Picosecond Optoelectronic Modulation of Millimeter-Waves in GaAs Waveguide

Abstract

Optically controlled microwave or millimeter-wave devices have been a topic of great interest recently. Utilizing a laser induced electron-hole plasma in semiconductor waveguide to control the propagation of an RF signal, we have previously demonstrated the switching, gating and phase shifting of millimeter-wave signal in Si-waveguide with picosecond precision [1]. Phase shifts as large as 300°/cm at 94 GHz were observed. In the experiment involving switching and gating of RF waves, millimeter-wave pulses with pulsewidth as short as 1 ns and variable to tens of nanoseconds have also been generated. In these earlier experiments, high resistivity Si was used as the waveguide material. Since the carrier lifetime in pure silicon is in the millisecond range, to generate a short RF pulse, one generally requires two separate laser pulses, one to “turn on” and the other to “turn off” the millimeter-wave signal. Furthermore, the repetition rate of the device is limited by the carrier recombination rate to less than 10 KHz. In this work, we will report on our most recent study of this type of device by using Cr:doped GaAs as the waveguiding medium. Due to rapid carrier recombination, only a single picosecond optical pulse is needed to produce an ultrashort millimeter-wave pulse. This feature has been utilized to construct a high speed millimeter-wave modulator with a repetition rate well in excess of 1 GHz.