Abstract The Cooper-pair breaking efficiency quantifies the effectiveness with which incident energy, such as that delivered by photons or phonons, is converted into quasiparticles—a process central to the response of superconducting pair-breaking detectors. We measure the pair-breaking efficiency in two far-infrared kinetic inductance detectors (KIDs) operating at 4.3 THz and 15 THz by comparing photon noise and thermal generation-recombination noise. Both KIDs are lumped-element devices consisting of a tantalum interdigitated capacitor in parallel with an aluminum inductor meandered line, which also acts as a low-volume absorber. The measured pair-breaking efficiencies are 0.42 and 0.07 for the 4.3 THz and 15 THz devices, respectively. The extremely low efficiency at 15 THz, a frequency well above the aluminum Debye frequency, is mainly attributed to rapid anharmonic decay of phonons, with additional phonon relaxation in the tantalum layer further reducing the available phonon population for pair-breaking. Furthermore, a relationship between the pair-breaking efficiency and detector resolving power is derived.
Abstract The Cooper-pair breaking efficiency quantifies the effectiveness with which incident energy, such as that delivered by photons or phonons, is converted into quasiparticles—a process central to the response of superconducting pair-breaking detectors. We measure the pair-breaking efficiency in two far-infrared kinetic inductance detectors (KIDs) operating at 4.3 THz and 15 THz by comparing photon noise and thermal generation-recombination noise. Both KIDs are lumped-element devices consisting of a tantalum interdigitated capacitor in parallel with an aluminum inductor meandered line, which also acts as a low-volume absorber. The measured pair-breaking efficiencies are 0.42 and 0.07 for the 4.3 THz and 15 THz devices, respectively. The extremely low efficiency at 15 THz, a frequency well above the aluminum Debye frequency, is mainly attributed to rapid anharmonic decay of phonons, with additional phonon relaxation in the tantalum layer further reducing the available phonon population for pair-breaking. Furthermore, a relationship between the pair-breaking efficiency and detector resolving power is derived.
Future far-infrared space observatories equipped with cryogenic telescopes require ultra-sensitive detectors to enable measurements limited only by the sky background. We present the design, fabrication, and characterization of a superconducting kinetic inductance detector (KID) at 4.3 THz. The lumped-element KID comprises a tantalum interdigitated capacitor and a 100-nm-wide aluminum-line inductor connected in parallel. The inductor acts also as a low-volume absorber. We achieved photon-noise-limited performance with a noise equivalent power of 1.9 × 10−19 W/Hz referenced to absorbed blackbody power at a modulation frequency of 300 Hz. Under optical loading above 1.83 fW, the detector remains stable with white-noise spectra extending down to ∼100 mHz.
Low temperature Kinetic Inductance Detectors (KIDs) are attractive candidates for producing quantumsensitive, arrayable sensors for astrophysical and other precision measurement applications. The readout uses a low frequency probe signal with quanta of energy well-below the threshold for pair-breaking in the superconductor. We have calculated the detailed non-equilibrium quasiparticle and phonon energy spectra generated by the probe signal of the KID when operating well-below its superconducting transition temperature Tc within the framework of the coupled kinetic equations described by Chang and Scalapino.[1] At the lowest bath temperature studied Tb/Tc = 0.1 the quasiparticle distributions can be driven far from equilibrium. In addition to the low frequency probe signal we have incorporated a high frequency (~ 1 THz) source signal well-above the pair-breaking threshold of the superconductor. Calculations of source signal detection efficiency are discussed
The response of superconducting pair-breaking detectors is dependent on the details of the quasiparticle distribution. In Kinetic Inductance Detectors (KIDs), where both pair breaking and non-pair breaking photons are absorbed simultaneously, calculating the detector response therefore requires knowledge of the often nonequilibrium distributions. The quasiparticle effective temperature provides a good approximation to these nonequilibrium distributions. We compare an analytical expression relating absorbed power and the quasiparticle effective temperature in superconducting thin films to full solutions for the nonequilibrium distributions, and find good agreement for a range of materials, absorbed powers, photon frequencies and temperatures typical of KIDs. This analytical expression allows inclusion of nonequilibrium effects in device models without solving for the detailed distributions. We also show our calculations of the frequency dependence of the detector response are in agreement with recent experimental measurements of the response of Ta KIDs at THz frequencies.
Abstract Understanding the detailed behaviour of superconducting pair breaking photon detectors such as Kinetic Inductance Detectors (KIDs) requires knowledge of the nonequilibrium quasiparticle energy distributions. We have previously calculated the steady state distributions resulting from uniform absorption of monochromatic sub gap and above gap frequency radiation by thin films. In this work, we use the same methods to calculate the effect of illumination by broadband sources, such as thermal radiation from astrophysical phenomena or from the readout system. Absorption of photons at multiple above gap frequencies is shown to leave unchanged the structure of the quasiparticle energy distribution close to the superconducting gap. Hence for typical absorbed powers, we find the effects of absorption of broadband pair breaking radiation can simply be considered as the sum of the effects of absorption of many monochromatic sources. Distribution averaged quantities, like quasiparticle generation efficiency η , match exactly a weighted average over the bandwidth of the source of calculations assuming a monochromatic source. For sub gap frequencies, however, distributing the absorbed power across multiple frequencies does change the low energy quasiparticle distribution. For moderate and high absorbed powers, this results in a significantly larger η –a higher number of excess quasiparticles for a broadband source compared to a monochromatic source of equal total absorbed power. Typically in KIDs the microwave power absorbed has a very narrow bandwidth, but in devices with broad resonance characteristics (low quality factors), this increase in η may be measurable.
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.
Understanding the detailed behaviour of superconducting pair breaking photon detectors such as Kinetic Inductance Detectors (KIDs) requires knowledge of the nonequilibrium quasiparticle energy distributions. We have previously calculated the steady state distributions resulting from uniform absorption of monochromatic sub gap and above gap frequency radiation by thin films. In this work, we use the same methods to calculate the effect of illumination by broadband sources, such as thermal radiation from astrophysical phenomena or from the readout system. Absorption of photons at multiple above gap frequencies is shown to leave unchanged the structure of the quasiparticle energy distribution close to the superconducting gap. Hence for typical absorbed powers, we find the effects of absorption of broadband pair breaking radiation can simply be considered as the sum of the effects of absorption of many monochromatic sources. Distribution averaged quantities, like quasiparticle generation efficiency η, match exactly a weighted average over the bandwidth of the source of calculations assuming a monochromatic source. For sub gap frequencies, however, distributing the absorbed power across multiple frequencies does change the low energy quasiparticle distribution. For moderate and high absorbed powers, this results in a significantly larger η–a higher number of excess quasiparticles for a broadband source compared to a monochromatic source of equal total absorbed power. Typically in KIDs the microwave power absorbed has a very narrow bandwidth, but in devices with broad resonance characteristics (low quality factors), this increase in η may be measurable.