Transition-metal dichalcogenides (TMD), MoS2, MoSe2, WS2 and WSe2 and post-transition metal chalcogenides (PTMC), InSe and GaSe are materials where a strong covalent bonding of atoms inside individual layers coexists with a weak van der Waals coupling of the consecutive layer of the bulk crystal. Such peculiar bonding makes it possible to isolate mechanically, or to grow epitaxially atomically thin films of such compounds with a precise number of atomic layers. At the same time, van der Waals nature of these crystals coexists with strong hybridizations of – separately – conduction and valence band orbitals in the consecutive monolayers in the film, making the band structure of few-layer atomically thin TMDs and PTMCs sensitive to the number of layers in them. In particular, few-layer films acquire multiple subbands in their electronic spectra, with a strong coupling of inter-subband transitions of carriers (electrons in n-doped and holes in p-doped materials) with out-of-plane polarised photons. Here, we show that, when n- or p-doped, few-layer films of TMDs and PTMCs become absorbers and emitters of infrared (IR) and THz light. Our density functional theory modelling and a specially designed hybrid k·p theory for the monolayers of these materials, combined with the tight-binding model description of the interlayer hopping (HkpTB), predicts that optical activity of few-layer films of these two classes of compounds densely covers the range from IR (1.5 micron) for bilayer films to THz for the films with 3-10 layers. In a way, these thin films are analogous to quantum wells in conventional semiconductors, and, by choosing the number of layers, and/or n- or p-doping in one of TMD and PTMC compounds, one can tune such inter-subband transition energy to the desirable application range, offering a new way how 2D materials can be harnessed for developing new technologies. 

Transition-metal dichalcogenides (TMD), MoS2, MoSe2, WS2 and WSe2 and post-transition metal chalcogenides (PTMC), InSe and GaSe are materials where a strong covalent bonding of atoms inside individual layers coexists with a weak van der Waals coupling of the consecutive layer of the bulk crystal. Such peculiar bonding makes it possible to isolate mechanically, or to grow epitaxially atomically thin films of such compounds with a precise number of atomic layers. At the same time, van der Waals nature of these crystals coexists with strong hybridizations of – separately – conduction and valence band orbitals in the consecutive monolayers in the film, making the band structure of few-layer atomically thin TMDs and PTMCs sensitive to the number of layers in them. In particular, few-layer films acquire multiple subbands in their electronic spectra, with a strong coupling of inter-subband transitions of carriers (electrons in n-doped and holes in p-doped materials) with out-of-plane polarised photons. Here, we show that, when n- or p-doped, few-layer films of TMDs and PTMCs become absorbers and emitters of infrared (IR) and THz light. Our density functional theory modelling and a specially designed hybrid k·p theory for the monolayers of these materials, combined with the tight-binding model description of the interlayer hopping (HkpTB), predicts that optical activity of few-layer films of these two classes of compounds densely covers the range from IR (1.5 micron) for bilayer films to THz for the films with 3-10 layers. In a way, these thin films are analogous to quantum wells in conventional semiconductors, and, by choosing the number of layers, and/or n- or p-doping in one of TMD and PTMC compounds, one can tune such inter-subband transition energy to the desirable application range, offering a new way how 2D materials can be harnessed for developing new technologies.