1T-TaS₂: A strongly correlated material for tunable nanophotonics
Naik, Gururaj Viveka
Master of Science
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.
strongly-correlated materials; 1T-TaS₂; tunable optical properties