Browsing by Author "Li, Weijian"

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    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.
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    1T-TaS₂: A strongly correlated material for tunable nanophotonics
    (2019-09-24) Li, Weijian; Naik, Gururaj Viveka
    Tunable nanophotonics have been demonstrated for several decades and achieved to practical applications in real world in multiple ways such as imaging, sensing, signal processing, information communication and etc. Many different mechanisms have been involved in such modulating, gate tunable Fermi energy of graphene, MEMS devices, chemical reaction. However the tunabilities of all of mechanisms are either small or slow due to small capacitive gaps and large stimulus. Fortunately materials with especially electric tunable optical properties are supposed to be one of the solution for overcoming this limitation. Hence, tunable optical materials are attracting more interests and under deeply investigations. Strongly correlated materials provides a group of candidates for tunable optical materials in which electronic and phononic structures are strongly sensitive to external environments. Transition Metal Dichalcogenide (TMDs), a prototype of two-dimensional (2D) compound, is one of the well studied strongly correlated materials exhibiting numerous different interesting phases such as superconducting, charge density wave (CDW) and spin liquid from liquid Helium temperature to above room temperature. Some of them even exhibit non-Fermi liquid behaviors raised from strongly interaction among localized sub-shell electrons of transition metals atoms. Because of the diverse phase transitions and non-Fermi liquid properties, TMDs provide possible larger tunabilities of optical properties of the materials compared with normal semiconductors. Although optical properties of materials hugely differ around phase transition point, low temperature makes almost all of them hard be implemented in dynamic world optical applications. CDW is one of the quantum ground states that can happen around room temperature which makes the host materials possible platforms for tunable nanophotonic applications. This quantum phase is a result of strong interaction between electrons and phonons of the materials producing a condensate that rearranges the lattice and produces a nested Fermi surface. Many TMDs support charge density waves such as NbSe₂ and TiSe₂, but 1T-TaS₂ supports CDWs at room temperatures which attracts increasing interests of physicists due to its non-equilibrium state that can be excited by electric field and light. In this thesis, we propose that 1T-TaS₂ is a promising candidate for tunable nanophotonics due to its tunable optical properties in visible by in-plane electric bias and therm-optical effect. We demonstrate that the refractive index can be tuned up to 0.1 in visible at room temperature by both DC and AC in-plane bias and up to 0.4 by white light excitation. By implementing this tunability of optical properties of 1T-TaS₂, we theoretical propose a grating design that shift the first diffraction angle at 516 nm by 15.4° under 2.5 mW/cm2 and 250 mW/cm2 white light excitation. Finally we experimentally vary this application of 1T-TaS₂ by showing up to 1 nm diffraction peak shift at around 558 nm.
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    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.
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    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.
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    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.