Macroscopic coherent states of excitons in semiconductors

Hoang Ngoc Cam

B. Fluegel

Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.

G. P. Golubev, V. Dneprovskij, E. V. Zimenko +2 more

Makoto Kuwata‐Gonokami

Abstract This chapter begins with a discussion of excitonic matter in semiconductors. It then describes experimental methods for generating highly degenerate excitonic particles by using energy- and momentum-selective optical excitation processes for biexcitons in CuCl and orthoexcitons in Cu2O. Phase-sensitive interference of particles demonstrates the macroscopic coherence of such ensembles of cold particles prepared in the quantum degenerate regime. This is followed by a discussion of the search for spontaneous Bose–Einstein condensation of excitons in a bulk crystal of Cu2O.

Leonid V. Keldysh

Leonid V. Keldysh