Optical absorption of gate-controlled twisted bilayer graphene at infrared frequencies

We report an infrared transmission measurement on electrically gated twisted bilayer graphene. The optical absorption spectrum clearly manifests dramatic changes such as the splitting of the interlinear-band absorption step, the shift of the inter-van Hove singularity transition peak, and the emergence of a very strong intravalence (intraconduction) band transition. These anomalous optical behaviors demonstrate consistently that electron band of the TBG undergoes a nonrigid modification against the Fermi level shift by ion-gel gating. We compare it with theoretical band calculation in which an electronic screening and layer-asymmetry are explicitely taken into account. We propose that this band modification may be universal to other twsisted 2d material.