Abstract.A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.

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Large improvements are reported in the sensitivity of optically addressed multiple quantum well spatial light modulators. In prior work with these materials the quantum well region has been made semi-insulating. It is shown that this is unnecessary and in fact detrimental to performance. By placing layers containing high trap concentrations at the ends of the structure and leaving the active quantum well layers intrinsic the speed of the device at a given illumination is improved by more than four times, diffraction efficiency is enhanced and spatial resolution is almost the same. © 1995 American Institute of Physics.Recent demonstrations of perpendicular geometry GaAs/ AlGaAs multiple quantum well ͑MQW͒ structures as optically addressed spatial light modulators ͑OASLM͒ have shown that these devices can offer high speed 1 and high resolution. 2 These devices use multiple quantum wells that have been made semi-insulating by ion implantation 2 or chromium doping. 1 An ac electric field is applied perpendicular to the plane of the wells at a rate faster than the dark screening time of the material. The surface of the quantum well is then exposed to a write beam that creates photocarriers which screen the applied field in the illuminated regions. The result is an internal electric field which is modulated in the same pattern as the write beam. The optical properties of the quantum well are then changed through the quantum confined Stark effect, and readout with a beam that is nearly resonant with the heavy hole exciton.All MQW OASLMs to date have had deep traps distributed throughout the MQW region. These ''bulk'' traps have been thought necessary to decrease the dark conductivity of the MQW layers so that screening is largely due to photogenerated, and not background, carriers. In addition, the bulk traps were included to restrict the lateral diffusion of carriers which degrades OASLM resolution. Both these motivations are valid for parallel field geometry MQW OASLMs where photogenerated carriers drift and are trapped within the quantum well layers. 3 This thinking may not, however, apply to the perpendicular geometry for two reasons: First, materials grown by molecular beam epitaxy ͑MBE͒, can have an unintentional doping on the order of 10 14 carriers/cm 3 . This doping concentration is sufficient to screen only about 10% of the high fields applied to these materials in the perpendicular geometry ͑ϳ50 kV/cm͒. Any further screening must be by photocarriers or thermally generated carriers, of which there are few due to the large band gap of GaAs. Thus, background carrier screening may not be a problem in good quality materials.Second, recent work also suggests that bulk traps may not be necessary to maintain OASLM resolution. Theoretical calculations indicate that the screening carriers accumulate in an extremely thin ͑5 nm͒ region at the semiconductorinsulator interface. 4 In addition, experimental measurements of MQW OASLMs indicate that the transit time of the carriers across the quantum well region is ϳ300 ps, 5...

We describe a new class of micro-opto-mechanical chemical sensors: A photonic microharp chemical sensor is an array of closely spaced microbridges, each differing slightly in length and coated with a different sorbent polymer. They are optically interrogated using microcavity interferometry and photothermal actuation, and are coupled directly to an optical fiber. Simultaneous measurements of the fundamental flexural resonant frequency of each microbridge allow the real-time detection and discrimination of a variety of vapor-phase analytes, including DMMP at concentrations as low as 17 ppb.