Nonlinear intensity dependence of photogalvanics and photoconductance induced by terahertz laser radiation in twisted bilayer graphene close to magic angle

TL;DR

This paper investigates the nonlinear intensity dependence of photogalvanic and photoconductive responses in twisted bilayer graphene near the magic angle, revealing complex absorption mechanisms and their dependence on Fermi level tuning.

Contribution

It demonstrates the nonlinear intensity behavior of photoresponses in twisted bilayer graphene and elucidates the role of various optical transitions influenced by gate voltage.

Findings

Photoresponses exhibit oscillations with gate voltage.

Saturation of photoresponses occurs at high intensities.

Different absorption channels have distinct intensity dependencies.

Abstract

We report on the observation of the nonlinear intensity dependence of the bulk photogalvanic current and photoconductivity in the twisted graphene with small twist angles close to the second magical angle. We show that terahertz radiation results in the photoresponses, which is caused by indirect optical transitions (free carrier absorption), direct interband transitions and optical transitions between Moir\'e subbands. The relative contribution of these absorption channels depends on the Fermi level position with respect to the multiple Moir\'e subbands of the twisted graphene. The interplay of these absorption channels results in oscillations of the photoresponses with variation of the gate voltage. We show that the photoresponse saturates at high intensities. For different absorption channels it has different intensity dependencies and saturation intensities. The latter depends non-monotonically on the Fermi level position, which is controlled by the gate voltage.