This thesis presents theoretical studies of the nonlinear optical properties of p-type semiconductors. Chapter 2 is concerned with the intensity dependence of the complex dielectric constant of p-type germanium for light with a wavelength in the 9-11 µm region. The nonlinear absorption is described by the imaginary part of the complex dielectric constant, and the nonlinear dispersive properties are described by the intensity dependence of the real part of the dielectric constant. Chapter 3 deals with the saturation characteristics of practically all Groups IV and III-V p-type semiconductors and includes a discussion of the systematic dependence of the saturation intensity on the material parameters. Chapters 4 and 5 are concerned with several "pump-and-probe" experiments. Here, the transmission of a low-intensity light beam (probe) can be altered by the presence of a high-intensity laser (pump). In these chapters the modulation of the probe transmission is analyzed as a function of the intensity of the pump laser. Chapter 6 treats the intensity dependence of the conductivity of p-Ge for light with a wavelength of 10.6 µm.
In Chapter 2, we present a theory of the saturation of heavy- to light-hole band transitions in p-type germanium by high-intensity light with a wavelength near 10 µm. The free-hole distribution function is modified by the high-intensity light, which leads to an intensity dependence in the absorption coefficient and the index of refraction. The absorption coefficient is found to decrease with intensity in a manner closely approximated by an inhomogeneously broadened two-level model. For temperatures and hole concentrations where hole-phonon dominates hole-impurity and hole-hole scattering, the saturation intensity is independent of the hole concentration. For larger hole densities, the saturation intensity is found to increase monotonically with increasing hole concentration. We calculate the saturation intensity as a function of excitation wavelength and temperature for p-Ge. The saturation intensity is found to increase with increasing photon energy and temperature. The calculated results for the absorptionsaturation are compared with the available experimental data and good agreement is found. In addition to the nonlinear absorption, there exist laser-induced changes in the index of refraction resulting from the saturation of the intervalence-band transitions. Calculations of the intensity dependence of the real part of the dielectric constant are performed for room temperature and for light with a wavelength of 10.6 µm. The index of refraction is found to increase monotonically with increasing intensity.
In Chapter 3, we present the results of the theory describing the saturation behavior of most p-type semiconductors with the diamond or zincblende crystal structure by high-intensity CO2 light. For materials with large spin-orbit splittings as compared to the excitation wavelength (as for Ge), the dominant absorption mechanism is direct intervalence-band transitions where a free hole in the heavy-hole band absorbs a photon and makes a transition to the light-hole band. For materials with small spin-orbit splittings as compared to the excitation wavelength (as for Si), direct intervalence-band transitions are allowed between the heavy-hole and light-hole, heavy-hole and split-off, and light-hole and split-off hole bands. In each material, values of the saturation intensity are reported as a function of the photon energy and temperature.
In Chapter 4, we present a theory to describe the enhanced transmission of a weak tunable probe laser with a wavelength near 3 µm in the presence of a high-intensity saturating beam with a wavelength near 10 µm in p-Ge. The mechanism responsible for the increasing transmission of the probe laser is the depletion of holes in the heavy-hole band by the saturating beam. Room temperature values of the absorption coefficient of the probe are predicted as a function of the intensity of the pump beam.
In Chapter 5, we present a theory of the absorption lineshape of a low-intensity probe laser which is tuned in the vicinity of a high-intensity pump laser with a wavelength of 10.6 µm.Values for the absorption coefficient of the probe are calculated at room temperature as a function of the intensity of the pump laser. We find the probe absorption can be divided into two contributions: one being due to the depletion of holes in the resonant region of the heavy-hole band by the saturable pump, and the other being due to a coupling of the pump and probe beams which allow the pump photons to be scattered into the probe and vice versa. The calculated results for the composite lineshape of the probe are compared with the experimental data and good agreement is found.
In Chapter 6, we show how the modification of the free hole distribution function by the saturating beam leads to a change in the conductivity of p-Ge. The photoconductive response is calculated as a function of the doping level, temperature and light intensity.
