Microwave and millimeter wave dielectric measurements with picosecond optoelectronics

Figure 1 depicts a typical set-up for transmission measurements which has been used to characterize low-loss dielectrics [4]. The radiating and receiving elements in our experiments are exponentially tapered coplanar stripline antennas fabricated on silicon-on-sapphire substrates, with standard photolithographic techniques. The hemispherical fused-silica lenses collimate the radiation diverging from the transmitter, and refocus it onto the receiver [3]. In addition, the high polarization sensitivity of the planar radiating elements in Fig. 1 allow the dielectric properties of anisotropic materials to be determined [SI. Recently, we have utilized the coherent microwave transient spectroscopy (COMITS) technique to measure the dielectric properties of composites, and to explore the validity of mixture theories often used to predict their dielectric response [SI. The "quasi-optical" nature of the COMITS technique can be exploited to characterize samples in reflection as shown in figure 2 [7]. In this case the signal reflected from a gold mirror acts as the reference. The reflection configuration is particularly suited to characterization of lossy materials. Results of recent measurements on silicon substrates with a range of doping densities will be presented, and compared with the predictions of a simple Drude model [SI.