The grating coupling technique is used to determine the index of refraction under an applied electric field. Light is coupled into a GaAs/AlxGa1−xAs multiple quantum well slab waveguide using a conducting grating which has been etched onto the waveguide. The coupling angle is measured with high precision and the effective index of the mode is calculated with the mode coupling equation. The technique is very sensitive, allowing the index to be determined to within ±2×10−5. The strong absorption of the quantum wells prevents measurements at photon energies near the quantum well absorption peaks. This is not a serious limitation as the measurable range is the most technologically important region for electro-optic devices which utilize refractive index changes. The linear and quadratic electro-optic coefficients, evaluated from the change in index with electric field, agree well with previous measurements by other methods.

We have determined the index of refraction of AlxGa−xAs over the wavelength range 0.76–1.15 μm, and the composition range 0≤x<0.33, using a grating to couple light into waveguides. We find the mode indices of multimode slab waveguides from the coupling angle and grating period, then calculate the bulk indices of the core and cladding materials by a root searching technique using the analytical formula for the effective index of a guided mode. The method gives the core index within ±0.001, and the cladding index within ±0.01. We are in agreement with high precision index values for GaAs in the literature, confirming the method. We are in substantial agreement with literature values for AlGaAs but find a significant systematic difference in the composition dependence. An analysis of measurement uncertainties shows that the determination of composition is the dominant variable.

We report the first GaAs/AlxGa1-xAs superlattice waveguide absorption modulators operating at approximately 860 nm which utilize the Wannier-Stark effect. The n = -1 Stark ladder peak is used, which is the transition from the valence band well to the nearest neighbor conduction band well. This peak shifts rapidly with applied electric field, resulting in drive voltages lower than can be achieved using the quantum-confined Stark effect for quantum-well waveguides of similar structure. For a 1000-mu-m long waveguide at 867 nm, we obtain an extinction ratio of approximately 20 dB and a 4 dB attenuation with a drive voltage of 2 V.

Impurity-free interdiffusion has been used to fabricate single mode quantum well ridge lasers from the same (Al)GaAs epitaxial material which differ in emission wavelength by as much as 11.7 nm. This represents a shift of approximately 80 laser linewidths, as measured under pulsed conditions. Threshold currents for the interdiffused and nondiffused lasers are nearly identical. However, the differential quantum efficiencies of the most interdiffused lasers are a factor of 2 lower than nondiffused lasers. The interdiffusion coefficients for the quantum well laser structure are approximately a factor of 6 smaller than those observed for intrinsic GaAs multiple quantum wells.

Signatures associated with electronic states confined in a parabolic quantum well in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructure have been observed in its piezomodulated reflectivity spectrum. The spectra also exhibit electronic transitions with states confined to the spin-orbit-split valence band as the initial states. A comparison of the relative intensity of 11H and 11L signatures in the piezomodulated and those in the photomodulated reflectivity spectrum emphasizes the contribution of the strain dependence of the energies of the confined states.

Intersubband transitions induced in a simple Faraday transmission geometry by metallic grating couplers have been studied in several GaAs/Al0.3Ga0.7As MQW samples lightly doped with donors and with well widths between 210 Å and 320 Å. A sensitive pumping and probing technique was employed in which chopped visible pump light and spatially modulated infrared radiation are transmitted simultaneously through the MQW. The excess free electron density in the well created by < 100 microW/cm2 of red light is estimated to be ≅ 109cm−2 per well. The measured E1 – E0 energies are in good agreement with a simple ID model calculatio found that the grating coupling efficiency drops from 15% to 5% when the ratio of the transition wavelength to the grating period is increased from 1.5 to 3.6.

Measurements of intersubband absorption and cyclotron resonance on lightly-doped GaAs/AlGaAs quantum-well structures with a sensitive optical pumping technique show that the observed intersubband transitions are due to free electrons and that the large photo-generated excess free electron density is due to compensation by acceptors.

The number of usable devices that can be obtained from an epitaxially-grown structure is often dependent on the uniformity of the growth on the wafer. Thus the spatial uniformity of an epitaxial growth and its relationship to growth conditions have been issues of continuing interest. Here, an apparatus for measuring the photoluminescence over an entire wafer is described. A specific example, the use of this data to relate indium coverage on the back surface of MBE-grown material to the properties of the epitaxial layers, is then discussed. A clear correlation between the details of the indium coverage and the local growth rate on the front surface of the wafer is found. >

Beryllium acceptors doped in the centers of GaAs/AlGaAs quantum wells with widths between 300 angstrom and 100 angstrom, as well as in a "bulk" epitaxial layer of GaAs for comparison have been studied by far infrared magnetospectroscopy. Results clearly show the effects of confinement on the acceptor, and the observed increase in transition energy is in qualitiative agreement with recent calculations. For the bulk acceptor the observed splittings in a magnetic field cannot be explained with calculated g-values.

Far infrared transmission has been used to study beryllium acceptors in AlGaAsGaAs quantum wells with widths between 200A and 100A. A bulk sample of GaAs was also studied and the binding energy was found to be 28.5meV. Our results clearly show the effects of confinement on the acceptor by an increase in binding energy in qualitative agreement with recent calculations.

We present a magneto-optical study of p-type GaAs/AlxGa1−xAs quantum wells doped with Be acceptors over the central one-third of the GaAs layers. Using magneto-reflectance and magneto-luminescence spectroscopies, we have investigated (a) interband Landau transitions and (b) transitions from the conduction band Landau levels to the Be acceptors. Binding energies of the acceptors were determined and the dependence of the impurity ground state energy on magnetic field has been studied.

Far infrared transmission has been used to study beryllium acceptors doped in the center of GaAs/AlGaAs quantum wells and bulk GaAs. From the infrared measurements a binding energy of 28.5meV was inferred, in agreement with photoluminescence measurements. The effects of confinement are evident in shifts to progressively higher frequencies of the dominant transitions as the well width is decreased from 200Å to 100Å. The magnetic field dependence of the observed transitions has been investigated between 0 and 9.0T at temperatures between 4.2 and 30K for the bulk sample and 200Å well width sample. The shifts of the transitions with magnetic field are much smaller than predicted by recent calculations for confined acceptors.