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Acta Physica Sinica

A THEORY OF COHERENT PROPAGATION OF LIGHT WAVE IN SEMICONDUCTORS (Ⅱ)

GAN ZI-ZHAO, Guo-Zhen Yang, (1)北京大学物理系; (2)中国科学院物理研究所

Published Jan 1, 1981• 0 citations
Publisher

Abstract

This paper is the second part of a theory of the coherent propagation of light wave in semiconductors. After considering the interaction between electrons, the problem of a semiconductor interacting with light is discussed and the equation of exciton polarization wave excited by light wave is obtained. We point out that the equation can be approximated by a set of linear ones and the exciton polarization wave can be regarded as a Bose field only in the limit of low excitation. In this paper, the problem of coherent excitation of discrete exciton level is also discussed specifically. It is shown that the discrete exciton level system excited coherently is approximately equivalent to a two-level atom system excited by light wave in which the difference of energy level is dependent on the degree of excitation and the density of equivalent "two-level atoms" is determined by features of the exciton state wave function.

DOI
10.7498/aps.30.1041

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This paper is the second part of a theory of the coherent propagation of light wave in semiconductors. After considering the interaction between electrons, the problem of a semiconductor interacting with light is discussed and the equation of exciton polarization wave excited by light wave is obtained. We point out that the equation can be approximated by a set of linear ones and the exciton polarization wave can be regarded as a Bose field only in the limit of low excitation. In this paper, the problem of coherent excitation of discrete exciton level is also discussed specifically. It is shown that the discrete exciton level system excited coherently is approximately equivalent to a two-level atom system excited by light wave in which the difference of energy level is dependent on the degree of excitation and the density of equivalent "two-level atoms" is determined by features of the exciton state wave function.
This paper contains the second part of a theory of the coherent propagation of a light wave in semiconductors. After considering the interaction among the electrons, we discuss the matter of the interaction of a semiconductor with light and derive equations of motion for the polarization wave of the excitons which have been excited by the light. We show how our equations can be approximated by linear ones and how the exciton polarization wave can be regarded as a Bose field in the limit of low excitation. In this paper we also discuss in detail the problem of the coherent excitation of discrete exciton levels. We show that the coherently excited discrete exciton is approximately equivalent to a two-level atom which is excited by a light wave; the atom energy-level spacing is a function of the degree of excitation, and the density of equivalent "two-level atoms" is determined by features of the exciton-state wave function.
This paper is the third part of a theory of the coherent propagation of light in semiconductors. Based on the previous papers (Ⅰ) and (Ⅱ), the Maxwell-Bloch equation describing the coherent propagation of light in semiconductors is derived; the equation is then used to discuss a number of possible effects of coherent propagation in both interband transitions and exciton transitions. In particular, the phenomena of self-induced transparency are analysed, and it is shown that the phenomena may occur in the above two transitions.
In this letter, we show that semiconductor excitons generated by coherent light may become squeezed. This is due to their interactions both with the light and among themselves. Based on a polariton theory and using a secular approximation, analytical expressions for exciton quadrature variances have been derived, which enable us to study easily the dependence of exciton squeezing degree on the pumping light intensity, frequency detuning and strength of exciton-exciton, as well as exciton-photon, interactions.
Summary form only given. Large-radius excitons in semiconductors suffer strong Coulomb interaction on close approach. This is a distinctive difference in comparison with two-level systems in quantum optics. A mean field theory of interacting excitons in semiconductors leads to purely repulsive interaction, and does not account for spin-dependent polarization mixing and the formation of excitonic molecules (biexcitons). We have developed a theory for the nonlinear optical response, which can account for both. The coherent polarization is driven by exciton-exciton correlation, which can be expressed in terms of spin-dependent retarded memory functions. The effective equation is a generalized Gross-Pitaevskii equation.
A formal, regorous theory of Frenkel excitons interacting with an external electromagnetic field is doveloped within the framework of a two-level-atom model in which the dipole approximation is employed in describing the atom-field (taken as a c-number) interaction. The model exciton Hamiltonian is shown to be identical in form with the Heisenberg model Hamiltonian for S=1/2 spins with anisotropic long-range exchange interactions. Instead of treating excitons as a non-ideal Bose gas, atomic polarization operators and level population defference are taken up as two fundamental quantities to describe Frenkel excitons. A pair of equations satisfied by these two quantities are derived. It is shown that Frenkel excitons are generally expressed as quantum-mechanical nonlinear polarization waves, the nonlinearity being characterized by the level population. A discussion is given on the interrelationship between such polarization waves and those derivable from the classical Drude-Lorentz model. With the aid of several approximation procedures similar to those employed in nonlinear optics, a set of equations describing nonlinear exciton-photon coupled modes are derived. Brief discussion is given on the eigenfrequencies of nonlinear Frenkel excitons and nonlinear polaritons. The existence of nonlinear exciton-photon coupled modes whose properties are different from those of polariton modes is suggested.