The planar Microwave Kinetic Inductance Detector (MKID) is very compact and allows a large number of detectors to be fabricated simultaneously and implemented in a focal plane of a telescope using a minimal number of wiring interfaces. In this detector, the incident power is guided through a planar transmission line and absorbed in a thin superconductor with high kinetic inductance. The amount of power absorbed per cubic volume of the detector determines the sensitivity of the detector and its ability to count number of photons. This invention is a detector design that has minimal volume of absorptive material, thus yielding a highly sensitive cryogenic detector. In addition, the design provides a very broad operating frequency range and has low crosstalk among detectors when placed in an array configuration. Using this invention, microwave power can be coupled to an absorptive volume as low as one cubic micron. Furthermore, this design minimizes the amount of trap charges in the detector thus increasing the accuracy in detecting small numbers of photons. The high quality factor of MKID is preserved at its resonance frequency in the microwave frequency band.
The MKID is a low-temperature detector whose sensitivity is mainly determined by the kinetic inductance value of the detector material. Its detection frequency is between sub-millimeter and far-infrared, and the same approaches can be applied in detector design for X-ray applications. To achieve a highly sensitive detector detecting a few numbers of photons, material with high kinetic inductance is used. In addition, a small volume of material must be used to maximize the temperature gradient in the material. Although obtaining high kinetic inductors for MKID can be achieved by selecting proper material and applying it to a large portion of the MKID area, reducing the material volume can be very difficult using the prior art approaches as it has impact on coupling efficiency and operating bandwidth of the detector.
The invention consists of the readout and the detection sections. In the readout section, at readout frequency (which is much lower than at the detection frequency), the detector can be visualized as two lumped inductors and two lumped capacitors connected in series where two inductors are placed between two capacitors. The lumped inductors represent the power-absorbing portion of the detector. They are made of a superconductor with a high kinetic inductance value whose value changes according to the received input power at the detection frequencies. This superconductor material is selected to have a critical frequency below the detection frequency band such that it behaves as a resistive element, and absorbs the incident power above its superconducting critical frequency. The capacitor portions are made of superconductors with lower kinetic inductance value, and have a superconducting critical frequency above the detection bandwidth. The capacitors represent the tuning element to produce a resonator with the resonance frequency suitable for readout by electronic circuits operating between 1 and 10 GHz.
This work was done by Samuel Moseley, Kongpop U-yen, Ari Brown, Thomas Stevenson, Wen-Ting Hsieh, Edward Wollack, and Negar Ehsan of Goddard Space Flight Center. GSC-16602-1