Microwave Properties of Anisotropic Materials Characterized with Picosecond Transient Radiation

Abstract

The recently developed coherent microwave transient spectroscopy (COMITS) technique is used to characterize the complex dielectric properties of anisotropic materials, over wide frequency range. COMITS is based on picosecond duration electromagnetic transients radiated and received by optoelectronically pulsed antennas [1,2]. The experimental setup is shown schematically in figure 1. The exponentially tapered coplanar stripline antennas are fabricated on implanted silicon-on-saphhire substrates [3]. Optical pulses from a mode-locked, pulse-compressed, and frequency- doubled, Nd:YLF laser are used in a standard pump/probe configuration to excite the dc-biassed transmitter and photoconductively sample the received waveforms. Hemispherical fused silica lenses collimate the radiation diverging from the transmitter and focus it onto the receiver. The measured signal has a 7 ps wide central peak, and frequency components from 0 GHz to 150 GHz. Since it is proportional to the field (and not power) incident on the receiver, phase information is preserved. The dielectric properties of a material are determined by making measurements with two different sample thicknesses. The corresponding Fourier spectra are divided in order to eliminate the effect of the sample surfaces and extract the net contribution due to the bulk [1,2]. Since COMITS is in effect a phase-sensitive microwave spectrophotometer, both real and imaginary parts of dielectric constants are obtained simultaneously, over a broad bandwidth.