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PICOSECOND HIGH-VOLTAGE PULSES GENERATION USING LOW-TEMPERATURE (LT) GaAs T. MOTET, J. NEES, S. WILLIAMSON AND G. MOUROU University of Michigan, Center for Ultrafast Optical Science, Ann Arbor, MI.

ABSTRACT We report on the generation of 825 V electrical pulses with 1.4 ps rise time and 4 ps FullWidth-at-Half-Maximum using pulse biased Low-Temperature-grown GaAs photoconductive switch triggered by submicrojoule, 150 fs laser pulses. Dependence of the temporal pulse shape on both the electric field and the optical energy is observed and discussed. Introduction 1

In recent years many applications have emerged for high-voltage, high-speed switching. Using photoconductive techniques both kilovolt switching and picosecond pulse generation have been achieved. High-voltage switching has been reported using high resistivity materials and high power lasers with large-dimension structures to achieve kilovolt switching. This resulted in pulses with duration down to 70 ps and rise time no less than 10 ps. 2 Conversely, high-speed switching has used short optical pulses with fast recovery time materials, small dimension structures and low power lasers to achieve generation of subpicosecond pulses with amplitude up to 6 V. 3, 4 To this day the competing needs of having a large gap to hold off high voltage and a small gap to maintain high speed have hitherto left kilovolt amplitude and single picosecond pulse generation decoupled. The breakdown voltage or maximum bias voltage that can be applied to a photoconductive switch is determined by the dimension accross which the bias is applied and the resistivity of the semiconductor material. This breakdown voltage may be increased by limiting the duration of the bias voltage. The fraction of bias voltage effectively switched depends on the energy of the optical pulse. The rise time of the electrical signal is determined by the duration of the optical pulse and by the switch bandwidth, which is a function of the switch dimensions. The pulse duration is limited by the carrier lifetime. Ultimately the geometry of the switch determines the maximum possible bias and the minimum rise time. With conventional high resistivity GaAs or Si, the electric field hold-off is around 104 V/cm. Therefore, the small dimensions necessary for picosecond signal generation limit the applied voltage to few hundred volts. However, Low-Temperature-MBE-grown GaAs (LT3 GaAs) has recently demonstrated extremely high resistivity and breakdown threshold. Furthermore, due to its subpicosecond carrier lifetime LT-GaAs satisfies the conditions for picosecond pulse generation. Using this material and the technique of pulse biasing, we have been able to apply up to 1.3 kilovolt to a 100 gxm switch making possible the generation of picosecond high-voltage pulses. Exnerimental configuration Efficient switching of kilovolt-level bias voltages requires an optical energy at the microjoule level, which necessitates the use of amplified laser pulses. Our laser (similar to that described in references 5 and 6) generates micro