Holey Silicon-based Thermopiles for High-Sensitivity Broadband Thermal Detection

Future planetary missions call for thermal detectors that can measure not only hot but also cold objects with high sensitivity over a wide range of temperature. For instance, ice giant planets such as Uranus and Neptune or icy regolith at the lunar poles may require measurements of cold objects with temperatures below 60 K, in which much of thermal emission takes place at wavelengths greater than 50 μm. However, conventional approaches based on photon detectors are limited to a narrow range of wavelengths and require active cooling for measurements of long-wavelength radiation. To measure thermal emission at wavelengths 50 μm and greater, photon detectors have to be cryogenically cooled below 10 K. On the other hand, thermal detectors based on thermopiles are sensitive to a wide range of wavelengths including far-infrared radiation and do not require active cooling or cryogenic operations. While thermopiles are favorable for measurements of long-wavelength radiation or thermal emission by cold objects, their detectivity is limited by the material’s thermoelectric response and heat losses. Here, we propose to develop a highly-sensitive broadband thermal detector for hot and cold objects in space using a novel thermopile technology based on holey silicon. Holey silicon is a thin membrane of silicon with a microfabricated arrangement of pores that can be optimized to minimize the heat losses and enable breakthrough performance. With holey silicon, the proposed thermal detector is expected to outperform the state-of-arts by significant margins. In terms of the specific detectivity (D*), a representative figure of merit capturing the signal to noise ratio, the holey silicon-based thermopile can be 9 times better than the previously flown instrument (MCS and Diviner) and 4 times better than the state-of-art instrument (PREFIRE) that has not launched yet. Furthermore, the proposed use of silicon for thermopiles instead of bismuth telluride compounds offers significant advantages with ease of fabrication, packaging, and compatibility, which will allow efficient and reliable processing. The outcomes of the project will enable a novel instrument that can measure objects over a wide range of temperature with unprecedentedly high sensitivity and detect even faint thermal signals from temperatures down to 30 K or wavelengths up to 100 μm and enable planetary studies of very cold objects that are not accessible by current instruments. We expect the proposed instrument will lead breakthrough scientific discoveries in the future by allowing a deeper look at the cold objects, and these findings could help scientists discover new information about the ice giants.

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