Nonlinear Optical Properties of Mono and Multilayer MoWSe₂ Alloys

Transition metal dichalcogenide (TMD) alloys provide a stable and reliable platform for broadband tuning of excitonic resonances. Here, the nonlinear optical response of Mo (1 − x) W x Se 2 , focusing in particular on second harmonic generation (SHG) and two‐photon photoluminescence (TP‐PL), is studied. It is found that alloys always display stronger nonlinearities compared to pristine TMDs. In addition, by comparing the resonant energies of SHG and TP‐PL, a non‐monotonic change of the energy difference between the 1s and 2p states of the A exciton, pointing toward the possibility of tuning the exciton binding energy by alloying and material composition, is found. Finally, layer‐dependent SHG and TP‐PL, which show an alternate broken/preserved space inversion symmetry for odd/even number of layers and a transition from indirect to direct bandgap when thinning down the layered samples to the monolayer limit, is reported. This work provides useful insights for a better understanding of the optical and electronic properties of TMD alloys, and thus for their use in future photonic and opto‐electronic devices.