Development of a microwave transmission setup for time-resolved measurements of the transient complex conductivity in bulk samples

Abstract

We describe and characterize a microwave transmission setup for the measurement of radiation-induced transient conductivities in the frequency range between 26 and 38GHz (Q band). This technique combines the virtues of two already existing techniques. On one hand, the microwave transmission technique is well established for the determination of (quasi)static conductivities, but requires adaptations to be suitable to the determination of transient conductivities with 1ns temporal resolution. On the other hand, the transient conductivity technique is well established, too, but in its present form (using a reflection configuration) it suffers from a poor signal to noise ratio due to unwanted interferences. These interferences are due to the circulator, which diverts part of the incoming microwave flux directly to the detector. We characterized the transmission setup by measuring the real and imaginary components of the conductivity of pulse irradiated CO2 gas at different pressures, and compared these results to predictions of the Drude model. CO2 was chosen as a test sample because of its well characterized behavior when irradiated with MeV electron pulses, and the fact that a wide range of the ratios of imaginary to real components of the conductivity are obtainable by just controlling the pressure. For intrinsic bulk isolators (either powders or in solution) pulse-induced conductivity changes as small as 10−8S∕m can be measured with nanosecond time resolution. The ratio of the imaginary to real part of the conductivity can be measured in the range from 0.084 to 28, which means that the dynamic range has been increased more than 100-fold with respect to the customary reflection setup.