Efficient mid-IR superconducting nanowire single-photon detectors for exoplanet science

We propose to improve the detection efficiency and dark count rates of superconducting nanowire single-photon detectors (SNSPDs) at mid-IR wavelengths from 10 to 25 µm. The mid-IR is a key region for exoplanet science due to the peak in thermal emission from earth-like planets in this range. Moreover, many biomarkers of interest have transitions between 10 and 25 µm, including CO2 at 15 µm and water vapor at 18 µm. Space telescopes like JWST have thus far relied on Si:As IBC detectors in this wavelength range, but these detectors have been out of production for over a decade, and it is not clear that production can be restarted. HgCdTe detectors work well at the shorter wavelengths in this range, but become more difficult to produce beyond 15 µm. Similarly, MKID detectors excel at far-IR wavelengths, but optical coupling becomes increasingly difficult below 25 - 30 µm. We have recently demonstrated SNSPDs with high internal detection efficiency at wavelengths as long as 18 µm, making them ideal detectors to bridge the gap between HgCdTe detectors and MKIDs. In particular, SNSPDs could be useful for direct imaging of exoplanets, such as on a nulling interferometry mission like the LIFE mission concept, or as ultra-stable detectors for exoplanet transit spectroscopy on a future IR flagship mission. Ultra-stable mid-IR detectors are currently on NASA’s technology gap list. Traditionally used in the NIR, SNSPDs have demonstrated 98% efficiency, dark count rates near 1e-5 counts/s/pixel with 0 read noise, maximum count rates > 1e7 counts/s/pixel, and kilopixel array formats at wavelengths near 1.5 µm. Their high timing resolution can be used to gate out false detections, such as cosmic ray events or perturbations from the null point in a nulling interferometer. Through previous APRA funding, we demonstrated single-photon sensitivity and high internal detection efficiency at wavelengths up to 18 µm, and we began to investigate how different metrics of SNSPD performance change at longer wavelengths. We found that the major challenges to producing high-performance SNSPD arrays suitable for mid-IR exoplanet science were achieving high optical coupling into the nanowire and measuring and improving the detector’s dark count rate. Therefore, the proposed work effort will focus on these two metrics of SNPSD performance. To enhance the mid-IR detection efficiency, we plan on modeling, fabricating, and testing both optical stack and antenna coupling schemes. We will perform a study to identify low-loss materials that are suitable for use as dielectrics in this range, and will characterize the optical properties of deposited thin-films in the mid-IR wavelength range. We will then fabricate test devices and measure the detection efficiency using an existing mid-IR testbed with a reference detector. To characterize the baseline dark count rate of the detectors, we will measure count rates in a shielded package to determine how dark counts depend on bias current, nanowire geometry, and operating temperature. We will use these results to adjust the detector design to minimize dark counts, and we will design a package that allows for optical coupling while still minimizing counts from background light sources. After three years, we expect to produce detectors with both high coupling efficiency and low dark counts.

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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