A. M. Fox

Quantum well carrier sweep out: relation to electroabsorption and exciton saturation

The authors studied the effects of changing the barrier design of GaAs-Al/sub x/Ga/sub 1-x/As quantum wells on the electroabsorption, exciton saturation, and carrier sweep-out times. Five samples with x values ranging from 0.2 to 0.4 and barrier thicknesses from 35 to 95 AA were studied. Within this range, the authors find that the electroabsorption is not very sensitive to the barrier thickness, but that the ionization field of the excitons approximately doubles for an increase of x from 0.2 to 0.4. The samples with high, thick barriers have lower internal quantum efficiencies than those with low, thin barriers. It was found that the exciton saturation intensity increases with increasing applied field, and decreasing barrier thickness or height. Time-resolved electroabsorption measurements confirm the variation in sweep-out rates between samples, and indicate that the escape mechanism at low field is probably a thermally-assisted tunneling process. >

Effect of gain saturation on the oscillating modes of optical masers

Because an active maser medium exhibits nonlinear gain saturation, the oscillating modes of an optical maser are expected to be somewhat different from those of the passive resonator. Statz and Tang [5] have obtained some numerical results for an active resonator with a pair of parallel-plane, infinite-strip mirrors. We have reformulated the problem for active resonators with circular mirrors of both parallel-plane and confocal geometries and have obtained numerical results using an iterative method of solution. We find the cardinal features of the active modes, such as mode patterns, diffraction losses, and resonant frequencies, to be essentially the same as those of the passive modes, even for unsaturated gains as high as three and a half dB per pass. The mode that predominates in an active Fabry-Perot resonator is found to be the lowest-order (TEM 00 ) mode. However, the predominating modes in an active confocal resonator are found to depend on the Fresnel number; the larger the Fresnel number, the higher is the mode order. The study includes computations of field distributions, diffraction losses, and phase shifts of the steady-state predominating modes and of their output intensities as functions of unsaturated gain, saturation parameter, mirror transmissivity, scattering loss, and resonator geometry.

Fast escape of photocreated carriers out of shallow quantum wells

We report that at room temperature the field‐induced escape of photogenerated carriers out of shallow GaAs/AlxGa1−xAs multiple quantum wells is as fast as for pure GaAs of the same thickness, if the value of x does not exceed 0.04. Our experimental findings can be explained by assuming that carriers are efficiently scattered into the unconfined barrier states by absorption of a LO phonon, as long as the effective barrier height is less than the LO‐phonon energy. The application of shallow quantum wells with x≤0.04 in self‐electro‐optic effect devices, providing not only strong excitonic electroabsorption but also fast sweep‐out times at small biases, should lead to shorter switching times.

EXCITON SATURATION IN ELECTRICALLY BIASED QUANTUM WELLS

We have measured the heavy hole excitation saturation intensity in GaAs/AlGaAs quantum wells as a function of applied electric field and AlGaAs barrier design. We find that the saturation intensity increased with increasing applied field, and decreasing barrier thickness or height, because of increased carrier sweep‐out rates. Time‐resolved sweep‐out time and temperature‐dependent saturation intensity measurement point out the roles of both thermionic emission and tunneling in the field and barrier‐dependent carrier escape time. By reducing the barrier Al composition from 30 to 20%, we achieved an increase in the saturation intensity by a factor of ∼6.

Computation of optical resonator modes by the method of resonance excitation

A novel computational method is used to calculate the modes of an optical resonator. The method is a computer simulation of the physical experiment of exciting a resonator externally and adjusting its length to resonate the various modes; the output at each resonance is purified by means of mode filters consisting of suitably adjusted resonators in tandem. The method is powerful and useful, and is applicable to resonators of arbitrary configuration. Calculated results for a Fabry-Perot resonator with a Fresnel number of ten are presented.

Role of electrorefraction in quantum‐well Fabry–Perot modulators

The effects of electrorefraction in quantum‐well Fabry–Perot (FP) modulators are discussed theoretically and illustrated experimentally. The large electrorefraction at the zero‐field heavy‐hole exciton is shown to produce strong modulation in FP devices operated at that wavelength. At longer operating wavelengths, the weakening electrorefraction plays a diminishing role. By taking into account the field dependent absorption α(λ,E) and dispersion n(λ,E) of the quantum‐well material, the measured reflectivity spectra are successfully modeled.

Excitons in resonantly coupled quantum wells

We have studied excitonic effects in resonantly coupled GaAs/A1GaAs multiple quantum wells. We use photocurrent spectroscopy to deduce the resonant field from the field-dependent exciton energies, and find an unexpected variation of -10% in the resonant field between different excitonic transitions involving the same coupled electron levels. We construct a variational model of the coupled excitons which explains the results in terms of Coulomb mixing of the delocalised electron states.