used optical materials. In this dissertation, I will demonstrate the need for alternative material platforms and show how these new platforms enable unconventional thermal light sources using the principles of phase, symmetry, and topology. First, I will discuss physical modeling to predict the optical properties of doped semiconductors at high temperatures. This analysis will demonstrate the role loss engineering plays in designing selective thermal emitters. Next, I will present experimental results of loss engineering in hybrid plasmonic-photonic resonators resulting in passive parity-time (PT) symmetry in thermal emission. Using the principles of non-Hermitian physics in such a loss asymmetric system provides a pathway for overcoming the trade-off between spectral linewidth and peak emissivity. Furthermore, controlling the coupling between horizontal and vertical modes in such a hybrid system allows for the observation of higher-order non-Hermitian phenomena. This control permits the creation of exceptional concentric rings and thermal emitters with non-trivial topology. Additionally, I will present an experimental demonstration of iron pyrite (FeS$_2$) as an ultrahigh index dielectric material for mid-infrared metamaterials. Iron pyrite has a very large refractive index, up to 4.4, with an optical band gap close to 1 eV far surpassing performance estimates using the Moss rule's common form. Finally, I will conclude with an experimental demonstration of a hyperbolic metamaterial using aligned films of single-walled carbon nanotubes. The optical anisotropy of the aligned films facilitates the creation of ultra-small thermal emitters with volumes below ~$\frac{\lambda^3}{700}$.Item In-plane electrical bias tunable optical properties of 1T-TaS2(Optical Society of America, 2019) Li, Weijian; Naik, Gururaj V.Electrically tunable optical properties have been demonstrated in many solid-state materials such as semiconductors, transparent conductive oxides and graphene. However, their tunability is limited in the visible range due to the requirement of extremely large charge build-up or high capacitive fields. Here, we propose strongly correlated materials for circumventing such limitations. 1T-TaS2, a strongly correlated material exhibiting charge density order at room temperature, allows tuning of its optical properties with an in-plane electrical bias. The electrical bias causes the charge density waves to slide and thereby alter their coherence and condensation. As a result, the optical conductivity or dielectric function of this layered material changes with an in-plane bias. Here, we report measured anisotropic dielectric functions of mechanically exfoliated thin films of 1T-TaS2 and their electrical tunability. We observe a maximum refractive index change on the order of 0.1 in the visible range with DC and AC in-plane biases.Item Non-Hermitian metasurface with non-trivial topology(De Gruyter, 2022) Yang, Frank; Prasad, Ciril S.; Li, Weijian; Lach, Rosemary; Everitt, Henry O.; Naik, Gururaj V.The synergy between topology and non-Hermiticity in photonics holds immense potential for next-generation optical devices that are robust against defects. However, most demonstrations of non-Hermitian and topological photonics have been limited to super-wavelength scales due to increased radiative losses at the deep-subwavelength scale. By carefully designing radiative losses at the nanoscale, we demonstrate a non-Hermitian plasmonic–dielectric metasurface in the visible with non-trivial topology. The metasurface is based on a fourth order passive parity-time symmetric system. The designed device exhibits an exceptional concentric ring in its momentum space and is described by a Hamiltonian with a non-Hermitian Z 3 ${\mathbb{Z}}_{3}$ topological invariant of V = −1. Fabricated devices are characterized using Fourier-space imaging for single-shot k -space measurements. Our results demonstrate a way to combine topology and non-Hermitian nanophotonics for designing robust devices with novel functionalities.Item Non-Hermitian metasurfaces for the best of plasmonics and dielectrics(Optical Society of America, 2021) Yang, Frank; Hwang, Alexander; Doiron, Chloe; Naik, Gururaj V.Materials and their geometry make up the tools for designing nanophotonic devices. In the past, the real part of the refractive index of materials has remained the focus for designing novel devices. The absorption, or imaginary index, was tolerated as an undesirable effect. However, a clever distribution of imaginary index of materials offers an additional degree of freedom for designing nanophotonic devices. Non-Hermitian optics provides a unique opportunity to take advantage of absorption losses in materials to enable unconventional physical effects. Typically occurring near energy degeneracies called exceptional points, these effects include enhanced sensitivity, unidirectional invisibility, and non-trivial topology. In this work, we leverage plasmonic absorption losses (or imaginary index) as a design parameter for non-Hermitian, passive parity-time symmetric metasurfaces. We show that coupled plasmonic-photonic resonator pairs, possessing a large asymmetry in absorptive losses but balanced radiative losses, exhibit an optical phase transition at an exceptional point and directional scattering. These systems enable new pathways for metasurface design using phase, symmetry, and topology as powerful tools.Item Reorganization of CDW stacking in 1T-TaS2 by an in-plane electrical bias(AIP Publishing, 2021) Li, Weijian; Naik, Gururaj V.1T-TaS2 is a 2D quantum material supporting charge density waves (CDWs) at room temperature. The strong correlations in this material make its electrical properties extremely sensitive to external stimuli such as an electrical bias and illumination. Recently, we demonstrated that the optical properties of this material also considerably change with electrical bias and light. With light, we showed that the CDW domains across layers stack differently and thus result in a unity-order change in the refractive index. Here, we demonstrate that an in-plane electrical bias also changes the CDW stacking in 1T-TaS2. However, the stacking change with electrical bias opposes that with illumination. Our experiments at room temperature suggest that an in-plane electrical bias sets the CDWs sliding and making way for the higher energy stacking configurations to switch to the ground-state stacking. The demonstration here sheds light on the origin of the giant electro-optical effect previously observed in 1T-TaS2 and paves the way for low-power MHz-fast electrically tunable optical devices.Item Temperature-dependent optical properties of titanium nitride(AIP Publishing LLC, 2017) Briggs, Justin A.; Naik, Gururaj V.; Zhao, Yang; Petach, Trevor A.; Sahasrabuddhe, Kunal; Goldhaber-Gordon, David; Melosh, Nicholas A.; Dionne, Jennifer A.The refractory metal titanium nitride is promising for high-temperature nanophotonic and plasmonic applications, but its optical properties have not been studied at temperatures exceeding 400 °C. Here, we perform in-situ high-temperature ellipsometry to quantify the permittivity of TiN films from room temperature to 1258 °C. We find that the material becomes more absorptive at higher temperatures but maintains its metallic character throughout visible and near infrared frequencies. X-ray diffraction, atomic force microscopy, and mass spectrometry confirm that TiN retains its bulk crystal quality and that thermal cycling increases the surface roughness, reduces the lattice constant, and reduces the carbon and oxygen contaminant concentrations. The changes in the optical properties of the material are highly reproducible upon repeated heating and cooling, and the room-temperature properties are fully recoverable after cooling. Using the measured high-temperature permittivity, we compute the emissivity, surface plasmon polariton propagation length, and two localized surface plasmon resonance figures of merit as functions of temperature. Our results indicate that titanium nitride is a viable plasmonic material throughout the full temperature range explored.