Development of an On-Chip Integrated Spectrometer for Far-IR Astrophysics

Far-infrared (IR) astronomy is one of the main tools astrophysicists use to understand the universe around us. Such observations probe objects and processes that are often invisible at other wavelengths, such as young stars and their formation; the birth and growth of supermassive black holes; and the nature and properties of dust in the interstellar medium (ISM) in local and distant galaxies. The far-IR hosts a rich and diverse set of atomic and molecular transition lines that are diagnostic of heating and cooling processes that trace the physical state of matter and radiation in the cosmos. Molecular transitions in protoplanetary disks trace the temperature, density, and composition of nascent solar systems outside our own. Observations of far-IR line emission and absorption from atomic gas including [OI], [NII], [CI], and [CII] yield insights into the processes responsible for the formation of stars at all epochs, ranging from clouds within our own galaxy all the way back to the first galaxies. Many other examples exist, which all point to the fact that spectral measurements in the far-IR are a crucial probe of the formation of structure in the cosmos at all distances and times.

At far-IR wavelengths, the Earth’s atmosphere is both absorptive of astrophysical photons, but also emissive, contributing both large Poisson noise and time-varying emission to observations at ground-based sites. The best solution to this problem is to perform measurements in space, but opportunities to do so are rare and resource-constrained. In general, space instruments be designed around the physical limitations of the platform, many of which make standard spectral dispersion technologies challenging to implement. Recent advances in semiconductor fabrication offer the potential for new, integrated devices that use quasi-photonic methods to disperse and sense the light. This kind of technology offers large scalability, ease of manufacture, size, weight, and power envelopes, and performance advantages that would allow us to envision instruments that are able to meet the demands of astrophysics in the next two decades.

Here we propose a short, focussed investigation whose primary objective is the production of an integrated on-chip spectrometer prototype operable at wavelengths between 100 and 200 microns. The spectrometer will be integrated with a butted kinetic inductance device (KID) detector array, integrating the light dispersion and detection on a single compact device. We target a spectral resolution of R>100, and plan to demonstrate 10 bands over the wavelength range. Spectral dispersion will be provided by waveguide etched into the Si coupled to harmonic resonators, similar to existing technologies operating at longer wavelengths. The spectrometer will be designed and fabricated by members of our team at the University of Illinois, and the detectors will be designed and fabricated at the University of Chicago. The Rochester Institute of Technology will build a spectral calibration apparatus, and all three institutions will be involved in the integration and test of the device.

Though this technology has applications across the entire range of astrophysics, to provide a concrete mission context we baseline the requirements of a sounding rocket platform concept that will search for axion-two photon decay in local dark matter halos. This challenging scientific goal will ensure the technology is robust, sensitive, and deployable on a small budget and constrained time scale. By the end of the 18-month investigation, we will deliver a TRL 3 prototype, and will be well-placed to design, fabricate, and fly a 2,000 detector device in the next 5 years. Beyond the reference sounding rocket mission, we expect this technology to have applications to a wide range of NASA strategic interests, including SOFIA, the Origins Space Telescope, possible Probe-Class missions, and elsewhere.

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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