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We describe a 4 by 4 array of embedded control two input, one output optoelectronic switching nodes based on the field effect transistor self electro-optic effect device (FET-SEED) technology. The arrays have electrical fan-out to remove the loss associated with optical fan-out in the system application. Extensive testing was done on several arrays at 156 Mb/s per channel with optical switching energies below 100 fJ, with the output driver limiting the maximum data rate to 400 Mb/s. Power dissipation, noise margin, crosstalk, sensitivity to stray light, and uniformity of both threshold and output waveforms are also discussed. >

Responsivity and thermionic dark current are studied in asymmetric stepped GaAs/AlGaAs multiquantum well infrared detectors with a single bound state. Although attributed to different transport properties, both the photoexcited and the thermally emitted carriers exhibit asymmetric transport properties with respect to the voltage polarity. The experimental characteristics are explained by the minibands-quasi continuum energy structure above the barrier.

The transport of hot electrons in the AlxGa1−xAs barriers above the wells in a multiple quantum well (MQW) structure is investigated. The structures that are studied are asymmetric quantum well infrared detectors. The transport of the hot electrons normal to the layers is strongly dependent on both voltage and well shape. It is suggested that the key parameter which affects the transport properties is the dwell time of the electrons in the continuum, above the well region. This is most readily seen in asymmetric MQW structures, in which the dwell time under an applied bias depends very strongly on bias polarity. Calculations of electron transmission coefficient and dwell time show that the electron mean free path in asymmetric wells is much larger in positive bias than in a negative one. Employing this model, we achieve a very good fit to experimental data.

We present a method for transparent watermarking using a custom bidirectional imaging device. The two innovative concepts of our approach are reflectance coding and multiview imaging. In reflectance coding, information is embedded in the angular space of the bidirectional reflectance distribution function (BRDF) and this information can vary at each surface point. In order to achieve a transparent watermark, reflectance coding is implemented using a spatial variation of the Brewster angle. The novel multiview imaging method measures the reflectance over a range of viewing and illumination angles in order to instantly reveal the unknown Brewster angle. Unlike typical in-lab measurements of the Brewster angle or the refractive index, this method does not require accurate prior knowledge of the surface normal so that imaging in non-lab conditions is feasible. Furthermore, a range of incident angles are examined simultaneously, eliminating the need for scanning incidence angles. The approach is well-suited for transparent watermarking where the observer cannot see the watermark because it is comprised of spatial variations of refractive index. The transparency and angular coding of the watermark has great utility in deterring counterfeit attempts. In this paper, we present the imaging device and demonstrate it's effectiveness in detecting and measuring changes in refractive index. This device acts as the decoder in a transparent watermark system.

Electron-sublevel-anticrossing effects have been studied in coupled quantum wells where the exciton binding energy is comparable to the minimum sublevel splitting. The anticrossing was induced by applying an electric field to align the first and second sublevels of adjacent wells. In this situation the electron-hole Coulomb interaction has a strong effect on the splittings measured by optical techniques, because the optical spectra typically measure exciton energies rather than single-particle energies. The most striking effect is that the minimum splitting of the excitons associated with each of the split electron levels does not occur at the same field as for the minimum splitting of the bare-electron levels. One unexpected but readily observable consequence is that when the same electron-sublevel splitting is measured using two different pairs of intrawell and interwell exciton transitions, the field for minimum exciton splitting can differ by up to \ensuremath{\sim}10% from one pair of transitions to the other. We have constructed a variational model of the coupled excitons that explains these effects in terms of Coulomb mixing of the delocalized electron states. We have measured the exciton splittings directly by photocurrent spectroscopy in three GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well structures. The samples were similar in design except that the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier thickness varied from 15 to 35 \AA{}. By fitting our variational model to the experimental anticrossing data, we have been able to deduce the actual bare-electron level splittings rather than the exciton splittings. Within the experimental accuracy, we find that the minimum splitting decreased exponentially with increasing barrier thickness, as would be expected for simple quantum-mechanical tunneling.

Clear evidence for resonant tunnelling of electrons in a p-i-n quantum well modulator is provided by picosecond pump-and-probe electro-absorption measurements. The temperature-independent escape times show a drastic reduction when an electrical field is applied perpendicular to the wells, with a pronounced minimum at the field corresponding to the resonance between the n=1 electron level in one quantum well, and the n=2 electron level in the adjacent one. The authors' calculated field dependence of the electron tunnelling times proves that this behaviour is the signature of resonant tunnelling.

