Figure 2: Investigating the origin of the upconversion process.
From: Enabling valley selective exciton scattering in monolayer WSe2 through upconversion
Sample temperature T=4 K. (a) Left panel: contour plot of the upconversion PL intensity for A:2s and B:1s as a function of excitation energy, with a clear resonance at 1.723 eV, the A:1s exciton transition energy. Right panel: The resonance in upconversion PLE has a FWHM of only 4 meV. (b) We show power dependence of the upconversion PL intensity at resonance and compare with power dependence of the neutral exciton excited at the A:2s state in standard PL. Slopes in the range of 10-100 μW are for A:2s=1.1, B:1s=1.25 and A:1s=0.56 (c) Scenario for upconversion PL based on 2-photon absorption aided by a real intermediate state is presented. (d) Evolution of upconversion PL of A:2s for T=4 K up to room temperature; the excitation laser energy is indicated following the shift of the A:1s exciton resonance with temperature.
