Ultra-Compact, Superconducting Spectrometer-on-a-Chip at Submillimeter Wavelengths

In this paper we will describe a novel, moderate-resolving-power (R~700), ultra-compact spectrograph-on-a-chip for millimeter and submillimeter wavelength which is currently under development. It's very small size, wide spectral bandwidth, and highly multiplexed detector readout will enable construction of powerful multi- beam spectrometers for high-redshift observations. The octave-bandwidth background-limited performance of this spectrometer is comparable to that of a diffraction grating, but in a photo-lithographically developed thin-film package with size ~10 λ x 10 λ. In general, even the most compact grating spectrometers are 2-D structures with size ~ λR/√e. The grating sizes for these spectrometers are prohibitive, approximately 1 meter for R=1000 at λ=1 mm in free space, and ~30 cm in silicon. This fundamental size issue is a key limitation for space based spectrometers for astrophysics applications. On the other hand, our photo-lithographic on-chip spectrometer camera is compact delivering 200 - 500 km/s spectral resolution over and octave bandwidth for every pixel in a telescope's field-of-view. The spectrometer employs a filter bank consisting of planar, lithographed superconducting transmission line resonators. Each mm-wave resonator is weakly coupled to both the feedline and to the inductive portion of a lumped element Microwave Kinetic Inductance Detector (MKID). Incoming mm-wave radiation breaks Cooper pairs in the MKID, modifying its kinetic inductance and resonant frequency, allowing for frequency-multiplexed readout. The design is realized using thin film lithographic structures on a Si wafer, with titanium nitride MKID resonators. In this paper, we will discuss the design and optimization of the MKID detectors for the spectrometer and the measured performance of a laboratory test device. We will also describe the ongoing development of a demonstration instrument which will consist of two 500-channel, R=700 spectrometers, one operating in the 1-mm atmospheric window and the other covering the 650 and 850 micron bands.