A multitude of existing technologies are based on the ability of converting light into electrical signals. Graphene has demonstrated a number of optical and transport properties which are promising for this type of optoelectronic applications and great efforts have been devoted to the development of graphene-based photodetectors. Being a gapless semiconductor, graphene enables light absorption over a wide energy spectrum, spanning from the ultraviolet to the far infrared. Moreover, its absorption is wavelength-independent and its optical properties are tunable via electrostatic doping. Finally, it displays low dissipation rates and high carrier mobility and it enables electromagnetic-energy confinement to extremely small volumes. However, graphene devices on standard SiO2-substrates display properties that are far inferior to that of the suspended graphene. This motivates the research for dielectrics that allow substrate-supported geometry while retaining the intrinsic quality of graphene. The photodetection efficiency is ultimately defined by the magnitude and the speed of the photoresponse of the detecting-material.
When light is absorbed by graphene, it creates a hot electrons distribution whose relaxation to equilibrium is ruled by electron-electron, electron-phonon and phonon-phonon interactions. According to supercollision theory, in SiO2-supported graphene, the defect-assisted scattering with acoustic phonons plays a major role in accelerating the cooling dynamics of electrons. The excess energy of photoexcited electrons is generally released within few ps. In order to increase the photo-excited carrier lifetime, Van der Waals heterostructures with graphene encapsulated by different layered isomorph materials have been proposed. In particular, hexagonal boron nitride (hBN) has demonstrated successful in decreasing defects density and doping, improving graphene’s electrical properties without any loss of functionality with respect to SiO2 substrates.
Here, we present an experimental study of the photoresponse of encapsulated graphene by high sensitivity ultrafast transient absorption experiments, in transmission and reflection geometry. We investigate the effect of the encapsulant material on the hot-electron cooling by comparing SiO2-supported graphene with graphene encapsulated into hBN and multilayer MoS2. In order to unveil the dominating cooling mechanism, we discuss the dependence of the relaxation dynamics on the lattice temperature and on the initial hot-electron temperature, which is tuned by changing the excitation power.