Tech Briefs

Microwave Kinetic Inductance Detector With Selective Polarization Coupling

Low-noise detector and readout functionality are combined into one device.

A conventional low-noise detector requires a technique to both absorb incident power and convert it to an electrical signal at cryogenic temperatures. This innovation combines low-noise detector and readout functionality into one device while maintaining high absorption, controlled polarization sensitivity, and broadband detection capability. The resulting far-infrared detectors can be read out with a simple approach, which is compact and minimizes thermal loading.

The proposed microwave kinetic inductance detector (MKID) consists of three basic elements. The first is the absorptive section in which the incident power is coupled to a superconducting resonator at farinfrared frequency above its superconducting critical frequency (where superconductor becomes normal conductor). This absorber’s shape effectively absorbs signals in the desired polarization state and is resonant at the radio frequency (RF) used for readout of the device. Control over the metal film used in the absorber allows realization of structures with either a 50% broadband or 100% resonance absorptance over a 30% fractional bandwidth.

The second element is a microwave resonator — which is realized from the thin metal films used to make the absorber as transmission lines — whose resonance frequency changes due to a variation in its kinetic inductance. The resonator’s kinetic inductance is a function of the power absorbed by the device. A low-loss dielectric (mono-crystalline silicon) is used in a parallel- plate transmission line structure to realize the desired superconducting resonators. There is negligible coupling among the adjacent elements used to define the polarization sensitivity of each detector. The final component of the device is a microwave transmission line, which is coupled to the resonator, and allows detection of changes in resonance frequency for each detector in the focal plane array.

The spiral shape of the detector’s absorber allows incident power with two polarizations to couple to the detector equally. A stepped impedance resonator was used that allows the incident power absorbed in the detecting membrane area to be uniformly distributed in the detector’s transmission line at the RF readout frequency. This maximizes the sensitivity of the detector. The signal is read out via a frequency multiplexing technique that requires a minimum number of interface transmission lines for readout. This reduces the packaging complexity and coupling to the device’s thermal environment.

This work was done by Edward Wollack, Kongpop U-yen, Thomas Stevenson, Ari Brown, Samuel Moseley, and Wen-Ting Hsieh of Goddard Space Flight Center. GSC-16342-1

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