Photorefractive Quantum Wells

Photorefractive quantum wells fully exploit the resonance represented by the excitonic absorption edge in semiconductors. The absorption coefficient for photon energies that generate bound excitons is approximately 104 cm-1. Interaction lengths for materials designed to operate at these wavelengths are therefore approximately 10-4 cm, or one micron. Such short lengths are compatible with epitaxial growth technology, such as molecular beam epitaxy (MBE), which can produce specifically engineered semiconductor materials, such as quantum wells and superlattices. Semiconductor quantum wells have enhanced optical resonances associated with quantum-confined excitons. The electroabsorption properties of quantum-confined excitons are also enhanced, making them attractive candidates for photorefractive applications.

Photorefractive quantum wells (PRQW) were first developed as an outgrowth of ultrafast photoconductor technology which used semiinsulating quantum wells for electroabsorption sampling. The semiinsulating properties coexisted with the enhanced electroabsorption properties of the quantum well, providing a material with both high trap concentrations and strong electro-optic effects. These conditions are also compatible with photorefractive effects, which were first demonstrated in an AlGaAs multiple quantum well structure [1].

Three PRQW geometries are classified according to the direction of the beam propagation (or grating vector) and the direction of the applied electric field. In terms of the grating geometry, there are two transmission grating geometries and one reflection grating geometry. In terms of the field geometry, there are two longitudinal-field geometries that use the quantum-confined Stark effect (QCSE) and one transverse-field geometry that uses the Franz-Keldysh effect.

[1] D. D. Nolte, D. H. Olson, G. E. Doran, W. H. Knox and A. M. Glass, J. Opt. Soc. Am. B7, 2217 (1990)

See Also:

Asymmetric Fabry Perot structure (pdf)
p-i-n Photorefractive Diode structure (pdf)

This figure shows the three different photorefractive quantum well geometries. They are categorized by the direction of the applied electric field, and by the direction of the hologram writing beams.