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Subgap Kinetic Inductance Detector Sensitive to 85-GHz Radiation

The energy spectrum of a superconductor possesses, in principle, a spectroscopic gap 2Δ that is the minimum energy required to excite the superfluid. Incident electromagnetic radiation with energy hν > 2Δ can break Cooper pairs, and this property has been used extensively to make ultra-sensitive radiation detectors: the Kinetic Inductance Detectors. In this work, we fabricated and tested an array of Subgap Kinetic Inductance Detectors that are sensitive in the 80–90 GHz frequency band well below the superconducting spectral gap of the film, at approximately 180 GHz. The intense sensitivity is interpreted as a divergence of the density of subgap modes at the plasma frequency.