The input/output ports of Smart Pixels are the optical signal receivers and transmitters. We have tested the simplest, lowest power dissipating, receiver‐transmitter pair circuit with mode‐locked (ML) laser pulses and find switching times as short as 200 ps. We also observe that the product of the set optical energy and switching time is a constant. In addition we present a comparison of the saturation properties of quantum well modulators when operated with short ML pulses, versus bit‐period length pulses (quasi‐continuous wave operation). Although the read pulse energy requirements of smart pixel optical signal transmitters are near saturation conditions, we conclude that with careful design, operation with short pulses, which gives the minimum switching time, should be possible.

We systematically studied microcavity enhancement and mode‐coupling effects in photo‐ and electroluminescence of an AlGaAs/GaAs vertical‐cavity light‐emitting diode (LED) by continuously changing the microcavity resonance with respect to the quantum well band gap. At mode overlap we obtained maximum photo‐ and electroluminescence intensities and a minimum emitted linewidth of 4.6 nm at 836 nm with a FWHM divergence of 62°. However, the electrical‐to‐optical efficiency was less than 1 μW/mA. Application issues for optical interconnects are presented.

Selectively excited room‐temperature photoluminescence spectra of InGaAs‐GaAs‐AlGaAs quantum well heterostructures reveal a broad line at midgap energies originating in the Si doped AlGaAs. When carriers are photoexcited directly in the wells, this line is dramatically enhanced at the expense of the quantum well line, indicating carriers escape from the wells. The broad emission and its enhancement can be explained in terms of recombination between electrons trapped at DX centers in AlGaAs and holes transferred into AlGaAs from the neighboring wells. We also observe the broad emission by direct over‐the‐gap photoexcitation of very highly doped Si:AlGaAs. The doping dependence consistently correlates this line to DX centers.

A new type of asymmetric stepped GaAs/AlGaAs multiquantum well infrared detector is reported. These asymmetric detectors utilize the usual bound‐to‐continuum transition. The current responsivity is remarkably asymmetric with regard to the voltage polarity. In contrast with rectangular wells, in which responsivity is saturated in both bias polarities, these wells exhibit saturation only for negative bias. The responsivity increases monotonously with positive electric field. This is attributed to changes induced by the field on the transport properties of the excited electrons. In particular, the bias affects the dwell time spent by the carrier wave packet in the well region. Employing this model, we achieve a very good fit with experimental data.

The effects of electrorefraction in quantum‐well Fabry–Perot (FP) modulators are discussed theoretically and illustrated experimentally. The large electrorefraction at the zero‐field heavy‐hole exciton is shown to produce strong modulation in FP devices operated at that wavelength. At longer operating wavelengths, the weakening electrorefraction plays a diminishing role. By taking into account the field dependent absorption α(λ,E) and dispersion n(λ,E) of the quantum‐well material, the measured reflectivity spectra are successfully modeled.

We find that GaAs‐AlGaAs quantum wells grown at low temperature (300 °C) by molecular beam epitaxy are nearly semi‐insulating and exhibit a broadened excitonic resonance. Studies of the femtosecond time‐resolved nonlinear optical saturation response and parallel‐field transport properties indicate that a subpicosecond nonradiative decay dominates the carrier recombination of this material. Annealing of low‐temperature‐grown quantum wells causes the arsenic point defects to form arsenic metallic precipitates within the quantum wells and up‐shifts the absorption edge.

Hydrogen passivation of Si δ‐doped GaAs grown by molecular beam epitaxy is studied. Just as in uniformily Si‐doped GaAs, exposure of the δ‐doped material to a low frequency (30 kHz) hydrogen plasma at 250 °C for 30 min deactivates the Si donors in the δ spikes. For samples with Si doping of (1–6)×1018 cm−3, carrier concentration in the spikes decreased by nearly three orders of magnitude following hydrogenation. Secondary‐ion mass spectrometric analysis of deuterated samples confirmed trapping of deuterium in the Si‐doped spikes. Consistent with the deactivation of Si donors following hydrogenation, changes were observed in the near‐band edge luminescence spectrum at 4.2 K, which showed in the hydrogenated sample the absence of Burstein–Moss shift that was observed in the as‐grown sample. This hydrogen‐induced passivation of Si donors in the δ spikes can be of benefit in selectively deactivating donor atoms in device applications, and also provide a method for tailoring the hydrogen distribution in an epitaxial structure.

