Grating-coupler-induced collective intersubband transitions in a quasi-two-dimensional electron system are investigated both experimentally and theoretically. Far-infrared transmission experiments are performed on samples containing a quasi-two-dimensional electron gas quantum confined in a parabolic quantum well. For rectangular-shaped grating couplers of different periods we observe a strong dependence of the transmission line shape and peak height on the period of the grating, i.e., on the wave-vector transfer from the diffracted beams to the collective intersubband resonance. It is shown that the line shape transforms with increasing grating period from a Lorentzian into a strongly asymmetric line shape. Theoretically, we treat the problem by using the transfer-matrix method of local optics and apply the modal-expansion method to calculate the influence of the grating. The optically uniaxial quasi-two-dimensional electron gas is described in the long-wavelength limit of the random-phase approximation by a local dielectric tensor, which includes size quantization effects. Our theory reproduces excellently the experimental line shapes. The deformation of the transmission line shapes we explain by the occurrence of both types of Wood's anomalies.

From absorption, emission, luminescence excitation and electron spin orientation studies of undoped GaAs-AlxGa1-xAs superlattices we demonstrate the intrinsic nature of the radiative recombination process. This is in direct contrast to recombination observed in similar purity thick GaAs material. Moreover, our results do not support a recent suggestion that enhanced LO phonon-electron coupling should occur in such superlattice structures.

We have studied room-temperature optical gating of ~5 ps pulses in GaAs/AlAs microresonators with diameters ranging from <0.5 μm to 1.5 μm by using a high-refractive-index lens system having a numerical aperture of 1.3. The larger devices show well-defined transverse modes and waveguide dispersion having fair agreement with calculations. The former effect causes decreased modulation of the probe beam. The smallest-diameter devices (<0.5 μm) show higher modulation, fully recover in <30 ps, and have their peak transmission wavelengths shifted by >500 Å compared to a large ~ 10 μm region.

The newly discovered even-denominator fractional quantum Hall effect at filling factor $\ensuremath{\nu}=\frac{5}{2}$ has been studied at ultralow temperatures. While ${\ensuremath{\rho}}_{\mathrm{xx}}$ is not found to vanish in the temperature range studied, the minimum in ${\ensuremath{\rho}}_{\mathrm{xx}}$ is seen to drop at the lowest temperatures. While this drop is insufficient to determine the energy gap, $\ensuremath{\Delta}$, it may be combined with the temperature dependence of the background resistivity to give a value of $\ensuremath{\Delta}\ensuremath{\sim}26$ mK. Because of the high electron-phonon relaxation rate, ${\ensuremath{\tau}}_{\ensuremath{\epsilon}}^{\ensuremath{-}1}=(2.9\ifmmode\times\else\texttimes\fi{}{10}^{3}){T}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{K}}^{\ensuremath{-}3}$, a minimum electron temperature of 9 mK could be obtained with a residual heat leak of 8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ W. It appears likely that ${\ensuremath{\rho}}_{\mathrm{xx}}$ approaches zero as $T\ensuremath{\rightarrow}0$.

Modulation-doped GaAs heterostructures with low-temperature electron mobilities of 5.0×106 cm2/V s at a two-dimensional electron areal density of 1.6×1011 cm−2 have been made. The mobilities are the highest ever observed in a semiconductor. Multiple quantum wells of GaAs prepared by similar methods showed electron mobilities up to 0.54×106 cm2/V s at an areal density of 5.3×1011 cm−2 per layer, which also exceeds any mobility value previously reported for multiple well structures. The structures were grown by molecular beam epitaxy with an atomic-plane sheet-doping technique.

A 2 × 2 array of individually driven MQW modulators has been fabricated. Because of the large electroabsorption effect in MQWs, good on/off ratios can be achieved in a single pass through a set of 50 MQWs. Each device has an on/off ratio of approximately 1.45 : 1, and the modulator displays rise and fall times of roughly 400 ps.

We have studied the injection of excess charge into the lightly p-doped compensated GaAs semiconductor layer of a metal-insulator-semiconductor heterostructure. Below a few Kelvin, the length over which excess carriers screen an applied voltage step grows dramatically as the step size is decreased. This nonlinear screening reflects the reduced density of states near the Fermi level caused by electron-electron interactions (the Coulomb gap). The nonlinearities persist for minutes or longer, demonstrating the existence of an electron-glass regime.