  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.
