Optical Efficiencies of Lens-Antenna Coupled Kinetic Inductance Detectors at 220 GHz

We have been developing a terahertz camera based on antenna-coupled superconducting resonators, the so-called microwave kinetic inductance detectors (MKIDs), and a silicon lens array. The MKID consists of a coplanar waveguide coupled to a double slot antenna and is patterned on a high-quality aluminum film grown by molecular beam epitaxy. The camera is sensitive at frequencies of 200-240 GHz. Its bandwidth is limited by the impedance properties of the double slot antenna. The design, fabrication, and optical evaluations of the planar antennas and silicon lens arrays are presented in this paper. The MKID camera has been evaluated both in dark conditions and under optical radiation in a 0.1-K dilution refrigerator. The electrical noise equivalent power was around 5x10(-18)W/root HZ in dark conditions and 4x10(-16)W/root HZ, which is much lower than the photon noise level, with the optical load. The optical efficiency of the camera was estimated by three independent methods, and the results were consistent with each other and equal to 20%-25% without an anti-reflection coating on the lens surface.