Carrier~activated nght modulation

Recent reports have highlighted the potential importance of intersubband transitions between the two lowest conduction subbands ( C 1, C 2) of a semiconductor superlatdce (SL) in optoelectronic applications. ! -3 Two factors are of key significance: intersubband transitions are spectrally tunable and exceptionally strong. The VB (valence band) -C 1 band gap and the C 1 -> C 2 subband gap are determined independently by choice of supedattice period and aHoy composition. The absorption coefficient and change in refractive index associated with the C 1 -C 2 transitions depend directly on the number of electrons in the C 1 subband and may be adjusted to exceed the values associated with the fundamental absorption of a direct gap bulk semiconductor. These considerations also apply to silicon-based SL's, although the effects are a factor of 10 smaller. The carrier dependence of the intersubband optical properties is the basis of a novel class of carrier-activated light modulators. A light beam tuned to the miniband gap energy propagates through an undoped supcrlattice without appreciable attenuation since there are no carriers in the C 1 subband. If, however, the beam is polarized perpendicular to the superlattice planes (the z direction) and electrons are electrically injected into or optically generated within the superlattice, the beam can be modulated by the induced intersubband absorptive and/or refractive effects. In view of the polarization restriction, carrier-activated modulation is most simply realized in integrated optical configurations where the light signals are guided by planar waveguide channels aligned paranel to the planes of the superlattice. One particularly promising configuration, which would be usefUl in communications and computer applications, is that of a crossed waveguide switch wherein the cross-channel coupling is controlled by carrier-activated index changes at the intersection of waveguides. ,5 A second promising configuration, which would be useful in signal processing applications, is that of a one-dimensional spatial modulator wherein a planar beam is diffracted by an induced spatially varying carrier distribution within the planar channe1. Picosecond response of these modulators is expected if the electrons are electrically injected normal to the supedattice planes by resonant tunneling. The response of optically activated devices would be limited to the nanosecond range by carrier recombination, but the independent tunability ofthe VB -> C 1 band gap makes possible three-dimensional stacking of switching structures in which the signal at a given level is controlled by