Search

A laser light source for high-resolution near-field optics applications with an output power exceeding 1 mW (104 times the power from previous sources) and small (300 nm square to less than 50 nm square) output beam size is demonstrated. The very-small-aperture laser (VSAL) tremendously expands the range of applications possible with near-field optics and increases the signal-to-noise ratios and data rates obtained in existing applications. As an example, 250-nm-diam marks corresponding to 7.5 Gb/in.2 storage density have been recorded and read back in reflection and transmission on a rewritable phase-change disk at 24 Mb/s with a 250-nm-square aperture VSAL. VSALs potentially enable data storage densities of over 500 Gb/in.2 (up to 100 times today’s magnetic or optical storage densities).

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.

The transmission and photocurrent of a multiple‐quantum‐well optically addressed spatial light modulator are measured to study the screening dynamics of the internal electric field under uniform illumination. The response is modeled with a numerical device simulator which solves the conventional drift‐diffusion transport equations. Evidence is found for bipolar transport, and the photocurrent response at low internal fields is shown to decay due to the transit time of carriers across the quantum well region. For large internal fields the photocurrent response saturates and the corresponding screening rate is estimated using an analytical device model.

Strong permanent gratings (index change Δn∼3×10−3) have been formed in bulk hydrogen treated germano‐silicate glass with 242 nm exposure. Diffraction efficiencies of 6% and initial sensitivity (Δn/incident energy density) of 2×10−5 cm2/J have been obtained. The observed sensitivity is comparable to the values achieved in other holographic storage materials such as LiNbO3. It is estimated that by more uniform hydrogen loading of the sample, diffraction efficiencies of 100% can be obtained in 3 mol % GeO2 samples of only 175 μm thickness. These materials are potentially useful as fast access time (sub‐μs), high capacity (∼0.8 Tbits), robust, inexpensive write‐one‐read‐many storage media, narrow (subangstrom) linewidth optical filters, and other optical components.

We report on the effect of carrier escape time on the performance of semi‐insulating photorefractive self‐electro‐optic effect devices by investigating three samples of Cr‐doped GaAs/AlxGa1−xAs multiple quantum wells of varying barrier thickness and height. Reduction of barrier thickness from 100 to 35 A and Al fraction from 0.42 to 0.29 results in a three orders of magnitude increase in diffraction efficiency at a given voltage. The effect of shorter carrier escape and sweep‐out times on the diffraction efficiency, resolution, and sensitivity of these devices is discussed.

Semi‐insulating multiple quantum well photorefractive devices using GaAs/Al0.29Ga0.71As with an electric field applied perpendicular to the layers are demonstrated. Semi‐insulating behavior is obtained by doping with Cr(1016/cm3) during epitaxial growth of the material. Diffraction efficiencies as high as 3% with an applied voltage of 20 V and microsecond response times are obtained in a 2 μm thick device. These devices are of importance for implementation of fast and sensitive two‐dimensional optical information processing systems at wavelengths compatible with current diode lasers without the spatial‐bandwidth limitations of thick photorefractive materials.

We present results on the operation of a high sensitivity semi‐insulating multiple quantum well device for optical image processing. This device operates in the visible spectrum using II‐VI CdZnTe/ZnTe multiple quantum well structures. Incident light creates charge carriers that screen an applied ac electric field modulating the absorption and refractive index of the structure through the quantum confined Stark effect. In this way, an incident intensity pattern is recorded as an absorption and refractive index variation. The semi‐insulating nature of the material eliminates the need for pixelation. In a wave‐mixing experiment, a peak diffraction efficiency of 0.25% was observed from 2.25 μm active layer of the device. Two‐beam‐coupling gain coefficients of ∼500 cm−1 at wavelengths longer than the exciton absorption peak should be possible.

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.

We report photorefractive two beam coupling gain coefficients as high as Γ=19 cm−1 and net gains of Γ‐α=14 cm−1 in InP:Fe at around 970 nm. This enhancement was achieved by combining band‐edge resonant nonlinearities with a second resonant enhancement derived from the bipolar transport in this material. Measurements of gain as a function of pump intensity, applied field, grating spacing, wavelength, and temperature are presented. No moving gratings were required.

We have for the first time demonstrated two‐beam coupling energy transfer at a wavelength of 1.5 μm. Beam coupling gain coefficients of 0.6 cm−1 have been obtained in vanadium ‐doped CdTe with only 5 mW/cm2 incident intensity. These gain coefficients exceed typical gain coefficients in GaAs at 1.06 μm wavelength by 50%. In preliminary measurements using the moving grating technique, we have measured a gain coefficient of 2.4 cm−1. Through adjustment of the doping level, CdTe:V can be used as a sensitive photorefractive material through the 0.9–1.5 μm spectral range.

We report here for the first time, two‐beam coupling energy transfer using the Franz–Keldysh electrorefractive effect in photorefractive semiconductors. A large beam coupling gain coefficient (Γ=2.8 cm−1) exceeding the absorption coefficient (α=2.0 cm−1) is obtained in undoped semi‐insulating GaAs using the new photorefractive process. The new photorefractive process differs from conventional photorefractivity in that the direction of energy transfer is dictated by the direction of an externally applied electric field. Using a moving grating technique and combining the electrorefractive grating with the conventional electro‐optic grating, a very large gain Γ=16.3 cm−1 (α=3 cm−1, crystal length l=4mm) resulting in net amplification by more than a factor of 200 has been demonstrated.

We show that photorefractive beam coupling can be used for deep level spectroscopy in semi‐insulating GaAs. In four samples cut from different locations in a single boule of GaAs, we find the same degree of compensation as determined from absorption spectra, Hall measurements, and the photorefractive effect. In the most compensated of our samples, the sign of the photorefractive beam coupling changes from electron to hole dominated as the optical wavelength is changed from 1.06 to 1.15 μm. Furthermore, we show that absorption spectra for semi‐insulating GaAs can be used to choose samples for optimal photorefractive behavior at wavelengths of 0.9–1.5 μm.