Research

M.Sc. Project : I explored the nonlinear rotation of the polarization of light propagating in a 2D system. I used a density matrix framework to perturbatively calculate the induced polarization density, up to first and second order in the electric field strength. Using this I calculated the linear and non-linear optical conductivity. These conductivities go into Maxwell’s equations as material parameter inputs, for calculating the reflection and transmission coefficients of the optical beam interacting with the 2D material. I found that finite off-diagonal terms in the optical conductivity matrix induce Faraday and Kerr rotations in the reflected or transmitted optical beams .

Resonant bulk photogalvanic effect in centrosymmetric metals: My research works focus on nonlinear transport and optical phenomena in Topological materials. Currently, I am calculating third-order susceptibility in centrosymmetric systems. In centrosymmetric systems, the second-order photogalvanic responses vanish due to symmetry constraints which makes the third-order response the leading-order photogalvanic effect. We present a complete formalism for third-order susceptibility in optical limit for metallic or semi-metallic systems where a finite Fermi surface is present. There is no prior research in the literature on these kinds of responses. In addition to the third-order Jerk, shift, and injection current contributions that are expected in insulators as Fermi sea contributions, we also predict a novel sort of current contributions. In centrosymmetric semi-metallic systems where the second-order photogalvanic effects are identically zero and finite Fermi surface is present, our study provides a new perspective on the photogalvanic responses. The new current contributions can be seen in the following figure.

Fig: Schematic diagram of all the current contributions in third-order photogalvanic effect.

Quantum Geometry Induced Third Order Nonlinear Transport Responses: Nonlinear transport phenomena offer an exciting probe into the band geometry and symmetry properties of a system. While most studies on nonlinear transport have looked at second-order nonreciprocal responses in noncentrosymmetric systems, the reciprocal third-order effects dominant in centrosymmetric systems remain largely uncharted. Here, we uncover two significant contributions to the third-order charge conductivity: one affecting longitudinal resistance and another impacting the Hall effect. We demonstrate that these previously unexplored contributions arise in time-reversal symmetry-broken systems from the band geometric quantities such as the Berry curvature and the symplectic connection. We prescribe a detailed symmetry dictionary to facilitate the discovery of these fundamental transport coefficients. Additionally, we unify our quantum kinetic results with the semiclassical wave-packet formalism to unveil all contributions to the third-order charge transport. We illustrate our results in antiferromagnetic monolayer SrMnBi2. Our comprehensive study significantly advances the fundamental understanding of reciprocal nonlinear responses.

Band geometry induced electro-optic effect and polarization rotation: Electric field-induced modulation of the optical properties is crucial for amplitude and phase modulators used in photonic devices. Here, we present a comprehensive study of the band geometry-induced electro-optic effect, specifically focusing on the Fermi surface and disorder-induced contributions. These contributions are crucial for metallic and semimetallic systems and for optical frequencies comparable to or smaller than the scattering rates. We highlight the importance of the quantum metric and metric connection in generating the phenomenon in parity-time reversal (PT) symmetric systems such as CuMnAs. Our findings establish the electro-optic effect as a novel tool to probe band geometric effects and open new avenues to design electrically controlled efficient amplitude and phase modulators for photonic applications.

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