Valley-Selective Polarization in Twisted Bilayer Graphene Controlled by a Counter-Rotating Bicircular Laser Field

The electron valley pseudospin in two-dimensional hexagonal materials is a crucial degree of freedom for achieving their potential application in valleytronic devices. Here, bringing valleytronics to layered van der Waals materials, we theoretically investigate lightwave-controlled valley-selective excitation in twisted bilayer graphene (tBLG) with a large twist angle. It is demonstrated that the counter-rotating bicircular light field, consisting of a fundamental circularly-polarized pulse and its counter-rotating second harmonic, can manipulate the sub-cycle valley transport dynamics by controlling the relative phase between two colors. In comparison with monolayer graphene, the unique interlayer coupling of tBLG renders its valley selectivity highly sensitive to duration, leading to a noticeable valley asymmetry that is excited by single-cycle pulses. We also describe the distinct signatures of the valley pseudospin change in terms of observing the valley-selective circularly-polarized high-harmonic generation. The results show that the valley pseudospin dynamics can still leave visible fingerprints in the modulation of harmonic signals with a two-color relative phase. This work could assist experimental researchers in selecting the appropriate protocols and parameters to obtain ideal control and characterization of valley polarization in tBLG.

Automated summary

In twisted bilayer graphene, the sub-cycle valley transport dynamics can be controlled by a counter-rotating bicircular light field, which consists of a fundamental circularly-polarized pulse and its counter-rotating second harmonic. The unique interlayer coupling of twisted bilayer graphene makes its valley selectivity highly sensitive to the pulse duration and leads to noticeable valley asymmetry. The manipulation of valley pseudospin dynamics can be observed through the valley-selective circularly-polarized high-harmonic generation and leave visible fingerprints in the modulation of harmonic signals.