Antenna-Coupled Superconducting Bolometers for Studying Dynamics with Terahertz Spectroscopy*

We report microwave and terahertz characterizations of antenna-coupled hot electron bolometers designed for laboratory-based terahertz spectroscopy. The active element is a superconducting niobium microbridge, and the incident signal is coupled to the microbridge by a planar double-dipole antenna. These devices combine sub-nanosecond response with high sensitivity and the ability to operate below saturation when viewing a room temperature background. The optimum small signal responsivity is 4.4 x 10$^{4}$ V/W, obtained at a bath temperature T$_{b}\approx $ 0.9T$_{c}$. The corresponding saturation power is 7 nW. The saturation power increases and the responsivity decreases as the bath temperature is lowered. The measured noise equivalent power is 2.0 x 10$^{-14}$ W/(Hz)$^{1/2}$, near the predicted thermal fluctuation limit. The unique combination of speed and sensitivity demonstrated by these detectors will enable new measurements of dynamic processes in the far-infrared on millisecond to nanosecond timescales.