Ultra-Sensitive Kinetic Inductance Detectors for Low-Background Space-Based Astronomy

Discovering the signs of life outside of our Solar system has profound implications for science and humanity, and is now a national science priority. However, achieving this goal requires unprecedented stability and control of the radiation and electro-thermo-mechanical environment in future space telescopes. Specifically, the ultra-low-background environment of the instruments demands nearly noise-free focal plane detectors with unprecedented stability to avoid false detections over long integration times. To address the needs of future space instruments operating across the visible-to-near-IR (VISIR), we propose a development program for improving the performance of superconducting kinetic inductance detectors (KIDs), which have been demonstrated in various forms for the detection of visible-to-millimeter photons in ground-based and sub-orbital astronomical instruments. In space, energy-resolving KIDs have the potential to enable and enhance the performance of (VISIR) instruments dedicated to searching for the signs of life in exoplanet atmospheres. These detectors are noise-free with virtually no dark current. However, the energy resolving power of KIDs at VISIR is more than a factor 3 lower than the theoretical limit. This gap must be closed to detect detect the narrowest bio-molecular spectral features across the 0.2 to 2 μm waveband. To improve the performance of energy-resolving KIDs, we propose a development program that targets the solid-state process limiting the optical responsivity. Specifically, the absorption of a VISIR photon creates high energy (athermal/hot) phonons in the KID. The athermal phonons leave the detector on timescales much faster than the detector response time. We aim to prevent the loss of athermal phonons by employing two specific strategies: (1) The optically active element of the KID will be suspended on a thin dielectric membrane. This strategy has been recently shown to lead to substantial improvement in energy resolving power in aluminum KIDs. (2) The membrane will be lithographically etched to form a meta-material phononic crystal (PnC), which surrounds the optically active part of the KID. The PnC opens up new ways to modify the electron-phonon interaction in a KID, which is otherwise not possible. For example, we have designed a PnC that, in addition to impeding the flow of athermal phonons, has a bandgap at the recombination energy of the Cooper pairs in the superconductor (in this case hafnium), and at the same time transmits thermal phonons with near unity transmission coefficient. We propose a focused three-year effort to develop hafnium lumped-element KIDs with energy resolution of R = 90 − 160 at wavelengths of 1500 - 400 nm, respectively. The resulting detectors will enable key science goals of future instruments searching for signs of life in exoplanet atmospheres. We also plan to measure R at mid- IR wavelengths (2-20 μm), where important spectral features resulting from bioactivity are located. An energy resolving power of 20 at 20 μm is possible with the proposed detector, which meets the requirement of mid-IR exoplanet mission concepts. The energy-resolving KIDs proposed in this effort open new opportunities for implementing small to medium scale exoplanet missions across the visible to mid-IR spectrum. The proposed KIDs retain all the desirable properties of a conventional KID, including the pair- breaking detection scheme, naturally high multiplexing factors, and immunity to ionizing radiation. These characteristics meet the goals of the APRA solicitation.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground-based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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