Giant electro-optic effect in chiral topological semimetal RhSi
We studied the linear electro-optic effect of chiral topological semimetal RhSi which is characterized by high-fold chiral fermions separated in energy space. We identify that the general second order conductivity $σ^{(2)}_{xyz}(ω=ω_{1}+ω_{2};ω_{1},ω_{2})$ includes a real symmetric component and an imaginary antisymmetric component, which are from the inter-band shift and intra-band injection current with frequency $ω$, respectively. The $σ^{(2)}_{xyz}$ is significantly enhanced by the high electron velocity and nontrivial band topology of chiral fermion, and modifies the phase velocity of light wave. We also predict that the electro-optic coefficient $χ_{xyz}^{(2)}(ω;ω,0)$ of chiral crystal RhSi is about 7000 pm/V at photon energy 0.01 eV and 1.1 eV, which is about 200 times that of widely used LiNbO$_{3}$ crystal. The giant electro-optic coefficient renders a relatively low half-wave voltage in order of hundreds volt, and demonstrates potential application as electro-optic crystals for the wavelength in the second telecom window of optical fiber communications.