Nonlinear optical response in multiband Dirac-Electron System

We study the dc photocurrent induced by linearly polarized light in a multiband Dirac-electron system, focusing on the organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$. Utilizing perturbation theory, we predict the dependence of shift current on the frequency of light in photodriven $\alpha$-(BEDT-TTF)$_2$I$_3$. Our findings demonstrate a strong correlation between the frequency of light and both the magnitude and direction of the shift current. Furthermore, we delve into the nonperturbative effects of nonlinear optical responses using Floquet theory and demonstrate how the sign of the optical response changes with increasing light intensity. Our results unveil remarkable optical phenomena in the multiband Dirac-electron system and are anticipated to be observed in future experiments.