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 magic angle. We show that terahertz radiation results in photoresponses, which are caused by indirect optical transitions (free carrier absorption), direct interband transitions, and optical transitions ...
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 magic angle. We show that terahertz radiation results in photoresponses, which are caused by indirect optical transitions (free carrier absorption), direct interband transitions, and optical transitions between moire subbands. The relative contribution of these absorption channels depends on the Fermi-level position with respect to the multiple moire 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 nonmonotonically on the Fermi-level position, which is controlled by the gate voltage.
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