Angle-tunable intersubband photoabsorption and enhanced photobleaching in twisted bilayer graphene

Eva A A Pogna,Driele Von Dreifus,Xianchong Miao,Giulio Cerullo,Cristian Manzoni,Marcus V O Moutinho,Thonimar V Alencar,Pedro Venezuela,Ana Maria De Paula,Minbiao Ji

doi:10.1007/s12274-021-3288-0

Abstract

Van der Waals heterostructures obtained by artificially stacking two-dimensional crystals represent the frontier of material engineering, demonstrating properties superior to those of the starting materials. Fine control of the interlayer twist angle has opened new possibilities for tailoring the optoelectronic properties of these heterostructures. Twisted bilayer graphene with a strong interlayer coupling is a prototype of twisted heterostructure inheriting the intriguing electronic properties of graphene. Understanding the effects of the twist angle on its out-of-equilibrium optical properties is crucial for devising optoelectronic applications. With this aim, we here combine excitation-resolved hot photoluminescence with femtosecond transient absorption microscopy. The hot charge carrier distribution induced by photo-excitation results in peaked absorption bleaching and photo-induced absorption bands, both with pronounced twist angle dependence. Theoretical simulations of the electronic band structure and of the joint density of states enable to assign these bands to the blocking of interband transitions at the van Hove singularities and to photo-activated intersubband transitions. The tens of picoseconds relaxation dynamics of the observed bands is attributed to the angle-dependence of electron and phonon heat capacities of twisted bilayer graphene.

Highlights

Van der Waals heterostructures composed of stacked singlelayer crystals represent a promising platform for novel electronic, photonic and spintronic devices [1,2,3,4,5,6,7,8]
The rotational misalignment of twisted bilayer graphene enables to explore new properties as topological valley transport [13], superconductivity [14] at the magic twist angle θ ~ 1.1°, or the topologically protected one-dimensional (1D) chiral states appearing at the moiré domain walls [15]
The Dirac cones associated to the two layers in twisted bilayer graphene (tBLG) are no longer aligned in the energy-momentum landscape and two saddle points appear at their low-energy intersections

Summary

Introduction

Van der Waals heterostructures composed of stacked singlelayer crystals represent a promising platform for novel electronic, photonic and spintronic devices [1,2,3,4,5,6,7,8]. We studied the out-of-equilibrium optical properties of tBLG as a function of the twist angle by femtosecond TA microscopy with high sensitivity, which allows to explore low excitation regimes in which the angle-dependent contribution from ISB transitions emerges.

Results

Conclusion