We have measured the heavy hole excitation saturation intensity in GaAs/AlGaAs quantum wells as a function of applied electric field and AlGaAs barrier design. We find that the saturation intensity increased with increasing applied field, and decreasing barrier thickness or height, because of increased carrier sweep‐out rates. Time‐resolved sweep‐out time and temperature‐dependent saturation intensity measurement point out the roles of both thermionic emission and tunneling in the field and barrier‐dependent carrier escape time. By reducing the barrier Al composition from 30 to 20%, we achieved an increase in the saturation intensity by a factor of ∼6.

We have devised a novel vertical‐cavity top surface‐emitting GaAs quantum well laser structure which operates at 0.84 μm. The laser combines peripheral current injection with efficient heat removal and uses only the epitaxially grown semiconductor layers for the output mirrors. The structure is obtained by a patterned deep H+ implantation and anneal cycle which maintains surface conductivity while burying a high resistance layer. Peripheral injection of current occurs from the metallized contact area into the nonimplanted nonmetallized emission window. For 10‐μm‐diam emitting windows, ∼4 mA thresholds with continuous‐wave (cw) room‐temperature output powers ≳1.5 mW are obtained. Larger diameter emitting windows have maximum cw output powers greater than 3 mW. These are the highest cw powers achieved to date in current injected vertical‐cavity surface‐emitting lasers.

We have fabricated high speed Si metal semiconductor metal photodetector using 19F+ ion implantation in low doped Si. Bandwidths in excess of 6 GHz have been obtained representing more than an order of magnitude improvement over unimplanted counterparts. Measurements using short optical pulses show that the increase in bandwidth is primarily due to shorter carrier lifetime in implanted devices. In the absence of implantation, the response under short optical pulse excitation has a long decay with a time constant of approximately 0.35 ns. An optical fiber transmission experiment using a GaAs ((lambda) approximately 0.85 micrometers ) laser source and the implanted Si photodetector was carried out. Error-free transmission (BER < 10-11) with good receiver sensitivity was obtained at 2 Gb/s. These results demonstrate implanted Si can be used as a detector for short wavelength fiber optic communication systems for speeds up to a few Gb/s. Monolithic integration of this detector technology with conventional Si processing offer the potential for low cost receiver designs.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Arrays of fast smart pixels are the building blocks of high speed, high throughput photonic switching systems and spatial light modulators. Our work describes the operation of smart pixels with picosecond mode-locked laser pulses, and addresses some of the issues in the fast operation of smart pixels: switching energy, switching speed, dynamic range.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Selectively excited photoluminescence in microcavity enhanced light-emitting diodes reveals that the coupling between the optical cavity mode and the quantum well exciton can be controlled by the exciting light itself. Optical pumping can also provide information on the internal electrical fields.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new type of asymmetric stepped GaAs/AlGaAs multi quantum well infrared detectors is reported. These asymmetric detectors utilize the usual bound to continuum transition. The current responsivity is remarkably asymmetric with regard to the voltage polarity. In contrast with rectangular wells, in which responsivity is saturated in both bias polarities, these wells exhibit saturation only for negative bias. The responsivity increases monotonously with positive electric field. The difference between polarities for the noise is much smaller at low temperatures. As a result, the highest D * in positive polarity is much higher than in the negative one. This is attributed to changes induced by the field on the transport properties of the excited electrons. In particular, the bias affects the dwell time spent by the carrier wave packet in the well region. Employing this model, we achieve a very good fit with experimental data. The transport asymmetry is further studied using identical asymmetric wells which were grown in opposite sequences. It is shown that the effect of the asymmetry in the interfaces is of the same order of magnitude as the structural asymmetry.

