Two-photon absorption in an indirect-gap semiconductor quantum well systems 2. Excitonic transitions

A. R. Hassan

Ferney J. Rodríguez, David Guzmán, Luis Eduardo Martínez Salazar +1 more

Alfredo Pasquarello, A. Quattropani

Vladimir S. Krivobok, С. Н. Николаев, Victor S. Bagaev +2 more

It is predicted that superradiant states can be formed in a degenerate exciton gas in a semiconductor with an indirect fundamental absorption edge. The superradiance results from four-particle recombination processes and occurs at photon energies approximately twice as high as the band gap energy. Experimental results supporting the possibility of the observation of superradiance from SiGe/Si quantum wells are presented.

V. A. Kochelap, A. V. Kuznetsov

Within the framework of a simple numerical model, we investigate the optical properties of a laser-excited direct-band-gap semiconductor (CdS) in the case when both free and bound electron-hole pairs are present. At low temperatures the ionization of excitons proceeds via a first-order phase transition, which is closely related to the Mott transition, and we argue that in direct-band-gap semiconductors it is not suppressed by the electron-hole liquid condensation, as it is in indirect-band-gap materials. The existence of the two corresponding ionization phases with different optical properties is shown to result in optical bistability (OB), which is quite similar to the increasing-absorption (resonatorless) OB but has some peculiarities of its own. Our model also predicts another optical manifestation of this ionization phase transition---the possibility of absorption oscillations at resonant excitation of excitons.

D. S. Chemla