Hybrid $mathbf{kcdot p}$-tight-binding model for intersubband optics in atomically thin InSe films

We propose atomic films of n-doped $gamma$-InSe as a platform for intersubband optics in the infrared (IR) and far infrared (FIR) range, coupled to out-of-plane polarized light. Depending on the film thickness (number of layers) of the InSe film these transitions span from $sim 0.7$ eV for bilayer to $sim 0.05$ eV for 15-layer InSe. We use a hybrid $mathbf{k} cdot mathbf{p}$ theory and tight-binding model, fully parametrized using density functional theory, to predict their oscillator strengths and thermal linewidths at room temperature.