Abstract

Many microwave systems call for components that can control the amplitude and phase, or reflection and transmission of microwave signals. Especially desirable are components that are simple and inexpensive, and have large area and minimum insertion loss. Semiconductor nipi structures have been used as all-optical spatial light modulators for devices that use one optical beam to control another.¹ Here we show that a nipi can also be used as an optically controlled microwave modulator with many ideal characteristics. The basis for the device is the variation of free carrier density over several orders of magnitude by the application of low intensity light.² In these preliminary measurements, the optical beam illuminates an area on the nipi which is roughly 1 /10 of the cross-section of the microwave beam. Despite the small size of the optical control beam, the transmission of a broadband microwave pulse through a GaAs nipi decreases by 30% when the nipi is excited with above band gap light from a cw argon ion laser operating in the multiline mode. We estimate that the optical excitation creates on the order of 10¹⁷ 10¹⁵ electron-hole pairs per cm³ in the nipi throughout a thickness roughly equal to the absorption depth of the exciting beam (~0.5 μm). The attenuation of the microwave signal is attributed to absorption/reflection by photo-induced carriers. The microwave signals are created and detected with photoconductive antennas in a broad band mm-wave spectrometer described below.

© 1993 Optical Society of America