Broadband kinetic inductance detectors for far-IR observations

The development of direct absorbing kinetic inductance detectors (KIDs) for broadband far-infrared (FIR) observations designed to meet the needs of present and future telescopes is presented. This development was initiated to investigate the potential for upgrading the High-resolution Airborne Wideband Camera Plus (HAWC+) instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) with KIDs; but also has applications generally for FIR detectors. These detectors consist of backside-illuminated KIDs on a silicon substrate where the hybrid TiN/Al inductor forms a resistive wire grid with broadband absorption. Detectors can be configured to absorb in either one or two polarizations with a pixel filling factor of around 80%. A novel two-layer meta material anti-reflection coating, and a sub-quarter-wavelength backshort allow for greater than 85% detection efficiency over the ultra-wide 1 to 6 THz bandwidth. These detectors require no focal plane focusing optics such as feedhorns or microlenses, do not require fragile membranes, and utilize proven and straightforward fabrication methods. The optical and microwave design of these detectors is presented. Additionally, the performance of test devices is quantified. This includes measurements of the AR coating effectiveness, detector noise equivalent powers, and detector internal quality factors under the relevant loading levels for the HAWC+ instrument. This information is used to assess the potential benefit of upgrading the HAWC+ instrument with these new detectors and to determine the applicability of this technology for other future FIR detectors.