It has been demonstrated that semiconductor doping superlattices (n-i-p-i structures or δ-doped sawtooth structures) exhibit targe changes in absorption at the band edge upon pumping with relatively small optical intensities. Photogenerated carriers screen the built-in fields resulting in changes in the absorption coefficient via the Franz-Keldysh effect1 or the quantum confined stark effect.2 Such devices may have application in optical computing. However, they suffer from a serious flaw. Since the modulation occurs via changes in absorption the modulated (probe) beam must be weak, or it will produce carriers itself which screen the fields and hence lock the device in one state, disallowing modulation. Therefore gain is not possible, and hence the device may not function as a logic device. Additionally, since the probe beam affects the state of the device there is no input-output isolation, a fundamental requirement for logic devices.
© 1990 Optical Society of AmericaPDF Article