Gate tunable optical absorption and band structure of twisted bilayer graphene

We report the 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 a nonrigid band structure modification created by ion-gel gating through layer-dependent Coulomb screening. We propose that this screening-driven band modification is a universal phenomenon that persists to other bilayer crystals in general, establishing electrical gating as a versatile technique to engineer band structures and to create different types of optical absorptions that can be exploited in electro-optical device applications.