Past Events

Abstract

Optical physics of layered transition-metal dichalcogenides (TMDs) have been attracting great interest in recent years. In this seminar, theoretical studies of optical transitions and excitonic effects in monolayer TMDs are presented. Firstly, the one-photon selection rules for excitonic series are discussed. It is found that the entanglement of the exciton envelope function with the pseudo spin texture in massive Dirac fermion model leads to anomalous selection rules for one-photon bright states besides the s-state. Secondly, two-photon absorption in monolayer TMDs is discussed. The valley-dependent two-photon selection rules of exciton states are deduced by not only symmetry arguments but also microscopic approach. By accounting for the effects of multiple bands, the reversal and crossover of circular polarization is displayed for non-interacting case. Excitonic effects in two-photon absorption are studied. Thirdly, based on one- and two-photon transitions, second-harmonic generation (SHG) is discussed. Valley-contrasting SHG selection rules are deduced, it is verified in both symmetry and microscopic views that only cross-circular polarized process is allowed in SHG. In addition, valley current injection by the one- and two-photon interference for different polarization configurations is studied. By considering the interference between s- and p- excitonic continuum states excited by the two light beams, valley current is calculated and its Coulomb enhancement is discussed.

Anyone interested is welcome to attend.