Second-order optical response of superconductors induced by supercurrent injection

We develop a theory of the nonlinear optical responses in superconducting systems in the presence of a dc supercurrent. The optical transitions between particle-hole pair bands across the superconducting gap are allowed in clean superconductors as the inversion symmetry breaking by supercurrent. Vertex correction is included in optical conductivity to maintain the $U(1)$ gauge symmetry in the mean-field formalism, which contains the contributions from collective modes. We show two pronounced current-dependent peaks in the second-harmonic generalization ${\sigma }^{(2)}(2\omega ,\omega ,\omega )$ at the gap edge $2\mathrm{\hslash }\omega =2\mathrm{\Delta }$ and $\mathrm{\hslash }\omega =2\mathrm{\Delta }$ and one in the photocurrent effect ${\sigma }^{(2)}(0,\omega ,-\omega )$ at $\mathrm{\hslash }\omega =2\mathrm{\Delta }$, all of which diverge in the clean limit. We demonstrate this in the models of a single-band superconductor with $s$-wave and $d$-wave pairings, and Dirac fermion systems with $s$-wave pairing. Our theory predicts that the current-induced peak in $\text{Im}[{\sigma }^{(2)}(\omega )]$ is proportional to the square of the supercurrent density in the $s$-wave single-band model, with the same order of magnitude as the recent experimental observation of second-harmonic generation in NbN by Nakamura et al. [Phys. Rev. Lett. 125, 097004 (2020)]. Supercurrent induced nonlinear optical spectroscopy provides a valuable toolbox to explore novel superconductors.