Optical transitions and excitonic effects in two-dimensional transition-metal dichalcogenides

Abstract

Optical physics of layeredtransition-metal dichalcogenides(TMDs)hasbeen attracting greatinterest in recent years. In this thesis, optical transitions and excitonic effects in monolayer TMDs arestudied theoretically.Firstly, one-photon selection rules for excitonic seriesare investigated. It is found that the entanglement of the exciton envelope function with the pseudospin texturein massive Dirac fermion modelleads to anomalous selection rulesfor one-photon bright states besides the 𝑠-state.The trigonal warping effect in realistic hexagonal lattices further renders more exciton states bright. The selection rules of intra-excitonic transitions are deduced.Estimationsof one-photon excitonic transition strengths based on different models are carried out. Besides the one-photoncase, two-photon absorption(TPA)in monolayer TMDs isstudiedtheoretically. The valley-dependent two-photon selection rules of excitonicstatesare deduced by not only symmetry argument but also microscopic approach. By accounting for the effectsof multiple bands intwo-photon band-to-band absorption, the reversal and crossover of circular polarizationaredisplayedfor non-interactingcase. Going beyond the independent particle approximation, attractive Coulomb interaction of electron andhole is considered.Two-photon absorptionofboth bound excitons and the unbound excitonic continuumstatesisstudied.The Coulomb enhancementofthe TPA above the band edgeis found to be much significant when comparedwith conventional semiconductors, which gives a qualitative change of physical picture of the crossover of circular polarization discussed in the non-interacting case. Based on the resultsof one-and two-photon transitions, second-harmonic generation (SHG)in monolayer TMDsis investigated. Valley-contrasting SHG selection rules are deduced,and it is verified in both symmetry and microscopic views that onlycross-circular polarized SHG processis allowed when excited by circular polarized light. SHG fromboth non-interacting electronsandexcitonic states are studiedand compared. And the microscopic explanation of linear SHG intensity pattern with respect tocrystal angle is consideredin the viewofoptical coherenceofthe two valleys. In addition, valley current injection by the one-and two-photon interference (“1+2” interference)is studied. Valley current injection rate is calculatedfor different polarization configurations of the two excitation beams. The effect of remote conduction bands insuch current generationisalsodiscussedand its contribution is found to be insignificant. Attractive Coulomb interaction effect of electron and holeinsuch valley current injectionscheme isdiscussed for suchtruly 2D semiconductors. By considering the interference between 𝑠 and 𝑝 unboundexcitonic continuum states excited by the two beamsrepectively, valley currentis calculated and its Coulomb enhancement isdiscussed.