Transient optical reflectivity in MoSe2 under 785 nm and 810 nm pump excitation

Abstract

Transition-metal dichalcogenide (TMDC) crystals have emerged as a new class of two-dimensional materials. TMDCs exhibit strong Coulomb interaction (E_B=50−500 meV), and at monolayer thicknesses, can exibit exotic valley- and spin- dependent optical selection rules. The TMDC MoSe₂ is semiconducting at room temperature, with a bulk bandgap of ~1.10 eV, and forms ground state A-excitons at its K-point (~1.6 eV). In this report, we present transient reflectivity measurements at 785 nm (1.57 eV) and 810 nm (1.53 eV) optical excitation of bulk MoSe₂. A signal inversion of the optical reflectivity was observed with wavelength. When excited near-resonance, the transient reflectivity shows no dependence on fluence and decay times are observed to be longer than the non-resonant case. When excited non-resonantly, a clear fluence dependence is observed in the excitation and decay dynamics. The difference is attributed to the possible formation of A-excitons.