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Journal of Applied Physics

Optical phase conjugation in narrow direct-gap semiconductors

P. K. Sen, P. K. Sen

Published May 1, 1988• 2 citations
Publisher

Abstract

Optical phase conjugation processes via degenerate four-wave mixing mechanisms have been investigated analytically in narrow direct-gap semiconductors duly irradiated by low-power near resonant lasers. The role of linear absorption phenomena is introduced intrinsically into the electromagnetic equations through complex first-order optical susceptibility of the crystal. A coherent radiation-exciton interaction model is employed to obtain the intensity-dependent crystal optical susceptibility. The band-to-band optical transitions into the true continuum above the fundamental absorption edge are considered. We have estimated qualitatively the conversion efficiency η for important III-V crystals like InSb, GaSb, and GaAs. The nature of variation of η with respect to detuning as well as excitation intensity are found to be in very good agreement with the experimental observations in ammonia, and in semiconductors like Si, Ge, and Hg1−xCdxTe. The analyses also reveal that large η can be derived by using submicrometer sized crystals. InSb is established as the most promising III-V semiconductor for optical phase conjugation device technology at very low cw resonant laser irradiation.

DOI
10.1063/1.340116

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Optical phase conjugation processes via degenerate four-wave mixing mechanisms have been investigated analytically in narrow direct-gap semiconductors duly irradiated by low-power near resonant lasers. The role of linear absorption phenomena is introduced intrinsically into the electromagnetic equations through complex first-order optical susceptibility of the crystal. A coherent radiation-exciton interaction model is employed to obtain the intensity-dependent crystal optical susceptibility. The band-to-band optical transitions into the true continuum above the fundamental absorption edge are considered. We have estimated qualitatively the conversion efficiency η for important III-V crystals like InSb, GaSb, and GaAs. The nature of variation of η with respect to detuning as well as excitation intensity are found to be in very good agreement with the experimental observations in ammonia, and in semiconductors like Si, Ge, and Hg1−xCdxTe. The analyses also reveal that large η can be derived by using submicrometer sized crystals. InSb is established as the most promising III-V semiconductor for optical phase conjugation device technology at very low cw resonant laser irradiation.
Using a coherent radiation-semiconductor interaction model based upon the time dependent perturbation technique, the occurrence of optical nutation has been analytically investigated in direct-gap semiconductors such as GaAs, GaSb, InAs, and Hg1-xCdxTe, duly irradiated by near resonant laser beams. The present approach is much more simplified and straightforward than the conventional Maxwell-Bloch approach used so far for the study of coherent optical transient effects in molecular and atomic systems. The dispersion and absorption expressed in terms of the real and imaginary parts of the crystal optical susceptibility\chi_{r}and\chi_{i}exhibit temporally damped ringing behavior of Neumann and Bessel types, respectively. These processes have been identified as dispersive and absorptive optical nutation. The theory is developed for arbitrary laser intensity in presence of damping due to nonoptical processes. To understand the mechanisms of dispersive and absorptive optical nutation, we have restricted only to low-power resonant band-to-band transitions neglecting excitonic and damping effects. While studying analytically the transient behavior of\chi_{r}and\chi_{i}for a given pump field strength, it is noticed that the absorptive component exhibit remarkable qualitative agreement with the experimental observations of optical nutation in13CH3F.
Abstract A coherent radiation‐exciton interaction model is employed to study analytically the phenomenon of nonlinear absorption in important III—V semiconductors (viz., GaAs, GaSb, InSb, InAs), the crystals being irradiated by suitable lasers with energies (hω) nearly equal to the band gap energies (hωg). The nonoptical processesare accounted for phenomenologically by introducing a damping parameter into the equation of motion of the probability amplitude of the excited electron—hole pair state. The Wannier‐Mott type of exciton wave function is found to be responsible for the giant nonlinearity of the crystal absorption coefficient in the near resonant optical interband transition regime with h | ω ‐ ωg | of the order of the crystal exciton binding energy. The effect of renormalization of the crystal band gap in the presence of Coulomb interaction and Starkbroadening is also discussed.
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Optical phase conjugation through nearly degenerate four-wave mixing (NDFWM) in semiconductor lasers has attracted considerable attention in recent years since being first demonstrated by Nakajima and Frey[ 1 ]. Such interest arises due to the very high conjugate reflectivity of order 30-40dB which can be achieved owing to the amplifying effect of the semiconductor gain medium and cavity enhancement of mixing process. However, most previous experimental and theoretical work has placed emphasis on the weak signal case, where the injection signal acts only as a small perturbation of free-running laser diode. It is only very recently that a theoretical analysis of NDFWM in a semiconductor laser subject to strong probe injection power has been presented[2].
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.