NASA Goddard developed an absorber-coupled far-infrared microwave kinetic inductance detector (FIR MKID) array that incorporates a cross-absorber pattern. It allows incident power to be absorbed equally for both horizontal and vertical polarization. The array enables the output of the detector to be frequency multiplexed at more than twice the frequency of the conventional MKID design. In addition, the technology allows power to be more evenly distributed across the detector area, increasing detector sensitivity. The detector also can be read out at higher operating frequencies than prior art designs, thus increasing the detector array channel capacity.
The FIR MKID consists of two major components: (1) the metal pattern for FIR absorption, and (2) the microwave transmission line resonator for RF readout. The crossbar metal pattern on the membrane provides identical power absorption for both horizontal and vertical polarization signals. The metal patterns are placed on both the top and bottom of the membrane to create a parallel-plate-coupled transmission line that acts as a half-wavelength resonator at readout frequencies. The parallel-plate transmission line in the membrane area is connected to a low-impedance microstrip line at the detector edges to form a stepped impedance. The parallel-plate transmission line on the membrane is split into four sections in a meandering cross pattern.
At IR frequencies, the detector’s superconducting metal pattern acts as an absorber. It is designed to have the effective area match with the characteristic impedance of free space, resulting in minimum return loss at the center of the operating frequency. This maximizes the power absorption in the metal pattern, causing the temperature of the metal to increase. This enables detection of very low-power far-infrared frequency signals with both horizontal and vertical polarizations.