Development of Virtually Imaged Phased Array Devices: Enabling efficient velocity-resolved (delta v < 3 km/sec) background limited spectroscopy in the far-infrared (VIPA)

We propose to create Virtually Imaged Phased Array (VIPA) spectrometers with resolving powers >100,000 (delta v<3 km/sec) at wavelengths between 60 and ~300 um in the far-infrared. The primary science application for these VIPA devices is velocity resolved spectroscopy of water, H2, and HD and [OI] line emission from protoplanetary disks, where the velocity information together with Newton’s orbital laws reveal the locations of the emitting gas within the disk at AU or even sub-AU scales. These far-IR lines are uniquely powerful tracers of the building blocks of planetary systems and can only be observed from high altitude balloon or space-based telescopes. Note that for the nearest protoplanetary disk, TW Hya at ~ 54 pc, 1 AU corresponds to 0.019” which would require a 1 km aperture or space interferometry to be spatially resolved in the far-IR. Velocity resolution in the far-infrared can be achieved with a variety of instrument and detector technologies. Due to quantum noise inherent in coherent detection, direct detection systems are superior to coherent (heterodyne) receivers in the far-infrared, so the primary competition includes grating spectrometers (GS), Fourier Transform spectrometers (FTS) and Fabry-Perot Interferometers (FPI). Interference path length requirements mean that GS and FTS spectrometers are prohibitively large for balloon or space-born platforms. The FPI resonant cavity shrinks the required interference path length by factors > 50, so the FPI is a suitable choice. However, FPIs have the disadvantage that they need to be scanned to obtain a spectrum. The VIPA is at first glance similar to a FPI: it has metalized mirrors forming a resonant cavity. However, unlike the FPI is in a pupil of the optical system, the VIPA it is illuminated along a slit on its edge by a cylindrical mirror. The long-slit direction is in the pupil plane while its narrow width is in the image plane. The VIPA is slightly tilted in the image plane direction so that light passes down the resonant cavity. This opens continuous resonant paths for a rainbow of frequencies so that the VIPA has the very powerful advantage over the FPI in that it delivers an instantaneous spectrally multiplexed spectrum: no spectral scanning is required. The VIPA therefore promises greatly improved sensitivity and ease of operation over the more traditional methods. It is compact, its operation does not require any moving parts, and it delivers an instantaneously spectrally multiplexed spectrum of up to ~33 spectral resolution elements. With today's detector technology, a VIPA-based spectrometer promises background limited performance from balloon altitudes and unrivaled sensitivity from space-borne platforms such as a far-IR probe mission. Here we plan to design, build and test two VIPA designs to increase their technical readiness levels for NASA missions. At wavelengths longer than 70 um, cryogenic ultra-pure silicon is very transmissive, so we will work with VIPA’s based on solid silicon resonant cavities. This work is centered at Cornell. At wavelengths shorter than 70 um, silicon is absorptive so we need to build VIPA’s based on free-space cavities. This work is centered at GSFC. Shared design, manufacture and testing strategies are key to program success. This involves electromagnetic design work (GSFC and SRON) to understand and optimize the VIPA operation, manufacturing VIPA parts out of silicon (Cornell and NIST), and the construction of a cryogenic long wavelength test bed based on a tunable far-IR laser at Cornell and a room temperature short wavelength VIPA test bed at GSFC. Activities will involve the PI and Co-I and graduate students at Cornell and personnel at GSFC, NIST, Boulder and SRON. We also have an Inclusion Plan that involves exchange of summer students from Georgia State University to Cornell. Collaborator Professor Misty Bentz will work with the PI to facilitate this effort.

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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