We report on Molecular Beam Epitaxial growth and properties of strained In0.2Ga0.8As-GaAs quantum well (QW) structures suitable for 980 nm lasers. The QW width was maintained at 80 A and the barrier thickness was varied from 50 A to 300 A. The effects of increasing the barrier width on the structural and optical properties of the QW were examined using double crystal x-ray diffraction rocking scans (DCXRD) and photoluminescence measurements. DCXRD rocking scans revealed satellite peaks from the strained layer quantum wells (SLQW). The linewidth of the peaks decreased as the barrier width was increased. Optical measurements indicate significant improvements in the internal luminescence efficiency in the thick barrier structures. We assign the improvements in the luminescence properties to the reduction of non-radiative centers in the thick barrier structures. The sources of the non-radiative centers are ascribed to structural defects that are generated as a result of strain relaxation in the thin barrier structures. A new broad photoluminescence feature at 0.9 eV was also observed and believed to originate in the AlGaAs:Si cladding region. We shall present these results and discuss the implications of increasing the barrier thickness of In0.2Ga0.8As-GaAs QW on the performance of 980 nm lasers.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Resonant enhancement of tunneling rates of electrons in p-i-n quantum well structures is observed with the technique of picosecond pump-and-probe electroabsorption. Results compare well with recently published time-resolved photoluminescence data. When measuring resonance fields with photocurrent spectroscopy, we find that there is a small, but measurable shift between the “resonance” fields of the two excitonic transitions involving the same hole state (n=1), and the two resonating electron states (n=1 and n=2). We explain these results in terms of electron-hole Coulomb interaction, which must be taken into account in any optical measurement.

Following the demonstration of the absorption reflection modulator [1], many workers [2-10] have used Fabry-Perot resonators to improve the limited contrast ratio of these multiple quantum well light modulators. Most experiments have been in the normally on (Non), spectral region where absorption increases with field. Using field dependent GaAs/AlGaAs quantum well absorption data, we have created a model that calculates the reflectivity and contrast ratio of resonant and non-resonant modulators in both the normally off (Noff), region where absorption decreases with field, and the normally on (Non) spectral regions for use with photonic devices such as the self-electrooptic effect device (SEEDs). The calculations include both electro-absorption and electro-refraction effects.

We have studied excitonic effects in resonantly coupled GaAs/A1GaAs multiple quantum wells. We use photocurrent spectroscopy to deduce the resonant field from the field-dependent exciton energies, and find an unexpected variation of -10% in the resonant field between different excitonic transitions involving the same coupled electron levels. We construct a variational model of the coupled excitons which explains the results in terms of Coulomb mixing of the delocalised electron states.

We examine, experimentally and theoretically, the effects of excitons in optical measurements of resonant coupling of quantum wells. We find that the exciton line splittings do not correspond directly to the underlying electron levels, which leads to differences in the bias fields for resonance of, e.g., \ensuremath{\sim}10%. We construct a variational model of excitons in coupled wells and successfully compare this model with measured splittings near the resonance between the first and second electron levels of adjacent wells, deducing the actual conditions for coupling of the ``bare'' electron levels.

Room-temperature lasing has been achieved in vertical-cavity surface-emitting laser structures for wavelengths as short as 699 nm under pulsed electrical excitation. The molecular beam epitaxially grown structures have p- and n-type semiconductor quarter wave mirror stacks enclosing an active region containing a short period GaAs-AlAs superlattice. >

We have fabricated high-speed Si metal-semiconductor-metal photodetectors using(19) F(+) ion implantation in low-doped Si. Bandwidths in excess of 6 GHz have been obtained that represent more than an order-of-magnitude improvement over unimplanted counterparts. Measurements with short optical pulses show that the increase in bandwidth is due primarily to a shorter carrier lifetime in implanted devices. In the absence of implantation, the response under short optical pulse excitation has a long decay with a time constant of ~0.35 ns. We carried out an optical fiber transmission experiment using a GaAs (lambda ~ 0.85 mum) laser source and the implanted Si photodetector. Error-free transmission (bit error rate10(-11)) with good receiver sensitivity was obtained at 2 Gbits/s. These results demonstrate that implanted Si can be used as a detector for short-wavelength fiber-optic communication systems for speeds to a few gigabits per second. Monolithic integration of this detector technology with conventional Si processing offers the potential for low-cost receiver designs.

We demonstrate the transmission of 1-Gbit/s data patterns through various GaAs/AlGaAs smart pixels by using free-space optics and trains of mode-locked picosecond laser pulses spaced 1 ns apart as optical inputs and outputs.

We extend our recent general discussion of electroabsorption and refraction in multiple-quantum-well modulators to determine the optimum modulator design for smart-pixel applications. In addition to the optimum operating wavelength shift, from that of the zero-voltage exciton, we determine the optimum number of quantum wells, and we calculate the reflectivity change and the contrast ratio obtainable. This analysis is undertaken for both simple detectors and modulators, meaning that they are antireflection coated, as well as for devices that include Fabry-Perot resonators. The optimization is performed on a figure of merit that is inversely proportional to the incident optical read energy required on a device to switch another, downstream device. We maximize the figure of merit to minimize the optical read energy. An interesting result is that there should be no significant improvement in our smart-pixel circuit figure of merit with the use of Fabry-Perot resonant modulators and detectors. Our results are, of course, material-system specific, but for the 850-nm AlGalAs/GaAs quantum-well system the optimum wavelength shift from the exciton location is approximately 6 nm. The general trends and approach are applicable to other material systems.

