Development of compact Superconducting On-chip Fourier Transform Spectrometers arrays for sub-millimeter astronomy and cosmology.

Utilizing tunable superconducting mm-wave planar circuits, we plan to fabricate and characterize on-chip Fourier Transform Spectrometers (FTSs) for mm and submm wavelength astronomy and cosmology. This novel technique uses the non-linear current dependent inductance of high kinetic inductance (Lk) thin-film materials to replace meter-scale mechanical FTSs and grating spectrometers with cm-scale devices. Such devices can be easily arrayed into wide-band imaging/spectral integral field unit (IFU) focal planes, enabling a new class of powerful orbital and sub-orbital instruments for measuring spectral distortions of the cosmic microwave background (CMB), studying the epoch of reionization and star formation histories. Submm telescopes, with a limited number of detectors and readout channels, must balance the number of spatial-pixels and the number of simultaneous observing bands. State-of-the-art wide-band spectrometers with moderate/high resolution employ a hand-full of pixels. Broadband cameras for CMB and submm-wavelength astronomy include thousands of pixels but a limited number of bands. Additional bands improve foreground sensitivity in CMB cameras, and a reduction in detector count per mm-wavelength channel enable spectrometers with a large number of spatial pixels. Thus a technology that enables instruments to increase the spatial or frequency-resolution of each detector channel would have immediate benefit for CMB instruments, submm-wavelength cameras, and spectrometers used for both pointed observations and intensity mapping applications. We intend to develop a novel IFU with broadband spectroscopy per pixel; each pixel being an on-chip FTSs. Our scheme is entirely solid-state, avoiding the issues which complicate mechanical FTSs whilst having the continuum broadband scanning advantage of an FTS on individual pixels. These devices drastically reduces the size of the FTS by using high kinetic inductance (L_k) superconductors as the phase delay path. We use non-linear L_k response to DC currents to dynamically tune the inductance and thus phase delay with current biasing. Tiling many such FTS-pixels on a focal plane a broadband spectrometric imager with good spatial resolution can be realized. Our high Lk material is Titanium Nitride deposited in nanometer thick layers using Atomic Layer Deposition. We have successfully tested resonators with thicknesses as small as 3nm and Lk as high as 8 nH/square. We have designed and operated a series of dark prototype devices including a DC current-biasing mechanism and transmission lines to characterize the phase-delay and phase-noise of these films. These results show that such compact and flexible FTSs are indeed feasible. This proposal will support device fabrication at the Pritzker Nanofabrication Facility at U. Chicago, support for the co-I who will lead the electromagnetic design, and a postdoc or student effort to complete the design, fabrication, and testing. Device testing will make use of existing cryogenic testbeds. At the conclusion of this two year program, we will produce a fully characterized prototype detector array consisting of several antenna-coupled pixels, each measuring continuous broadband mm and submm wavelength spectra.

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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