The strong optical nonlinearity of biexcitons in semiconductors necessitates a nonperturbative calculation of the nonlinear-optical susceptibility of the semiconductor and of the propagation equation of the light wave. Such calculations are possible for all light-beam geometries within a simple model in which excitons and biexcitons are treated quantum mechanically as bosons, with no reference to their internal structure. This simple model can account for most experimental observations, while when dephasing and population effects are considered it predicts the appearance of a dephasing-induced extra resonance analogous to that observed in gases or molecular crystals.
The strong optical nonlinearity of biexcitons in semiconductors necessitates a nonperturbative calculation of the nonlinear-optical susceptibility of the semiconductor and of the propagation equation of the light wave. Such calculations are possible for all light-beam geometries within a simple model in which excitons and biexcitons are treated quantum mechanically as bosons, with no reference to their internal structure. This simple model can account for most experimental observations, while when dephasing and population effects are considered it predicts the appearance of a dephasing-induced extra resonance analogous to that observed in gases or molecular crystals.
The recently reported high excitonic and biexcitonic binding energies in GaAs/GaxAl1-xAs quantum wells and quantum wells wires suggests the possibility of strong optical nonlinearities around the biexcitonic resonance. We calculate using a simple boson model the magnitude of the absorptive and dispersive nonlinearities in GaAs, around the biexciton resonance comparing bulk, quantum well and quantum well wire structures. An assessment of inhomogeneous line broadening due to a Gaussian distribution of quantum well wire radii is also presented.
Turbulence and chaos in a semiconductor crystal? These are just some of the intriguing predictions of nonlinear optics with coherent excitons. In this chapter we discuss some aspects of nonlinear coherent optics with the participation of the excitons, photons, and biexcitons. As in many nonlinear systems, these effects arise because of feedback mechanisms, i.e., the light impinging on the system affects the electronic states, which in turn affect the dielectric constant of the medium through which the light passes. These effects are closely related to the phenomena of self-organization [1-8] and optical bistability [7-10] and self-pulsations and the appearance of chaos [2-6]. The cooperative nonlinear coherent processes in optical systems have recently attracted much attention [7-20]. These investigations have stimulated the development of new mathematical methods [21-31] and have opened up possibilities for many new applications. The theoretical and the experimental investigations in these directions give rise to the theory of solitons [32∧2] and lead to the new developments in the optics of ultrashort pulses [43-58]. Ultrafast phenomena such as self-induced transparency and nutation are related to the propagation of solitons and pulse trains, whereas the transient stages of the ultrashort lightpulse penetration in the media are described by area theorems. These coherent ultrafast phenomena typically take place during intervals of time less than the relaxation time, i.e., femtoseconds to picoseconds in typical solids. Collisions and incoherent scattering processes destroy the coherent evolution of the excited particles. But if coherent macroscopic states of the excitons, photons, and biexcitons have been formed because of spontaneous or induced BEC, in this case one can consider their time evolution over intervals of time much greater than the relaxation time.
The study of the nonlinear optical response of excitons in semiconductors is important for both scientific and technological applications. Although the nonlinear optical properties of bulk semiconductors and heterostructures have been studied for almost three decades, with most of the research focusing on heterostructures, the mechanisms leading to the nonlinearities in these systems are not completely understood. Determining these mechanisms and developing a theoretical picture which describes the most dominant effects remains a challenge and an active area of study. Although optical interactions in semiconductors is a genuine many-body problem, a description of this system similar to the optical Bloch equations used in atomic studies has shown some success, but more work is needed to make this model complete. Experimental work, particularly in the bulk system uncomplicated by the interface disorder present in heterostructures, will provide the guidance to extend this formalism. In this dissertation, the frequency domain resonant nonlinear optical response of excitons in bulk gallium arsenide at low density is studied to gain insight into the dominant mechanisms leading to the response, as well as the dynamics of the nominally three-dimensional excitonic system. Specifically, the nonlinear response was probed with differential transmission (DT) and four-wave mixing (FWM) experiments, which measure to lowest order the third-order susceptibility, $\chi\sp{(3)}.$ Using these various tools, three specific areas were studied and the results will be presented. First, the role of the biexciton, the state of two bound excitons, in determining the bulk nonlinear response was explored. The observation of a red-shifted nonlinear resonance suggests the importance of this state at moderate densities. Second, a new response, characterized by a resonant increase in exciton absorption induced by optical pumping with initial superlinear growth and a transition to behavior described by simple line broadening with increased pump intensity, was studied in detail. A model considering increased polariton absorption with increased exciton-exciton interactions explains these effects. Finally, nonlinear optical spectroscopy was used to study exciton transport, where near-ballistic behavior was revealed. The effect of exciton transport, thermalization and radiative recombination on the nonlinear optical response was explored.
The optical spectra of laser-excited semiconductors change with varying laser intensity. Close to exciton or biexciton resonances these optical nonlinearities are, e.g., large enough to obtain optical bistability. The cause for these large nonlinearities are many-body effects in the system of the electron-hole pair excitation. The use of nonequilibrium Green’s functions, or if a quasi-equilibrium exists of thermal Green’s functions, makes it possible to take radiative and Coulomb interactions consistently into account.
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.
The characteristic peculiarities of the self-reflection of a strong electromagnetic wave in a system of coherent excitons and biexcitons due to the exciton-photon interaction and optical exciton-biexciton conversion in semiconductors were investigated as one of the manifestations of nonlinear optical Stark-effect. It was found that a monotonously decreasing standing wave with an exponential decreasing spatial tail is formed in the semiconductor. Under the action of the field of a strong pulse, an optically homogeneous medium is converted, into the medium with distributed feedback. The appearance of the spatially separated narrow pears of the reflective index, extinction and reflection coefficients is predicted.