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Beam scanners are essential components in applications where it is necessary to form and scan an electromagnetic beam. In general, beam scanners are classified into mechanical and non-mechanical styles. Mechanical beam scanners employ a movable antenna. Generally they are heavier than desired and often expensive. Non-mechanical beam scanners are based on phased array antennas and employ phase shifters to individual elements or groups of elements[1]. The phased array approach offers many advantages, but the multiplicity of phase shifters makes such systems costly. Therefore, developing much inexpensive, light and speedy beam scanners will attribute practical value in delivery and reception of electromagnetic waves[2-8]. Recently, one group have developed a non-mechanical beam scanner which forms and steers a beam of microwave or higher frequency radiation using light-modulated photoconducting antennas[4-5,7-8]. Scanners based on this approach promise to be fast and inexpensive.
Control of the electronic properties of photoconductors with light has both a long history [9] and is actively being investigated. Recently, phase control of propagating radiation in dielectric wave guides has been studied [10]. Biased photoconductors were shown to emit wide band, broad but steerable beams extending to THz [11]. Femtosecond laser pulses controlled by spatial light modulators impinging on semiconductor surfaces have been used to produce steerable narrow beams of submillimeter radiation [12].
The goal of this thesis is using a semiconductor or photoconductor wafer in which a spatially varying density of charge carriers has been created by optical injection. The induced charge carriers, electrons and/or holes, alter the dielectric constant of the wafer locally and thereby attenuate and reflect incident electromagnetic radiation. With suitable parameters, unilluminated parts of the semiconductor allow the incident electromagnetic wave to be transmitted. Because the wafer responds rapidly to changes in optical injection, it is possible rapidly change the diffractive conditions and thus rapidly change the beam direction.
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