Figure 1: Light-polarization-dependent optical Stark effect of excitons in ReS2.
From: Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2
(a,b) Comparison of excitonic optical Stark effect in conventional semiconductors (a) and ReS2 (b). X1 and X2 are the lowest two exciton states and G indicates the ground state. In conventional semiconductors (a), such as GaAs-based quantum wells, X1 and X2 correspond to heavy-hole and light-hole excitons, respectively. Pump photon energy should be smaller than that of the lower-lying X1 transition to avoid real transition (see black arrows). Dashed lines represent the energy levels of unperturbed states. Unlike conventional semiconductors, energy-selective optical Stark effect is possible in ReS2 by choosing proper polarization configurations of pump and probe light (see main text). (c) A schematic illustrating the pump-probe experiment of few-layer ReS2. Bluish-green (yellow) dots are Re (S) atoms. Two electron (−)—hole (+) pairs represent anisotropic excitons, X1 (blue) and X2 (red). The light-polarization-dependent optical Stark effect is measured by varying polarization of the linearly polarized pump (red field) and probe (white field) pulses. Inset: optical image of few-layer ReS2. Yellow thick lines (main panel and inset) indicate the crystal b axis. Red graph shows the AFM height profile of the ReS2 sample along the red dashed line. Scale bar, 15 μm. (d) Polarization-dependent absorption spectra of few-layer ReS2. (e) Corresponding spectral weights of Lorentzian contributions of X1 (blue dots) and X2 (red dots). Yellow line represents the b-axis. Solid fit lines are proportional to cos2(θ−θmax)with offset, where θmax values are polarization angles of the excitons (19° for X1 and 87° for X2).
