One of the major goals of modern astronomy is the atmospheric characterization of small exoplanets to learn about their diversity, habitability and ultimately, whether they harbor life. The Large Interferometer For Exoplanets initiative aims to perform atmospheric characterization of these planets in the mid-infrared (MIR) wavelength regime (4 to 18.5 micron). Extremely sensitive and highly efficient detectors are required to detect the faint signal from these small exoplanets. Kinetic Inductance Detectors (KIDs) are a promising candidate as they are able to count single photons with no readout noise or dark current. In this work we experimentally show that KIDs are able to do photon counting at 4 wavelengths between 3.8 and 24 micron. We also compare the performance of two KID designs to investigate what design would be optimal across the MIR band.
Using disordered superconductors, like β-Ta, TiN, Hf and PtSi, for MKIDs has the potential to improve the photon absorption efficiency, η, over a broad wavelength range. However, the resolving power, R, of these MKIDs is not higher than 20 at 1µm, while the fundamental Fano limit is around 65. To improve R, the signal-to-noise ratio must be increased. We measure the single photon pulse response of β-Ta MKIDs and show that the pulse duration is limited by disorder in two ways. First, the initial pulse decay is faster than exponential, which we attribute to slow quasiparticle diffusion. Second, the decay time of the pulse tail is faster due to low energetic, localized quasiparticles. Both these effects do not occur in conventional Al at the same experimental conditions. These results imply a trade-off between η and R and shows that improving MKIDs by using disordered superconductors is not straightforward.
KEYWORDS: Superconductors, Quasiparticles, Photon counting, Single photon, Signal to noise ratio, Sensors, Quantum efficiency, Aluminum, Tin, Temperature metrology
Over de past decade, conventional superconductors (like Al), have been replaced by disordered superconductors (like beta-Ta, PtSi, Hf and TiN) in visible/near-infrared MKIDs, improving the responsivity and quantum efficiency. However, important detector characteristics, such as the single photon pulse decay time and generation-recombination (GR) noise levels, of disordered MKIDs are presently not understood.
We present temperature dependent GR noise and single photon (1545 – 402 nm) pulse measurements in MKIDs made from beta-Ta (Tc=1.0 K, ρ=240 μΩcm) and show that it does not follow theory. We hypothesize that quasiparticle trapping causes this deviation.
Superconducting Microwave Kinetic Inductance Detectors (MKIDs) are superconducting, photon-counting, and energy-resolving detectors for visible to near-infrared wavelengths.
We present the analysis of a hybrid Lumped Element (LEKID) design with a beta-phase Ta inductor. We show that the measured KID response matches a response model based on the complex conductivity and material parameters.
The signal-to-noise is limited by the devices' microwave power handling, the very fast pulse decay, and saturation of the KID phase response. We will present our follow-up design optimized for higher power handling and with a larger inductor volume to reduce the response saturation.
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