Towards Bose-Einstein Condensation of Semiconductor Excitons: The Biexciton Polarization Effect

We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells.

Figure 1

(top) Degree of circular polarized biexciton luminescence $|P|$ calculated within the seven site model. (a)–(c) Calculated PL spectra for different points in the temperature-density space (see color plot above). (d) Polarized PL spectrum for $T=0\mathrm{K}$ corresponding to the BEC case with a total of four spin $+1$ and two spin $-1$ excitons (density of 0.86 excitons per site).

Figure 2

(a) Transient, polarization-resolved PL spectra of a 10 nm ZnSe quantum well after resonant pulsed laser excitation. The early spectra exhibit negatively polarized biexciton emission consistently evidencing the biexciton polarization effect. After 120 ps, exciton spin polarization has disappeared (with equal ${\sigma }^{+}$- and ${\sigma }^{-}$-PL intensity). (b) Polarization-resolved PL spectra for increasing lattice temperatures. The absolute degree of biexciton polarization decreases from 5% at 10 and 30 K to 0% at 50 K.