Phys. Rev. Lett. 114, 097403 (2015)

Monolayers of transition metal dichalcogenides, such as WSe2 and MoS2, are potentially ideal materials for nonlinear optics because of their broken inversion symmetry and strong light–matter interaction. Gang Wang and colleagues from Université de Toulouse in France have now demonstrated an enhancement by up to three orders of magnitude of the efficiency of second harmonic generation (SHG) when a monolayer of WSe2 is excited by an electromagnetic wave. The wave needs to be in resonance with the energy of the exciton states of strongly Coulomb-bound electron–hole pairs below the electronic band gap. Wang et al. achieved this by tuning the optical excitation on and off resonance with respect to the ground (1s) and excited (2s, 2p and so on) exciton states. When the excitation laser is tuned in resonance with the 1s exciton states, for which two-photon absorption is parity-forbidden, strong SHG is observed. They attribute this to the unusual combination of electric dipole and magnetic dipole transitions. When the 2s or 2p exciton states are excited resonantly, k-valley coherence and valley polarization in two- and one-photon absorption can be maximized.