Figure 3. Ultrafast MEG-induced biexciton transients before the intra-band relaxation.

Transient E_xx dynamics are shown as a function of Δt for the 1.66E_x pump (a) and for the 3.3E_x pump (b) with various 〈N₀〉. (c) Schematic illustration of the PA caused by the transient Stark shift of the single-exciton absorption (black line) and the corresponding photo-induced biexciton broadening (red line). The gray line indicates the single-exciton absorption without the pump. The differential PA spectra exhibit an energy-shifted broader feature (blue line) due to the MEG-induced exciton scattering compared to the case of no MEG (green line). The spectrally-resolved PA spectra with the probe range from 0.78 to 1.08 eV measured at Δt = 400 fs are shown for the 1.66E_x pump (d) and for the 3.3E_x pump (e) with two different 〈N₀〉. Solid lines represent numerical fits using equation (1). The obtained biexciton Γ for the 1.66E_x pump with 〈N₀〉 = 1.1 and 2.2 are 106 meV and 107 meV, respectively, and those for the 3.3E_x pump with 〈N₀〉 = 1.1 and 2.2 are 118 meV and 123 meV, respectively. (f) Spectrally integrated areas of the broadened biexciton absorption. (g) The biexciton Γ broadening linearly increases with increasing the total number of excitons. The experimentally determined Γ (black filled squares) from the Figs. 3d and e are compared to the equation (2).