Hyper-efficient superconducting filterbanks with impedance-defined spectral resolution for millimeter-wave spectroscopy

Abstract

We present a high-efficiency, high resolution on-chip filterbank spectrometer designed for line intensity mapping and broadband wave-like dark matter searches. This spectrometer maximizes sensitivity to the faint, aggregate cosmic signal for line intensity mapping while providing redshift precision necessary to resolve the clustering of large-scale structure. For broadband dark matter searches, it partitions broadband signals into narrow frequency bins that directly correspond to dark matter masses, allowing signal frequency to be mapped with high precision to the particle's mass. Existing superconducting filterbank architectures used by the mm-wave community are limited by a 50\% inherent efficiency limit and are highly sensitive to thin-film dielectric loss. The design presented in this paper addresses these bottlenecks by eliminating the termination resistor and employing a niobium-on-silicon coplanar waveguide resonant structures for the filterbanks. Sonnet electromagnetic simulations of a 10-channel device around 90 GHz demonstrates >74\% per channel efficiency and a resolving power of $R=1211\pm105$. Sensitivity analyses confirm that the design is robust against typical fabrication uncertainties with the exception of dielectric thickness, providing a scalable technology solution for the next generation of millimeter-wave spectroscopic experiments.

Figures (7)

• Figure 1: (a) A design schematic of the filterbank spectrometer and (b) a CAD drawing of two adjacent filterbanks, with light purple representing niobium strips and olive representing exposed silicon substrate between the Nb signal line and ground plane of the coplanar waveguide. This design is inspired by the cochlear signal partitioning in the human ear.
• Figure 2: Circuit model and resulting filter efficiency using Quite Universal Circuit Simulator (QUCS) for (a) a representative circuit of existing technologies with a termination resistor and (b) the circuit principle employed for this technology without a termination resistor. While (a) shows that power is split evenly between the filter and termination resistor at the resonant frequency with $|S_{21}|^2$ capped at 0.5, (b) shows that all of the power is transmitted through the filter with $|S_{21}|^2$ = 1. Port impedances are set for maximal power transmitted to Port 2.
• Figure 3: Impedance ratio as a function of dielectric thickness, assuming a SiNx dielectric interlayer ($\epsilon_r = 8.6$). Tab. \ref{['tab:sim_sizes']} shows the parameters used in these $Z_0$ calculations. $Z_0^\mathrm{MS}$ decreases with SiNx thickness and therefore the ratio $Z_0^\mathrm{CPW}/Z_0^\mathrm{MS}$. Because the resolving power of these filterbanks scales with this ratio, decreasing the SiNx thickness leads to higher $R$.
• Figure 4: Cross-sectional illustration of the (a) MS and (b) CPW architectures utilized in this filterbank design. Labels correspond to the design parameters and material layers detailed in Tab. \ref{['tab:sim_sizes']}.
• Figure 5: Fractional transmitted power (solid) and reflected (dashed) of a 10 channel filterbank design for (a) realistic and (b) lossless dielectric layers, simulated using Sonnet simulation environment. The individual channels of the realistic design show $\eta>0.74$ and $R=1211\pm105$.