optical signal processing and communication.1-3 Various forms of bulk semiconductor bistable devices have been reported.2 Recently, multiple-quantum-well (MQW) structures have been shown to have potential as modulation and switching devices.3-5 We have observed cross modulation of a light beam at wavelength λ1 (below and near the band gap) induced by a second light beam at wavelength λ2 (above the band gap) in bulk GaAs in the presence of an applied electric field. This effect might be used for optical logic functions such as an AND gate. The effect is also expected to exist in other direct band gap III-V compound semiconductors such as InP and perhaps to be much larger in MQW structures. In this Letter we present the first results on cross modulation effects in semi-insulating (SI) GaAs.
In the initial experiments, we have used an undoped SI GaAs sample with optically polished surfaces; a free carrier density of ~108/cm3 is normally assumed for the samples. The samples are ~400 μm thick. A transparent indium tin oxide (InSnO) layer of ~600 Å was sputtered onto the SI GaAs surfaces to form electrodes. Since the resistance of such an InSnO electrode was fairly high (the sheet resistance was ~1000-10,000 Ω/p) and since the high resistance may cause distortion of the electric voltage pulses appearing across the GaAs, we evaporated an Al layer on top of the InSnO layer, leaving just a small window for the light to pass through the transparent InSnO electrode. The basic experimental setup
Fig. 1. Schematic of the cross modulation circuit.
Fig. 2. Wavelength spectrum of the LED in relation to the absorption edge of GaAs in the ideal situation. The LED spectrum was obtained from the RCA data sheet. The GaAs sample behaves as a
short wavelength filter.
Fig. 3. Effect of the optical pulse of 1 kW/cm2 on the LED absorption. The bottom trace is the voltage pulse with a 15-μsec/cm horizontal scale and 150-V/cm vertical scale. The upper trace is the enhanced absorption caused after the optical pulse with a 75%/cm ver
tical scale.
is illustrated in Fig. 1. A beam of light from a LED source (10 mW) centered at λ1 = 0.88 μm is transmitted through the window in the sample. The LED light source has an aperture lens which gives a half angle beam spread of 6°. After transmission through the GaAs, the LED light is focused onto a photodetector. Some part of the LED spectrum fell below the absorption edge (see Fig. 2). A second light beam at a wavelength λ2 = 0.8850 ± 0.01 μm, obtained from the tunable Chromatix CMX-4/IR optical parametric oscillator which is driven by a Chromatix CMX-4 dye laser, is also available. This light is transmitted through the same window as the LED light but at an angle of incidence of 30° to the substrate normal. This radiation source has a maximum intensity of 1 kW/cm2 and 1-μsec pulse duration with λ2 tunable from a visible light wavelength to 2 μm in wavelength. The optical energy per pulse can be adjusted from 10 -8 to 10 -4 J with a pulse-to-pulse energy fluctuation of 20-30%. In addition, a voltage pulse generator with variable pulse width (few tens of microseconds) and height (maximum 400 V) is used to provide the applied voltage to the GaAs sample. The pulsed radiation at λ2 can be triggered in synchronization with the voltage pulse so that the optical pulse can be controlled to occur at certain specific positions in time during the voltage pulse. Triggering for the voltage pulse generator has also been provided so that two consecutive voltage pulses can be applied to the sample with a variable time delay between the two voltage pulses.
