Millimeter-Wavelength Dual-Polarized Lens-Absorber-Coupled Ti/Al Kinetic Inductance Detectors

TL;DR

Millimeter-wavelength astronomical instruments require highly sensitive, robust direct detectors that can operate with low-energy photons. The authors develop Ti/Al bilayer absorber-coupled MKIDs with lens-coupled dual-polarized spiral absorbers to achieve broadband, polarization-insensitive sensitivity at 85 GHz. Through simulations, they predict lens-aperture efficiencies above 70% across an octave for a 4×4 spiral-array and demonstrate two devices (a 9-pixel test chip and a 253-pixel large-format demonstrator) with a target NET near 1 mK√Hz at 1 kHz and high yields. They also implement a MKID-shuffling scheme to mitigate cross-talk and discuss pathways toward scalable, large-format mm-wavelength cameras with potential NbTiN-capacitor improvements and antenna-coupled variants.

Abstract

This work presents Ti/Al bi-layer Microwave Kinetic Inductance Detectors (MKIDs) based on lens-coupled spiral absorbers as the quasi-optical coupling mechanism for millimeter-wavelength radiation detection. From simulations, the lens-coupled absorbers provide a 70% lens aperture efficiency in both polarizations over an octave band with a spiral array absorber and over 10% relative bandwidth with a single spiral. We have fabricated and measured two devices with bare Ti/Al MKIDs: a 3x3 cm chip with 9 pixels to characterize the optical response at 85 GHz of the two variations of the absorber; and a large format demonstrator with 253 spiral-array pixels showing potential towards a large format millimeter-wavelength camera. We find a sensitivity of 1 mK/sqrt{Hz} and a detector yield of 95%.

Figures (11)

• Figure 1: Panel (a) shows the absorbing spiral and its dimensions, where the green square is just a visual aid to show the extend of the unit cell. Panel (b) shows a lumped element resonator with a single spiral as inductive element. Panel (c) shows a resonator with a $4\times4$ spiral array as inductive element. Both designs use an interdigitated capacitor. The yellow circle is the footprint of the lens clear aperture on top of each MKID.
• Figure 2: Geometry and aperture efficiency for two orthogonal polarizations for (a) the lens-coupled double-spiral and (b) the lens-coupled double-spiral $4\times4$ array, in both cases analyzed with and without broadband AR coating The absorbers (in red) are at the lower focus of the synthesized elliptical lens. The frustras are not conformal to the lens shape to be able to use current capabilities in laser ablation technology Bueno2022. The orange sheet has a sheet impedance to perfectly absorb a free-space plane and are included in the simulations to limit the input power to the lens clear aperture.
• Figure 3: Broadside plane-wave response of the frustra AR-coating unit cell shown in the inset. Port 1 is above the frustra and port 2 is below, embedded in silicon. The dimensions are given as function of the free-space wavelength at the central frequency of operation, here 92.5 GHz. These dimensions are achievable with current laser ablation technology.
• Figure 4: Photograph of the light-tight Al holder for $3\times3$ cm chips. The different parts of the holder are: (1) the top lid with an aperture stop of $\diameter$3cm, (2) the lid holding a 85GHz $\diameter$2" Fabry-Pérot band-pass filter, (3) the part holding the 9 MKID chip wire-bonded to the grounded coplanar waveguide PCB launchers interfacing with the SMA connectors, and (4) the bottom lid to close the assembly. The bottom lid and the joins of the parts are blackened with a thin stray-light absorbing layer.
• Figure 5: Photograph of the large format demonstrator fabricated on a 4" wafer and hosting 253 MKIDs. The device is placed on an Al holder, where only two of its SMAs connectors are employed to read out the device in a frequency-multiplexed fashion.