Virtually Image Phased Array (VIPA): demonstration of the next generation direct detection spectrometer for velocity resolved spectroscopy in the far-infrared

We have created and demonstrated a Virtually Imaged Phased Array (VIPA) device for velocity resolved spectroscopy in the far-IR and will present the first cryogenic (LHe) measurements of the spectral profile of a prototype VIPA at 115.7 µm. A VIPA is a compact spectral filter without moving parts that consists of a resonating cavity that generates angular dispersion due to constructive interference, and that can deliver an instantaneous spectrum with a resolving power of 100,000 or more. Coupled with superconducting direct detection detectors, the VIPA promises unsurpassed sensitivity at velocity resolutions > 3 km/s, making it a prime choice for spectroscopic instruments to observe e.g. protoplanetary disks to trace the gaseous building blocks out of which planets form. Due to their compactness and absence of moving parts, VIPAs are optimal for balloon and space-borne astronomical instruments like the NASA Pioneer Mission POEMM and the proposed NASA Probe Mission FIRSST. The prototype we have measured was made of a 3 × 5 × 1 cm block of high-purity float-zone silicon. The entrance and exit sides of the VIPA are highly parallel, and the entrance side was fully metallized (gold-coated) except for a narrow entrance slit resulting in a reflectivity of 100%, while the exit side was metallized with an inductive mesh resulting in a reflectivity of about 88%. The measurement was done using a custom testbed that included separate cryostats for a cryogenically cooled THz Quantum Cascade Laser (QCL) and the VIPA, as well as a warm pyroelectric detector. The QCL was tuned to a number of different frequencies near 2.59 THz and the detector was scanned along the dispersion direction. Our VIPA prototype achieved the designed resolving power of 15,000 and the spectral profile matched extremely well with the expectations from our simulations. We are now building a full testbed to straightforwardly measure and characterize VIPAs and other GHz/THz filters with resolving powers of order 100,000 using tuneable THz lasers.