Large values of the exponential gain coefficient Γ are obtained (Γ≂8–12 cm−1) when recording with a moving grating in photorefractive BSO crystals (nearly degenerate two‐wave mixing; drift recording mode). The resolution of the Kukhtarev’s equations with a moving grating shows a resonance effect which at the optimum velocity makes the modulation of the photoinduced space charge field Esc higher. An optimum of the grating spacing also exists: Λopt:2π(E0/NA)(μe/eγR)1/2. In such conditions, the space charge field is phase shifted by π/2 with respect to the incident fringe pattern; this allows an efficient beam coupling between the two recording beams. The dependence of the gain Γ versus the incident beam ratio β of the two interfering waves is interpreted by including the second‐order term in the Fourier development of Esc. The conditions allowing one to obtain a reasonable agreement between the theory and experiments are presented and discussed, as well as the adopted values of the crystals’ parameters.

Two‐beam coupling in photorefractive Bi12SiO20 crystals with moving grating: Theory and experiments

Cr2+-doped II–VI materials for lasers and nonlinear optics

Spontaneous, highly periodic, often permanent surface gratings or "ripples" can develop on the surface of almost any solid or liquid material illuminated by a single laser beam of sufficient intensity, under either pulsed or CW conditions. The grating periods are such that the incident laser beam is diffracted into a tangential wave which skims just along or under the illuminated surface. These spontaneously appearing surface ripples are generated by a runaway growth process analogous to stimulated Brillouin or Raman scattering or small-scale self focusing, but having many of the same properties as Wood's anomalies in diffraction gratings. Hence, it seems appropriate to refer to these spontaneous surface structures as "stimulated Wood's anomalies."

Stimulated Wood's anomalies on laser-illuminated surfaces

Metal-semiconductor-metal photodetectors (MSM PDs) with finger spacing and width as small as 25 nm were fabricated on bulk and low-temperature (LT) grown GaAs and crystalline Si using ultra-high-resolution electron-beam lithography. High-speed electrooptic characterization with a 100-fs pulsed laser showed that the fastest MSM PDs had finger spacing and width, full width at half maximum response time, and 3-dB bandwidth, respectively, of 300 nm, 0.87 ps, and 0.51 THz for LT-GaAs; 100 nm, 1.5 ps, and 0.3 THz for bulk GaAs; and 100 nm, 10.7 ps, and 41 GHz for crystalline Si. Monte Carlo simulation was used to understand the impulse response of the MSM PDs and to explore the ultimate speed limitation of transmit-time-limited MSM PDs on GaAs and Si. Factors that are important to detector capacitance were identified using a conformal mapping method. Based on the experimental data, Monte Carlo simulation, and calculation of detector capacitance, scaling rules for achieving high-speed MSM PDs are presented. >

Nanoscale tera-hertz metal-semiconductor-metal photodetectors

Coherent image amplification by two-wave coupling in a crystal of photorefractive BaTiO3 is analyzed. This amplifier is optimized with respect to such operational characteristics as gain versus amplified image quality and space-bandwidth product. Experimental results that demonstrate coherent image amplification of 4000, space-bandwidth product of 1,000,000, and gray-level dynamic range of greater than 100 are presented.

Optimal coherent image amplification by two-wave coupling in photorefractive BaTiO3

The photorefractive effect has been observed for the first time in semi‐insulating InPe:Fe and GaAsCr. These materials are sensitive and versatile recording media for high bit rate parallel optical processing in the 0.8–1.8‐μm spectral region using injection lasers of milliwatt power levels.

Four‐wave mixing in semi‐insulating InP and GaAs using the photorefractive effect

We report the first study of the room‐temperature electroabsorption effects in CdZnTe/ZnTe multiple quantum well structures which exhibit sharp excitonic absorption peaks. The magnitude of the Franz Keldysh and quantum‐confined Stark Effects are found to be comparable to those of III‐V semiconductors. With optimized structures we expect II‐VI semiconductors to be important components for information processing in the visible spectrum.

Electroabsorption in II‐VI multiple quantum wells

Amorphous silicon Schottky barrier photodetectors with internal quantum efficiencies of 36% and sampling oscilloscope limited response times of 40 ps (full width at half‐maximum) have been fabricated. Utilizing ultrathin films of rf glow discharge deposited hydrogenated amorphous silicon, carrier sweep‐out was achieved in a new microstrip transmission line structure. The performance of these devices, for picosecond pulse detection, is now comparable to that of crystalline semiconductor detectors.

High quantum efficiency amorphous silicon photodetectors with picosecond response times

Enhanced optical absorption has been obtained in thin (∼30 nm) layers of hydrogenated amorphous silicon by depositing films on periodically structured substrates. At the appropriate angle of incidence, light is coupled to a surface wave in the abosrbing ‘‘artificial dielectric’’ region formed by the structured interface. This effect is used as the basis of high‐speed metal a‐Si:H metal detectors to overcome the conflicting requirements of high optical absorption and short transit times in thin layers. Peak response as high as 0.16 V is achieved for 50‐pJ incident pulses of picosecond duration. This is comparable to that obtained with much higher mobility crystalline semi‐insulators.

D. H. Olson

Papers

Four‐wave mixing in semi‐insulating InP and GaAs using the photorefractive effect

Electroabsorption in II‐VI multiple quantum wells

High quantum efficiency amorphous silicon photodetectors with picosecond response times

Periodically structured amorphous silicon detectors with improved picosecond responsivity