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An analytical model that includes lateral drift of photocarriers is developed to explain the experimental resolution of photorefractive multiple quantum well (MQW) devices. The model is in excellent agreement with a phenomenological fit proposed earlier and it follows a small intensity modulation model closely. Charge distributions with and without lateral drift of carriers at the MQW interfaces are assumed in order to calculate the peak diffraction efficiency as a function of grating period. An effective mobility-lifetime product is used to account for the effect of lateral drift in the MQW region. It is shown that good agreement with experiment is obtained when lateral drift is taken into account. Moreover, the model is applied to the study of design tradeoff between resolution, sensitivity, and diffraction efficiency which are all of crucial importance in the performance of practical devices.

Photorefractive multiple quantum well (PRMQW) devices can achieve simultaneously higher resolution and greater sensitivity by distributing several highly trapping low temperature growth (LTG) layers at different depths in their intrinsic region. This article extends a previously derived analytical model of PRMQW devices to distributed photorefractive devices (DPDs) that consist of a cascade of smaller MQW regions sandwiched between LTG layers. This enables an understanding of how multiple trapping layers affect the resolution and sensitivity in these DPDs. In addition to an improvement in resolution, the model predicts an enhancement in diffraction efficiency at small grating spacings as the number of subdevices increases for a fixed total DPD length. This result is of significance in designing compact image processing systems that can operate at small grating periods but still be able to achieve a large signal-to-noise ratio for image processing and sensing applications.

We present results on carrier transport in shallow multiple-quantum-well GaAs-Al/sub x/Ga/sub 1-x/As p-i-n diodes (x=0.02, 0.04, 0.08) at various bias voltages. We show that only carrier drift and enhanced diffusion dominate response times of these devices. We also emphasize that the drift of holes plays different roles in determining the response times: at low bias, the slow drift of holes adds to enhanced diffusion, slowing down the decay-times; at high bias, the drift time of holes can be comparable to the time of electrons and contribute to the rise-times. From picosecond time-resolved pump/probe electroabsorption measurements, we obtain the drift times, effective drift velocities, and effective mobilities of electrons and holes. The effective drift velocities (especially for holes) appear rather insensitive to the Al concentration in the barriers.

The purpose of this paper was to present the management of a rarely encountered transposition anomaly involving the mandibular permanent lateral incisor and the canine. In the literature, the treatment of this type of anomaly involves fixed orthodontic appliances. The treatment of the present case, however, was performed with removable appliances. Transposition of the permanent mandibular right lateral incisor and the permanent mandibular right canine was detected in a 12 year old girl. The primary mandibular right canine was extracted to enable the distal movement of the permanent lateral incisor by the eruption force of the permanent canine. The transposed teeth were then aligned in the arch by using removable appliances. The forms of the teeth were changed with each other with composite restorations.

We demonstrate that a resolution approaching their fundamental drift-free diffraction limit can be achieved in photorefractive multiple quantum well (MQW) devices by proper choice of the growth and anneal conditions. Previously reported devices are compared with four MQW samples grown at low temperature and annealed after growth at 620 °C for over 3 h. A simple analytical model with and without lateral drift of photocarriers is used to explain the observed experimental resolutions. The growth temperature and annealing procedure of the low-temperature-grown MQW are shown to significantly influence the device resolution by reducing lateral drift.

We report a monolithically grown optically addressed multiple-quantum-well spatial light modulator with high-frame rate operation (>44 kHz) and large sensitivity ( 50) can be achieved at the expense of modulation depth. In addition, by using a voltage pulse, we reduce the recovery time of the device from 90 ms to 1 μs such that high-frame rate operation can be achieved. We demonstrate 60% modulation of the reflected readout beam at 44 kHz by only using 43 mW/cm2 cw writing illumination.

We report an optically addressed multiple quantum well spatial light modulator with large modulation gain which can be used in applications such as amplified incoherent-to-coherent conversion. The novel device design utilizes separate detector and modulator regions to obtain large gain. The design also incorporates low-temperature-grown quantum wells in the modulator region to reduce the effects of high-intensity readout. Our preliminary data show that modulation gain of 45 and modulation depth of 40% can be obtained by simply applying a dc bias. However, our data also show that the modulation gain decreases at higher writing frequencies, indicating that refreshing the device is required after each write-read operation.

Using picosecond time‐resolved four‐wave mixing we have investigated the turn‐on and turn‐off mechanisms in a photorefractive multiple quantum well spatial light modulator with Stark geometry. We show that holes contribute to the turn‐on time as a slower rise (∼300 ps) of the diffraction efficiency. Furthermore, the diffraction grating is dominated by either electrons or holes, depending on the applied voltage polarity relative to the diffraction of illumination. The grating decays in 0.4–8.5 ns, depending on the grating spacing. We show that the decay is due to lateral transport of carriers at the multiple quantum well/dielectric interfaces, where low‐temperature grown layers can increase the diffraction efficiency and device resolution.

Semiconductor photorefractive quantum well devices are prime candidates for high speed real-time image processing applications because of their high speed, small thickness and large electro-optic nonlinearities1,2,3. When used in the Stark geometry, where a field is applied perpendicular to MQW layers, the optical nonlinearity arises from longitudinal field screening in the illuminated areas. Lateral spatial modulation of the field screening is translated to lateral spatial modulation of refractive index and absorption by the quantum confined Stark effect. Device performance is characterized by spatial resolution and sensitivity, both of which strongly depend on carrier transport. The spatial resolution decreases with increased lateral carrier transport4,5, while the sensitivity increases with increased longitudinal transport. Highly trapping materials have been successfully used to increase the device resolution down to 5-7 μm6,7.

A tunable 1550 nm vertical cavity surface emitting laser with 2 mW of tunable CW output power and a 50 nm tuning range has been demonstrated. The device is a micro-electromechanical structure where the top curved mirror is displaced by a voltage-induced electrostatic force. The cavity is designed for operation under optical pumping at 980 nm wavelength.