UNIVERSITÄT ZU KÖLN

Introduction

Microwave Kinetic Inductance Detectors (MKID) are superconducting power detectors suitable for a wide frequency range from about 50 GHz up to the X-ray frequencies. MKIDs utilize the dependence of the inductive surface impedance of a superconductor on the number density of Cooper pairs. If Cooper pairs are split into quasiparticle excitations ("single electrons") by irradiation of the superConductor, the number of Cooper pairs decreases and the kinetic inductance increases. The surface impedance, of which the kinetic inductance is a part, can be measured sensitively using a superconducting resonator circuit (left image) with a resonance frequency in the GHz range, patterned into a thin superconducting film. If the inductor of the resonator is illuminated with photons of an energy larger than the superconducting gap energy, Cooper pairs are split up, the kinetic inductance increases and the resonance frequency shifts towards lower frequencies (right image).

By monitoring the resonance feature the absorbed power can be detected. Sensitivities of MKIDs around 10^-19 W Hz^-1/2 have already been demonstrated. One great advantage of this detector type is the easy implementation of readout multiplexing by attaching many resonators with different resonance frequencies, each representing a single pixel of the detector, to a single readout line. This is incontrast to the traditional bolometer camera approach where each pixel needs a readout line. This facilitates the construction of large-format camera arrays.

A 350 GHz Waveguide Coupled KID design

We investigate the sensitivity and noise of Microwave Kinetic Inductance Detector (MKID) devices that are coupled to a waveguide utilizing a proven waveguide probe antenna. This 350 GHz design originates from a SIS mixer and guarantees good radiation coupling to the detector, and facilitates analysis of the MKID device performance itself.

The detector design is that of an antenna coupled KID. The signal is coupled via a high performance smooth-walled spline profile feedhorn to a waveguide and superconducting Niobium probe antenna. The RF circuitry is fabricated onto a device chip with a 9 µm silicon membrane as substrate using the DRIE/MEMS fabrication technology developed for our HEB and SIS mixers. The RF signal is split into two coplanar waveguide (CPW) absorber lines with a 40 nm thick aluminum centerline as the absorbing part and Niobium ground planes. The CPW centerline forms the lumped element inductive section of a 2.5 GHz superconducting resonator, together with a niobium interdigital capacitor (see title image). It is capacitively coupled to the readout line. Since the deposition of the centerline is a separate lithography step, the design also enables us to exchange the Al material and use the circuit as a testbed for kinetic inductance and loss measurements with other absorber materials. In addition to this design an identically coupled quarter-wave transmission line resonator is designed and fabricated and will be compared to the detector design (picture above). The devices are fabricated in our in-house microfabrication lab. Both circuits are designed with CST Microwave Studio^TM for the high frequency parts of the circuit and Sonnet EM^TM for the readout circuit.

Within a collaboration with the IRAM institute in Grenoble we have measured devices intended for the NIKA2 MKID camera. These devices are made out of aluminum and allow us a thorough verification of the measurement setup. Our setup allows measurement of the fast dynamic response of the detectors when illuminated with short optical pulses. These measurements are currently not possible at IRAM, therefore out goal is the measurement of dynamic material parameters like the quasiparticle lifetime.

The MKID measurement setup consists of the ADR cryostat which cools the samples down to 40mK. Resistance measurements at different temperatures are possible down to 40 mK with currents in the range of microamperes using lock-in amplifiers. These measurments are vital for material development. For reading out the MKID detectors, low noise analog high frequency readout electronics are used with an IQ mixer as the main element allowing to separate amplitude and phase noise. In contrast to the digital solutions, the analog setup can only measure one detector at a time, but allows to measure the dynamic response and the noise up to 2 µs.

Acknowledgements

MKID development is carried out within the Collaborative Research Centre 956, sub-project D3 , funded by the Deutsche Forschungsgemeinschaft (DFG).