Fast, Multi-spectral, Polarization-Sensitive IR Detectors for Solar Astronomy from 5 to 100 Microns

The emission of the Sun at wavelengths from 3 to 100 um is a relatively unexplored region of the solar spectrum, but one that has raised some intriguing questions. We propose to develop a new class of high-cadence imaging detectors in this wavelength range that has novel polarization and spectral properties that can address these questions and characterize the layer of the solar atmosphere that is sensitive to these wavelengths. We propose to push the state-of-the-art in infrared (IR) imaging to a) extend the spectral range of focal plane arrays (FPAs) out to 100 um, b) provide multi-spectral imaging at these wavelengths, c) provide polarization sensitive imaging, and d) do so at high cadence. The detectors discussed in this proposal can provide imaging to further explore these questions and in particular at multiple mid/far-IR wavelengths to measure the spectrum of solar irradiance and flare emission at high cadence. Additionally, they provide a crucial capability missing from all previous mid/far-IR observations: the ability to measure polarization. Measurement of polarization is the key to studying magnetic fields and the ability to do so at mid/far-IR wavelengths routinely would help us to understand the evolution of magnetic fields with height in the lower solar atmosphere and the role of chromospheric magnetic fields in solar flares. The capabilities of the detectors we're developing have never before been available for spectral irradiance measurements or for flares of any class. This proposed project will focus on detecting infrared (IR) electromagnetic (EM) radiation at wavelengths between about 3 um and 100 um using novel dipole nanoantennas developed at UND. These unique devices are capable of multi-spectral, high-cadence, polarization-sensitive detection across a wide range of wavelengths and are uncooled and inexpensive to fabricate. Nanowire antennas resonantly absorb EM waves; the radiation-induced antenna currents heat the center of the antenna where the hot junction of the antenna-coupled nanothermocouple (ACNTC) is located. The resulting temperature difference between the hot and cold junctions of the NTC results in a measurable open-circuit voltage. The antenna plus wires determine the resonant frequency and polarization characteristics. UND has already demonstrated that ACNTCs suspended over cavities offer high gain compared with other nanoantenna configurations. The cavity eliminates most of the thermal losses to the Si (leaving only heat conduction through the lead lines and air) as well as reflects the EM waves and causes positive interference at the antenna. The capabilities of ACNTCs have already been demonstrated: high gain by being suspended over an air-filled cavity; capable of high cadence (beyond 160 kHz) due to their nano-size; polarization sensitivity due to the nature of half-wave dipole antennas; fabricated in thermopiles of multiple antennas whose output scales with the number of antenna components. In this work, antenna performance will be improved, and Sun-as-a-star imaging will be performed to test at various wavelengths. Here, the full sensor will be one pixel at one wavelength. High cadence will also be possible yielding time-dependent flux at selected wavelengths. Laser measurements will be used to develop imagers in the 60 to 100 um range in preparation for later space missions. Multi-polarization and multi-spectral FPAs will be fabricated and tested. As proof of concept, the proposed FPAs will provide a resolution of up to 16 x 16 pixels for one imaging wavelength or fewer pixels split amongst various wavelengths and/or polarization angles. The goals are to improve the sensor fabrication process; improve the sensitivity across the spectrum; perform Sun-as-a-star measurements on single-pixel sensors; integrate multiple pixels into FPAs; demonstrate multi-spectral and multi-polarization imaging at high-cadence, and validate operation using a high-quality amateur telescope.

Support NASA's Heliophysics strategic science objectives to understand the Sun and its interactions with Earth and the solar system, including space weather. This will be achieved by developing/demonstrating instrumentation technology necessary to address the following science goals: Explore the physical processes in the space environment from the Sun to the Earth and throughout the solar system; Advance our understanding of the connections that link the Sun, the Earth, planetary space environments, and the outer reaches of our solar system; Develop the knowledge and capability to detect and predict extreme conditions in space to protect life and society and to safeguard human and robotic explorers beyond Earth.

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