When a voltage pulse of few tens of microseconds duration at 400 V (corresponding to an electric field of 10 kV/cm) is applied to the InSnO/Al electrodes in the absence of λ2 radiation, the attenuation of the LED light is increased due to the electroabsorption effect. The measured increase in attenuation is 4-5% at 400 V; it agrees only approximately with other available published electroabsorption data,6 because the LED has an extended spectral width and because the absorption edge of SI GaAs is not very abrupt. The electroabsorption is also in approximate agreement with our own electroabsorption data described elsewhere.7 When the sample is illuminated within the voltage pulse duration by the pulsed radiation at λ2 at an intensity of 1 kW/cm2, the increase in attenuation due to electroabsorption is reduced to zero as shown in Fig. 3 when λ2 is present. In other words, under a large applied electric field, the LED light is transmitted to the detector with no electroabsorption, only when both the radiation λ1 and λ2 are present; this illustrates a cross modulation effect which might serve for AND gate logic functions. This modulation effect is presumed to be due to the photo-current generated by the λ2 pulse which has caused a reduction in voltage as shown in Fig. 3. Such a reduction in voltage caused by a photocurrent has already been reported in MQW structures.4 However, the electrical and optical behavior of SI GaAs is very complex, caused possibly by mechanisms such as the charging of the deep level traps. A number of side effects have been observed by us which may be related to these mechanisms, (a) We have measured an increase of attenuation at λ1 without the pulsed radiation at λ2 as a function of the applied voltage and at a variable time delay from the leading edge of the voltage pulse. The attenuation increased as the delay time is lengthened and saturates after ~25 μsec of delay. When we measured the attenuation of λ1 produced by two consecutive voltage pulses, we observed that the attenuation during the second pulse is affected significantly by the first pulse when the two pulses are close to each other (<5-μsec delay between pulses), but the attenuation during the second pulse is not affected by the first pulse when the separation is more than 25 μsec. It appears that this effect is influenced by the RC time constant of the external pulse
Fig. 4. Enhanced absorption of the LED light after the optical pulse is plotted against the electric field: (a) 600-W/cm2 optical pulse in
circuit which is ~10 μsec. (b) In Fig. 3 the attenuation at λ1 is increased after the λ2 pulse. We have measured the change in attenuation at λ1 due to the applied electric field after the occurrence of the optical radiation pulse of λ2. We have seen that the attenuation at λ1 is increased substantially after theλ2 pulse, beyond what was possible without a radiation pulse at λ2. If a second voltage pulse is applied, the attenuation at λ1 during the second voltage pulse will also increase with the presence of the λ2 radiation pulse during the first voltage pulse when the separation of two voltage pulses is <5 μsec. (c) Moreover, the increase in attenuation at λ1 after the λ2 pulse has a nonlinear dependence on the intensity of the λ2 radiation. Figure 4 shows the attenuation at λ1 as a function of the applied voltage when the intensity of the λ2 radiation is 600 and 8 W/cm2. Thus it appears that carriers generated by the λ2 pulse have introduced additional absorption for λ1· We do not have enough data to suggest the physical mechanism for this increased attenuation. It is too large for free carrier absorption.
To summarize: we have demonstrated a new cross modulation effect in SI GaAs. A number of the side effects have not yet been explained.
This work is supported in part by AFOSR grant 80-0037.
References 1. P. W. Smith, "On the Physical Limits of Digital Optical Switching
and Logic Elements," Bell Syst. Tech. J. 61, 1975 (1982). 2. D. A. B. Miller, S. Des Smith, and C. T. Seaton, "Optical Bistability
in Semiconductors," IEEE J. Quantum Electron. QE-17, 313 (1981).
3. D. S. Chemla, D. A. B. Miller, P. W. Smith, A. C. Gossard, and W. Wiegmann, "Room Temperature Excitonic Nonlinear Absorption and Refraction in GaAs/AlGaAs Multiple Quantum Well Structures," IEEE J. Quantum Electron. QE-20, 265 (1984).
4. D. A. B. Miller, D. S. Chemla, T. C. Daman, A. C. Gossard, W. Wiegmann, and T. M. Wood, "Novel Hybrid Optical Bistable Switch: The Quantum Well Self-Electro Optic Effect Device," Appl. Phys. Lett. 45, 13 (1984).
5. A. Migus, A. Antonetti, D. Hulin, A. Mysyrowicz, H. M. Gibbs, N. Peyghambarian, and J. O. Jewell, "One-Picrosecond Optical NOR Gate at Room Temperature with GaAs-AlGaAs Multiple-Quantum-Well Nonlinear Fabry-Perot Etalon," Appl. Phys. Lett. 46, 70 (1985).
6. G. E. Stilman, C. M. Wolfe, C. O. Bozier, and J. A. Rossi, "Elec-troabsorption in GaAs and its Application to Waveguide Detectors and Modulators," Appl. Phys. Lett. 28, 544 (1976).
7. T. E. Van Eck, L. M. Walpita, W. S. C. Chang, and H. H. Wieder, "Electrorefraction and Electroabsorption in InP and GaAs Near the Bandgap," to be published.
