Ultrafast nonlinear optics in semiconductors
The nonlinear optical phenomena which occur in semiconductor materials on a femtosecond to picosecond timescale have recently generated much interest, especially in the field of telecommunications where the development of all-optical switching devices based on semiconductors promises a considerable reduction in the complexity of design coupled with a large increase in the speed of operation. This thesis examines the underlying ultrafast physical processes with the aim of providing a clear understanding of the mechanisms involved. The two main regimes of operation are investigated, namely off-resonance excitation where virtual processes are important and on-resonance excitation where real carriers are photogenerated, and in each case a particular system of interest is studied. For the virtual regime of operation, a recent proposal is examined which suggests the use of bandstructure engineering for a semiconductor quantum well in order to enhance the nonlinear optical response by the introduction of additional resonant transitions between subbands. A number of descriptions of the device are presented, and it is concluded that the technique does not necessarily lead to an improved response. An example of on-resonance phenomena is provided by the modelling of the fast refractive index changes in semiconductor laser amplifiers which have been observed in recent experiments. A simple physical model is developed which predicts the behaviour seen in the experimental observations. The nonlinear optical response of the laser amplifier promises the development of fast all-optical switching based on these devices. The thesis also examines the difficulties associated with describing the interaction of semiconductor material and electromagnetic field, and in particular looks at the formulation of a gauge invariant procedure for calculating values of the susceptibility. The propagation of a light beam along the plane of a semiconductor quantum well is discussed, and the gauge invariance of susceptibility calculations performed in the so called A.p and E.r gauges is explicitly demonstrated. Finally, a brief exploration is undertaken of the effects of bandstructure on the optical response of a semiconductor, and two quantum well models for the calculation of a more realistic bandstructure are presented which employ infinite and finite wells respectively.