Millimeter-Wave Diffraction by a Photo-Induced Plasma Grating

Optical gratings are used extensively for beamsteering in the visible and IR range of the spectrum. Change in the dielectric permittivity of a semiconductor medium resulting from the excitation of a nonequilibrium electron-hole plasma makes it possible to extend this technique to MMW frequencies. A photo-induced plasma grating (PIPG) can be easily rewritten by changing the illumination pattern, so this technique can be used in an optically controllable MMW antennas. Initial experimental work studied the diffraction of MMW propagating along a dielectric waveguide containing a PIPG [1]-[41. This paper reports on the diffraction of MMW propagating in free space, steered by the PIPG. I. SEMICONDUCTOR MATERIAL HIGH RESISTIVITY ( p > lo3 ohm . cm) silicon ingot A was grown by the float-zone technique. Its long carrier lifetime (T > s) enabled it to obtain a high electron-hole plasma density at moderate illumination levels. A 75"diameter, 1.9-mm-thick slab was cut from the ingot. To decrease surface recombination, both of the flat sides of the slab were finished by chemical-mechanical polishing. 11. EXPERIMENTAL SETUP To produce a nonequilibrium plasma, we illuminated the slab with a pulsed xenon lamp through a grating mask (Fig. l), fabricated by printing opaque strips on transparent film, and .entirely transparent to MMW. The incident MMW beam was formed by a horn antenna. The combination of the MMW frequency, 92 GH7, the refractive index of silicon, 3.45, and the slab thickness, 1.9 mm, satisfied the conditions for suppressing Fresnel reflection at normal incidence. The MMW beam that passed through the silicon slab was detected by a GaAs Schottky diode coupled with a second horn antenna. The detector was mounted on a rotating arm to measure the angular distribution of the diffracted beam. The pumping light pulses were monitored using a reference photodiode that detected pumping light reflected from the slab. Both the rectified signal Manuscript received January 13, 1995; revised March 20, 1995. This work was supported in part by a contract from ONR (Project Monitor, W. Miceli). V. A. Manasson and L. S. Sadovnik are with Physical Optics Corporation. Torrance, CA 90501 USA. A. Moussessian and D. B. Rutledge are with the California Institute of Technology, Pasadena, CA 9 I 125 USA. IEEE Log Number 9413700. F Gratlng