We present a novel, free-standing low-temperature GaAs ͑LT-GaAs͒ photoconductive switch and demonstrate its femtosecond performance. A 1-m-thick layer of a single-crystal LT-GaAs was patterned into 5-10-m-wide and 15-30-m-long bars, separated from their GaAs substrate and, subsequently, placed across gold coplanar transmission lines deposited on a Si substrate, forming a photoconductive switch. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an

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... ctro-optic sampling system. Using 810-nm optical radiation, we recorded an electrical transient as short as 360 fs ͑1.25 THz, 3-dB bandwidth͒ and established that the photo-carrier lifetime in our LT-GaAs was 150 fs. Our free-standing devices exhibited quantum efficiency of the order of ϳ7%, and their photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the excitation power, below a well-defined saturation threshold.