Electromagnetic modeling and optimization of photoconductive switches for terahertz generation and photocurrent transient spectroscopy

The frequency response of photoconductive switches critically determines the performance of terahertz generating photoconductive antennas and of fast photocurrent measurements. Width and shape of the electrical transient depend on both the photoconducting material and on the geometry of the switch. In this work, the response of the photoconductive switch is described by a hybrid model based on the intrinsic response of the active material and the electromagnetic model of the switch geometry. The transmission line with its gap is expressed in terms of transconductance. Hence, its response can be optimized with respect to bandwidth, and moreover be taken into account by deconvolving the transconductance transient from the measured transient to determine more accurately the intrinsic properties of the active material. The method is illustrated for photoconductive switches with organic semiconductors as the active material. Incompatible with standard lithographic techniques, photoconductive switches based on organic semiconductors are typically embedded in microstrip lines and have feature size limitations imposed by the electrode deposition technique. An alternative structure with coplanar access is shown to have a bandwidth greater than 70 GHz, which is almost one order of magnitude higher than the state of the art.