Electromagnetic model for twin slot terahertz mixers

Twin slot antennas coupled to superconducting devices are currently being developed for terahertz mixers and direct detectors for astronomical observations. Although these mixers show promising performance in terms of noise temperature, they usually also show a considerable downward shift in the center frequency, especially when compared with calculations obtained with commonly used simplified models. This discrepancy is actually due to a variety of reasons. The impact of the bolometer-to-CPW transition, the RF choke filter and the radiation losses in the CPW line have been demonstrated in a recent study to be some of the most important reasons of the disagreement between calculations and measurements. In this paper we discuss these effects and other features, such as the silicon lens, obtaining a more robust model suitable not only for analysis but also for the synthesis of these circuits. A complete set of terahertz mixers at four different center frequencies, between 600 GHz and 2.5 THz, have been fabricated following the guidelines provided by our model and are currently being tested. The preliminary results of power coupling measured at 2.5 THz are still not as good as expected, however they show that our model is going in the right direction. With respect to previous designs, the new mixers have now a much better efficiency in terms of power coupling to the superconducting device and the center frequency shift is now decreased roughly from 30% to 10%. By the time that this paper will be published in the conference proceedings we will have more data, and we will be able to present a more extensive comparison between our calculations and measurements.