Abstract in Englisch:

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₎WₓSe₂, 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.