Submillimeter-wave spectral response of twin-slot antennas coupled to hot electron bolometers

The spectral response of quasioptical hot-electron bolometer (HEB) mixers has been measured with a Fourier transform spectrometer. In this study we have measured mixers with a wide range of twin-slot antennas with slot lengths ranging from 26 μm up to 152 μm. For all designs, the measured center of the direct detection response was lower than predictions based on a Method-of-Moment (MoM) calculation of a simplified mixer embedding circuit model. This model took into account the slot antennas and the coplanar waveguide embedding circuit, but not the parasitic effects associated with the couplings of these elements or the HEB device geometry. The frequency shift becomes particularly significant at frequencies beyond 1 THz, and is in excess of 20% for the smallest antennas. Such large shifts reduce the radiation coupling efficiency by more than 3 dB, and hence increase the mixer noise temperature unnecessarily. To resolve the discrepancy between the observations and the model predictions we have introduced two essential refinements into the MoM model which account for (1) the coplanar waveguide (CPW) to slot-antenna transition, and (2) the inductance associated with the effective narrowing of the CPW center lead as represented by the HEB bridge. The results of these model refinements prove to have a small effect on the real-part of the embedding impedance but a dramatic effect on the imaginary-part. In fact, the inductance of the narrow microbridge dominates the estimated frequency shift. Thus we find that properly modeling the entire embedding circuit and the device geometry leads to a better agreement with the measured results.