A new type of optical transition in GaAs quantum wells has been observed. The dipole occurs between two envelope states of the conduction-band electron wave function, and is called a quantum well envelope state transition (QWEST). The QWEST is observed by infrared absorption for two structures with 65-Å-thick- and 82-Å-thick wells. The transitions exhibit resonant energies of 152 and 121 meV respectively, full width at half-maximum linewidths as narrow as 10 meV at room temperature, and an oscillator strength of 12.2. The material is anticipated to have subpicosecond relaxation times and be ideal for low-power optical digital logic.

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High-frequency gratings with rectangular-groove profiles are used to generate high-efficiency beam splitters and beam deflectors. The effects of the grating design parameters, i.e., period, groove depth, duty cycle, number of phase levels, and polarization state (TE and TM) of the incoming signal, are considered. The case of the binary beam splitter grating is analyzed by using rigorous electromagnetic grating analysis. Fabrication techniques are presented in which three different lithographic techniques are considered (optical contact, deep-UV stepper reduction, and electron-beam direct write). Experimental results of 97% efficiency for the beam splitter grating and up to 80% for the beam deflector grating are reported.

Six heretofore unobserved envelope state transitions within the conduction band of an AlAs/GaAs quantum well are reported, two of which are forbidden in a symmetric quantum well. The highest energy transition is resonant with 2.86 μm wavelength light. These resonant energies and absorption strengths agree with predictions based on a many-body theory of electrons in nonparabolic energy bands. A new type of infrared modulator is possible via absorption changes in the ‘‘forbidden’’ transition between the first and the third conduction subbands.