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The utilization of optical computing will ultimately depend on the development of very efficient optical nonlinear materials that can process optical information rapidly using very low optical switching energy per operation (of the order of 10 to 100 fJ). We will describe both an all-optical and an electro-optical approach to optical bistable devices. In the all-optical approach, the nonlinearities associated with excitons bound to neutral donors in good quality CdS platelets are utilized. Oscillator strengths of about 7 with a consequent radiative decay of 500 ps have been measured. Under ideal conditions it becomes possible to approach the fundamental limits to optical switching energies with this system. Switching energies of the order of 10 femtojoule are predicted. Experimentally, we have demonstrated a 4 pJ optical switching energy, the lowest ever reported for an all-optical bistable device. The switching time was measured to be less than 1 ns and was limited by the response time of our detector . In the electro-optical approach, we use a bistable InGaAsP/InP diode laser amplifier. An optical switching energy of less than 1 femtojoule, the lowest ever reported (about 3000 photons), is sufficient to cause the device to switch. In addition to the low optical switching energy, these nonlinear optical devices also offer the following advantages: low total energy consumption, high gain, room temperature operation and wavelength compatibility with diode laser sources. The switching-on and off occurs in about half a nanosecond. Optical gain has been measured to be 10-20 dB. The gain is a very important characteristic that enables simple cascadability. The observed switching results from a change in the index of refraction of the order of 10-4 Large two-dimensional arrays of these bistable devices would open new possibilities in the area of digital optical signal processing.
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