For the past few decades, charge-coupled devices (CCDs) have been a dominant technology in optical to near-infrared (NIR) astronomy, but more recently, instruments incorporating superconducting detector technologies, particularly microwave kinetic inductance detectors (MKIDs), have been built for a number of astronomical applications. MKIDs have multiple advantages over CCDs in the optical to near-IR regime, including single photon detection with microsecond timing, broadband wavelength coverage, and intrinsic energy resolution. These qualities are critical for observations of multi-wavelength, time-variable sources, such as compact binaries, exoplanet transits, and millisecond pulsars. MKIDs can also be naturally read out using frequency division multiplexing (FDM) techniques that are standard in the telecommunications industry, enabling large detector arrays.
Another type of superconducting detector technology, the transition-edge sensor (TES), shares in many of these same advantages, but because TES performance is less sensitive to fabrication processes and additional material layers, the per pixel performance (energy resolution and quantum efficiency) of a TES can be much simpler to improve. This can be seen, for example, in the detector quantum efficiency, where near unity efficiency at 1550 nm has been demonstrated in TESs. Reading out TES devices has traditionally involved the use of superconducting quantum interference devices (SQUIDs), which are not innately straightforward to multiplex for microsecond scale signals. Due to this limitation, we instead propose to multiplex optical to near-IR TESs using the novel kinetic inductance current sensor (KICS). The KICS utilizes a resonator fabricated with high kinetic inductance superconductors and geometries, allowing it to exploit the kinetic inductance’s quadratic current dependence. By coupling a TES with such a device, photon events that cause a change in the TES current can be measured as shifts in the resonant frequency of the readout resonator, and a large number of devices can be multiplexed using FDM techniques in much the same way as MKIDs.
This program will comprise of three main research thrusts. First and foremost, we will develop and optimize KICS devices to read out integrated optical/NIR TES arrays, with the goal of multiplexing a kilopixel scale array through a single microwave transmission line. Next, we will improve TES energy resolution, primarily by reducing the superconducting critical temperature and dynamic range of existing devices, and by fabricating devices on membranes and phonon barriers. Finally, we will optimize optical stacks to maintain high photon quantum efficiency across a broader wavelength band. All in all, by the end of this program we aim to demonstrate the multiplexed readout of kilopixel scale integrated array of optical/NIR TES detectors with resolving power approaching 90 at 1550 nm and quantum efficiency > 90 % between 400 nm to 2000 nm. Upon further development, optical to near-IR TES arrays would not only be significant for general purpose instruments for multi-wavelength, time domain astronomy, but they could be groundbreaking in the study of exoplanets, a focus area of the recent decadal survey. Here, a large array of TESs with high efficiency and resolution paired with a large space telescope could be used for the search of biosignatures in exoplanet atmospheres.