Ultrafast electron dynamics reveal the high potential of InSe for hot-carrier optoelectronics

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the \l=M_\ valley. Subsequently, the electronic cooling is dictated by Frohlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the \l=M_\ valley could travel through a multilayer flake of InSe with a lateral size of 1 mu m. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.