Femtosecond Study of the Interplay between Excitons, Trions, and Carriers in (Cd,Mn)Te Quantum Wells

We study the absorption by neutral excitons and positively charged excitons (trions) following a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the gas of free holes is created by the formation of trions. The evolution of these populations is described, including spin flip and trion formation. We evaluate the contributions of phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blocked.

Figure 1

Optical transitions for pump and probe pulses.

Figure 2

(a) Optical density for an empty QW for negative delay ($-8\mathrm{ps}$, solid line) and very short positive delay ($0.5\mathrm{ps}$, dashed line), for copolarized beams; (b) neutral exciton oscillator strength vs delay. Open circles denote copolarized beams, closed circles cross polarized.

Figure 3

(a) Optical density for a QW with hole density $p\approx 2.5\times {10}^{10}{\mathrm{cm}}^{-2}$, at negative delay ($-9\mathrm{ps}$, solid line) and short positive delays ($0.5\mathrm{ps}$, dashed line), for a copolarized pump pulse tuned to $X^{+}$. (b) Evolution of the $X$ and $X^{+}$ oscillator strength with pump pulse tuned to $X^{+}$. (c) Evolution of $X$ and $X^{+}$ with pump pulse tuned to $X$. (d) Evolution of the $X$ oscillator strength for different hole densities, with the pump beam tuned to the $X$ line.