We demonstrate that GaAs/AlGaAs smart pixels can be operated with picosecond and subpicosecond laser pulses. Switching times as short as 200 ps are measured. Our results confirm the theoretical prediction that input mode-locked pulses are more advantageous than the more typical square-wave pulses. They also suggest that use of picosecond mode-locked pulses for optical output could produce operating speeds in the gigahertz range.

Optically bistable symmetric self-electro-optic-effect devices that use an asymmetric Fabry-Perot resonator are demonstrated and assessed. Contrast ratios exceeding 30 for low bias levels (under 6 V) are measured for low intensities. The Fabry-Perot self-electro-optic-effect device is also shown to saturate readily with intensity, with the contrast ratio decreasing by a factor of 3 for an irradiance of 10 microW/microm(2). Wavelength sensitivity measurements and calculations are also presented.

Highly efficient (1.2 mW/mA, >70% CW differential quantum efficiency), top-surface-emitting, vertical cavity lasers are achieved at room temperature. Buried damage layers by proton implantation are used for efficient current funnelling. The CW threshold currents are 3.5–8.0 mA, at 3.7–4.2 V bias, for 10.30 μm diameter lasers. The lasing wavelengths are 845–848 nm.

Resonant tunneling of electrons in GaAs-AlGaAs quantum wells is resolved by picosecond pump-and-probe electroabsorption measurements. The temperature-independent tunneling escape times are dramatically affected by the applied electric field, with a pronounced minimum at the field corresponding to the resonance between the n=1 electron level in one quantum well, and the n=2 electron level in the adjacent one. The calculated field dependence of the tunneling times is in qualitative agreement with the data.

We investigate the dynamics of non-thermal carrier distributions in undoped and modulation doped GaAs Quantum Well Structures (layer thickness ≈ 100 A) with near bandgap-resonant intense femtosecond light pulses. We are able to study selectively the carrier-carrier scattering process up to very high densities and we carry out the first optical investigation of non-thermal carrier generation in the presence of a thermalized Fermi-sea of electrons. In undoped quantum wells we find a reduction of the thermalization time from about 100 fs to about 30 fs as the photocarrier density increases from N ch ≈ 2 × 10 10 cm −2 to ≈ 10 12 cm −2 . The thermalization time is found to be sensitive to background doping with excess electrons in modulation doped samples.

The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples. A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band. This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results. The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities. >

Experimental and theoretical investigations of the absorption in a single-modulation-doped quantum well (QW) used as conducting channel of a field-effect transistor are presented. By applying a voltage to the gate, the electron concentration can be varied between 0 and approximately 10/sup 12/ cm/sup -2/. The continuous transition can be optically followed from an undoped to a highly doped QW. Effects of band filling are observed, along with renormalized effects at the first subband edge and electrostatic effects at the higher ones. It is shown that optical techniques can give in situ information on the electron density and temperature as well as on the electrostatic fields inside field-effect structures. >

By studying the temperature dependence of absorption and luminescence in modulation doped GaAsAlGaAs quantum wells, we show experimentally that the lowest energy absorption peak has a behavior strikingly different from the corresponding excitonic peak in undoped quantum wells, which is alos unexpected in the frame of the single particle picture. We find good qualitative agreement with the many-body model that attributes this “correlation singularity” to the interaction between the photoexcited hole and the sea of electrons.

Theoretical and experimental results from a new optical logic device are presented which indicate that it has speeds of many GHz, sub 100 femtojoule energies, and is easily fabricatable.

We study the effects of excess thermalized electron and hole populations on femtosecond carrier thermalization in optically excited GaAs quantum wells. In modulation-doped samples we find that the presence of 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ electrons produces an extraordinarily fast carrier thermalization (faster than 10 fs); however, the presence of the same density of holes yields a thermalization time close to that of undoped samples (\ensuremath{\cong}60 fs). These results give the first direct evidence that electron-electron interactions yield the dominant contributions to carrier thermalization in GaAs and also show that the electron distribution function itself influences thermalization dynamics.