A High Efficiency Superconducting On-chip Filterbank with Directional Filters for Integral Field Units in the Sub-millimeter Regime

Abstract

Integrated superconducting spectrometers are developing to the point that they are enabling integral field units, providing large area spectral mapping capabilities for astronomy in the sub-millimeter band. However, these integral field units are only worthwhile if they have a high efficiency, but to date the efficiency of on-chip filterbanks has been quite poor. Here we demonstrate a filterbank with high efficiency by using directional filters. Using a cryogenic thermal load and a noise measurement in combination with a continuous-wave terahertz source to obtain the spectral response of the filters, we are able to accurately measure the filterbank efficiency, accounting for all quasi-optical elements within our setup. We experimentally obtain an average peak coupling efficiency to the detectors of 75% in a filterbank that sparsely samples between 125 GHz to 220 GHz using filters with a mean loaded quality factor of 19.6. Our results demonstrate that a filterbank with a high efficiency is achievable using directional filters, giving a clear route towards efficient integral field units.

Figures (9)

• Figure 1: a) Fabricated chip (42x14 mm) in holder. b) Optical micrograph of a single directional filter. See \ref{['fig:design_filter']} for a schematic of the filter design. c) A scanning electron microscope (SEM) micrograph of part of the coupler. (without top dielectric) d) A SEM micrograph of a focussed ion-beam (FIB) cross-section of the coupler, showing the stratification of the dielectric layers and the metals. e) Stitched optical micrograph of the fabricated sparse filterbank. The highest frequency filter, which is the first in line from the antenna, is to the left. Note the pairs of wideband-coupled KIDs at the start and end of the filterbank.
• Figure 2: Transmission line circuit of the directional filter (reproduced from martingDirectionalFilterDesign2024a) and its implementation in our device. The KID is connected to port 3 and 4 of the filter. The bottom part of the filter is part of the quarter-wave KID.
• Figure 3: A SONNET simulation of a co-planar microstrip coupler with and without capping dielectric. The horizontal dash-dot and dotted lines indicate the coupling strength required for a certain $Q_\mathrm{l}$ following \ref{['eq:SQc']}. The coupling structure, with the definitions of ports a and b, can be found in \ref{['fig:design_filter']}.
• Figure 4: a) Simulation of the sparse filterbank design using a transmission line model martingDirectionalFilterDesign2024apascuallagunaTerahertzBandPassFilters2021. b) Stacked filter responses with normalized frequency. The dips in the lorentzian-like response at fixed intervals are due to filter suckouts of preceding filters.
• Figure 5: a) Cryogenic blackbody setup with filterstack. b) Total transmission through the filterstack for two different configurations. c) Schematic of the measurement setup, indicating all the parts considered and their efficiencies. See the text for details.