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Completed TRL 2 (started at 2, targeting 3)
In this work, we propose to merge two fabrication techniques for sensitive detectors currently in development at GSFC to produce game changing sensor arrays for the mid and far IR (MIR/FIR). The first technique is the Longitudinal Proximity Effect Transition Edge Sensor (LoPE TES) – a small volume of any metal, linking two superconducting leads, can be designed for high speed and sensitivity. Devices produced at GSFC have demonstrated world record NEPs as well as single photon counting [Nagler+20]. The second technique is a high efficiency photon absorbing metal film in the same layer of the cavity as the sensor. Because of the flexibility of the LoPE technique, any metal used to create the absorber can also have a small region dedicated to the formation of the detector. The LoPE, its wiring, and the metamaterial absorber can be patterned on GSFC’s new stepper and e-beam lithography machines that are coming online now and promise access to the ultrafine (submicron) features needed to achieve these devices.
As we set our sights on observing more distant, fainter objects, future NASA observatories require higher sensitivity detector arrays than ever before. When targeting faint IR astrophysical sources, a limiting technological factor in achieving higher sensitivity is the higher temperature control and stability required to minimize background radiative loading. Instruments like the proposed MIRECLE, which plans to perform spectroscopy on exoplanets in the MIR, require extraordinary thermal and electrical stabilities for the detector typically spanning weeks to take the measurements. However, if one were to utilize a detector as an energy resolving photon counter (such as have yet to be achieved at the target wavelengths of 2-20 microns) rather than a bolometer, single photon counts would always comprise events above the thermal fluctuations observed in the detector. This would prevent the need for extraordinary integration times to build up spectra. In addition to demonstrating the sensitivity (which is well underway with the ongoing technical maturation of LoPE TESes), the issue for SPDs then becomes developing a low-loss absorber that collects energy to the detective element within the intrinsic time constants of the device.
We are proposing for a single year IRAD to fabricate LoPE TES arrays and integrate them with novel metamaterial absorbers. The world’s best IR SPDs have achieved ~97% absorption with a cavity-based W (or MoSi) TES in a fiber optic coupled configuration optimized for 1.55 microns. While these demonstrated single photon counters have extraordinarily accurate determination number of photon number (i.e., distinguishing the number of photons that arrive simultaneously), the detectors are not easily arrayed - with a maximum demonstration currently at 12 sensors with a separate fiber for each detector. Our concept is to transform the cavity-based sensor single photon absorption through a micropatterned (or otherwise impedance matching) metal film to couple the single photon to the sensor without need for a fiber optic. The extents of the absorber film define the pixel dimensions for absorption with the LoPE TES and its attendant wiring being vanishing small compared to the absorber area. Notably, the LoPE TES can be made from the same layer as the absorber by micropatterning a region of it to the right scale for lateral proximity and creating the ultrasensitive device by attaching the superconducting leads in a second layer.
Ultimately, we aim to mature LoPE TESes into testable arrays with a high QE absorber for use as single photon detectors. In the process of fabricating LoPE TES arrays we will also reduce the size of the superconducting leads. Such advances in narrow lead width will improve device designs with better control over their superconducting properties while simultaneously preventing unwanted flux trapping near the interfaces in the LoPE devices. High-density arrays require ultranarrow wires to reduce space and decrease the detectors’ sensitivity to magnetic fields by reducing area of trapping interfaces for magnetic flux. Such demonstrations of narrow superconducting leads are expected to be beneficial to other superconducting device programs at GSFC.
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