Abstract

Time-domain device characterization using the laser-driven electro-optic or photoconductive sampling techniques has proven to be a valuable tool for directly measuring high-frequency behavior that can not be accessed using traditional network analyzers.^1-3 A typical time-domain test fixture using optically-based techniques is composed of a photoconductive switch, used to generate wide-band input pulses, and a transmission line, used to connect the source to the device under test and to guide the electrical transients to be measured. By characterizing incident, reflected, and transmitted electrical waveforms, the transmission and reflection coefficient—known electronically as the S- parameters—can be determined. Although the bandwidth of both the pulse generation and the waveform measurement techniques are in the terahertz regime, the device characterization bandwidth has been limited to around 100 GHz. This results from attenuation and dispersion in the transmission line, the latter arising from the high permittivity of the GaAs substrate (ε_r ≂ 13.1). We have alleviated this problem by lifting off an ultrafast-response low-temperature-grown GaAs (LT- GaAs)₄ thin film from its substrate and grafting it onto fused-silica or quartz substrates (ε_r ≃ 3.8) using Van der Waals bonding.³ As a simple preliminary example of the usefulness of such a structure, the transmission and reflection coefficients of a capacitive gap in a coplanar stripline (CPS) were characterized over a 200-GHz bandwidth.

© 1993 Optical Society of America