Ultrafast Recovery of Hetero n-i-p-i's by Enhanced In-plane Diffusive Transport

Hetero n-i-p-i's are one of several semiconductor quantum-well devices that use second-order electro-optic effects and that continue to attract interest for possible applications to low-power two-dimensional switching arrays and all-optical spatial light modulators. The turn-on time for a hetero n-i-p-i device is usually determined by transport perpendicular to the quantum wells. More specifically, it is determined by the time required for carriers generated in the quantum wells to escape the wells and to move to screen the built-in electric field, thus shifting the exciton. Consequently, typical turn-on times are of the order of a few ps. By contrast, when used in the conventional single-beam geometry, the recovery (or turn-off) time of hetero n-i-p-i's is determined by the slow recombination of the spatially-separated charges in the doped regions and is typically in the range of µs-ms. If instead, however, we use a two-beam mixing geometry for the device (where gratings are written in the material by the interference of the two beams), then the decay or turn off of the signal is determined by the decay of the gratings by in-plane transport over micron dimensions. Here, we use transient grating techniques to measure the recovery of such photorefractive and photoabsorptive gratings written in all-binary hetero n-i-p-i's. In this geometry, we show that the separation of photo-generated charge actually speeds the recovery by enhancing the effective in-plane ambipolar diffusion coefficient by roughly an order of magnitude (in contrast to the single beam geometry where charge separation elongates the recombination and recovery time).