30 April 2009 Demonstration of enhanced emission and time delay beam steering using photoconductive terahertz source with multiple spot feed
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Excitation of electrically biased photoconductive switches with femtosecond optical pulses is a well established method of generating wide bandwidth (0.5 to 2.5 THz) pulses of terahertz frequency radiation. This method of pulse generation draws energy from the bias power supply to accelerate optically injected charge carriers, giving rise to a current pulse that can radiate into free space; nevertheless, the output power is limited by both poor coupling of the radiation out of the substrate and by charge carrier screening. By optimizing the electrode structures and illumination geometries it may be possible to address both of these limitations. To explore this idea, we have experimentally studied illumination with coherent array of two sources spaced by less than a wavelength and operated near saturation in a semi-insulating GaAs source. By illuminating with an array of N=2 spots, we demonstrate a doubling in output terahertz power with no increase in input optical power. This result is consistent with results that have been shown for optical excitation that has been stretched along the anode using cylindrical lenses; however, the array of two spots permits steering the beam by adjusting the relative time of arrival of the two exciting pulses. Experimental evidence of this beam steering is presented. With proper electrode design, this approach may ultimately enable an N-fold increase in output power with an array of N spots, the formation of narrow beams, and the adjustment of beam direction by control of the relative time of arrival of the exciting optical pulses.
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Michael E. Knotts, Michael E. Knotts, Douglas R. Denison, Douglas R. Denison, } "Demonstration of enhanced emission and time delay beam steering using photoconductive terahertz source with multiple spot feed", Proc. SPIE 7311, Terahertz Physics, Devices, and Systems III: Advanced Applications in Industry and Defense, 73110X (30 April 2009); doi: 10.1117/12.819149; https://doi.org/10.1117/12.819149

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