We present a theoretical model of a superconducting kinetic inductance detector which promises high sensitivity and energy resolution from submillimetre to X-ray wavelengths. Cooper-pair breaking photons are absorbed in a superconductor, exciting quasiparticles which change the surface inductance. By arranging the detector in a resonant circuit we can measure the resulting phase-shift of a microwave probe signal. Software has been created to model the superconducting characteristics of the detector and its behaviour when a photon is absorbed. The model predicts the position sensitivity of the detector and calculates how quasiparticles diffuse and recombine to a thermal background level. This temporal evolution of quasiparticle dynamics gives rise to a measurable phase-shift pulse, which will allow the energy and time of a photon to be measured. Pulse shapes have been simulated for photon energies of 1-5 keV being absorbed at the sensitive ground-end of the detector.
We present a theoretical model of a superconducting kinetic inductance detector which promises high sensitivity and energy resolution from submillimetre to X-ray wavelengths. Cooper-pair breaking photons are absorbed in a superconductor, exciting quasiparticles which change the surface inductance. By arranging the detector in a resonant circuit we can measure the resulting phase-shift of a microwave probe signal. Software has been created to model the superconducting characteristics of the detector and its behaviour when a photon is absorbed. The model predicts the position sensitivity of the detector and calculates how quasiparticles diffuse and recombine to a thermal background level. This temporal evolution of quasiparticle dynamics gives rise to a measurable phase-shift pulse, which will allow the energy and time of a photon to be measured. Pulse shapes have been simulated for photon energies of 1-5 keV being absorbed at the sensitive ground-end of the detector.
We are starting to investigate a novel multiplexable readout method that can be applied to a large class of superconducting pair-breaking detectors.This readout method is completely different from those currently used with STJ and TES detectors, and in principle could deliver large pixel counts, high sensitivity, and Fano-limited spectral resolution.The readout is based on the fact that the kinetic surface inductance L s of a superconductor is a function of the density of quasiparticles n, even at temperatures far below T c .An efficient way to measure changes in the kinetic inductance is to monitor the transmission phase of a resonant circuit.By working at microwave frequencies and using thin films, the kinetic inductance can be a significant part of the total inductance L, and the volume of the inductor can be made quite small, on the order of 1 //m 3 .As is done with other superconducting detectors, trapping could be used to concentrate the quasiparticles into the small volume of the inductor.However, the most intriguing aspect of the concept is that passive frequency multiplexing could be used to read out ~ 10 3 detectors with a single HEMT amplifier. DETECTOR CONCEPTKinetic inductance detectors operate on the principle that a change in the number of quasiparticles in a superconductor will produce a change in the kinetic inductance.This occurs because the quasiparticles block Cooper pairs from occupying some of the electron states.Photons (or particles) are absorbed in the superconductor, producing quasiparticles, and the resulting change in the inductance is measured to produce the detector output.Bolometric detectors using kinetic inductance thermometers have been previously suggested [1, 2, 3, 4, 5, 6].However, such thermometers must be operated close to T c , since the variation of the kinetic inductance with temperature is most rapid near T c , and vanishes exponentially at low temperatures.Although it has been discussed previously [7], it is not widely appreciated that the the variation of the kinetic inductance with energy or quasiparticle number is actually fairly constant down to very low temperatures.The idea, then, is to use the kinetic inductance effect to read out changes in the quasiparticle population produced by the direct absorbtion of pair-breaking photons rather than changes in the temperature of the phonons in a bolometric absorber.How can one measure the change in the kinetic inductance of a superconductor?One way is to make the superconductor act as an inductive element in a resonant circuit.A change in the quasiparticle population will change the inductance of the superconductor, and therefore the resonance frequency of the circuit, which can be monitored by measuring the transmission phase of a constant-frequency microwave probe signal transmitted through (or reflected from) the resonator.This method is extremely attractive, due both to CP605, Low Temperature Detectors, edited by F. S. Porter et al.
The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations. This change may be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID). The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. The authors present results on position sensitive x-ray detectors made by using two aluminum MKIDs on either side of a tantalum photon absorber strip. Diffusion constants, recombination times, and energy resolution are reported. MKIDs can easily be scaled into large arrays.
Astrophysical photons with a sub-millimetre wavelength are of great value to research star and galaxy formation. Superconductors have a small energy gap with which these sub-millimetre photons can be measured. Superconductors can absorb photons so that quasiparticles are created, which will in turn change the complex conductivity: incoming photons change the electrical response of the superconductor. This principle can be used to realise a detector. This is done with the Microwave Kinetic Inductance Detectors (MKIDs) that consist of a superconducting film embedded in a resonant circuit, so that photons falling in will change the resonant frequency. This can be measured with a readout signal. To enhance the sensitivity of the MKID, we want to increase the responsivity of an MKID as much as possible.A previous study has proposed a figure of merit regarding the material properties of a superconductor to achieve a high responsivity. However, the readout frequency used is not included in this figure. The goal of this work is to extend the figure of merit so that the readout frequency of the MKID can be optimised for a higher responsivity.
Superconducting detectors are a modern technology applied in various fields. The microwave kinetic inductance detector (MKID) is one of the cutting-edge superconducting detectors. It is based on the principle of a superconducting resonator circuit. A radiation entering the MKID breaks the Cooper pairs in the superconducting resonator, and the intensity of the radiation is detected as a variation of the resonant condition. Therefore, calibration of the detector responsivity, i.e., the variation of the resonant phase with respect to the number of Cooper-pair breaks (quasiparticles), is important. We propose a method for responsivity calibration. Microwaves used for the detector readout locally raise the temperature in each resonator, which increases the number of quasiparticles. Since the magnitude of the temperature rise depends on the power of readout microwaves, the number of quasiparticles also depends on the power of microwaves. By changing the power of the readout microwaves, we simultaneously measure the phase difference and lifetime of quasiparticles. We calculate the number of quasiparticles from the measured lifetime and by using a theoretical formula. This measurement yields a relation between the phase responses as a function of the number of quasiparticles. We demonstrate this responsivity calibration using the MKID maintained at 285 mK. We also confirm the consistency between the results obtained using this method and conventional calibration methods in terms of the accuracy.
Kinetic inductance detectors (KIDs) are superconducting thin-film microresonators that are sensitive photon detectors. These detectors are a candidate for the next generation of experiments designed to measure the polarization of the cosmic microwave background (CMB). I discuss the basic theory needed to understand the response of a KID to light, focusing on the dynamics of the quasiparticle system. I derive an equation that describes the dynamics of the quasiparticle number, solve it in a simplified form not previously published, and show that it can describe the dynamic response of a detector. Magnetic flux vortices in a superconducting thin film can be a significant source of dissipation, and I demonstrate some techniques to prevent their formation. Based on the presented theory, I derive a corrected version of a widely-used equation for the quasiparticle recombination noise in a KID. I show that a KID consisting of a lumped-element resonator can be sensitive enough to be limited by photon noise, which is the fundamental limit for photometry, at a level of optical loading below levels in ground-based CMB experiments. Finally, I describe an ongoing project to develop multichroic KID pixels that are each sensitive to two linear polarization states in two spectral bands, intended for the next generation of CMB experiments. I show that a prototype 23-pixel array can detect millimeter-wave light, and present characterization measurements of the detectors.
While experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, these detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance, which can be measured by a sensitive SQUID readout. We modeled the kinetic inductance response of detectors made from disordered superconducting Nb, NbC, and MoRe films. Low phonon transparency of the interface between the superconductor and the substrate causes substantial re-trapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation-recombination noise can be as small as ∼10−19 W/√Hz at He4 temperatures.