CCAT: Optical Responsivity, Noise, and Readout Optimization of KIDs for Prime-Cam

TL;DR

This work addresses the challenge of optimizing readout for the large-scale KID focal plane of Prime-Cam under varying atmospheric loading. By lab-testing 280 GHz TiN and Al KID sample pixels with a blackbody cold load, the authors map optical responsivity and noise across optical loading, tone power, and tone placement, enabling accurate calibration with NEP defined as $ ext{NEP}(f)=\frac{\sqrt{S_{xx}(f)}}{R}$ and $R=\frac{\frac{\Delta f}{f_0}}{\Delta P_{\text{opt}}}$. The study finds TiN to exhibit near-linear responsivity while Al is nonlinear, and demonstrates that NEP degrades with detuning and reduced readout power, with small optical-load drifts capable of doubling the noise. These results inform commissioning, retuning cadence, and calibration procedures for Prime-Cam and motivate design refinements to ensure robust performance across multiple bands ahead of first light in 2026. The methodology and findings provide a concrete path to extending characterization to additional frequency bands and to maintaining optimal sensitivity in large KID arrays.

Abstract

The Prime-Cam instrument of the Fred Young Submillimeter Telescope (FYST) at the CCAT Observatory will conduct sensitive millimeter to submillimeter surveys for a range of astrophysical and cosmological sciences. Prime-Cam will use kinetic inductance detectors (KIDs) sensitive to multiple frequency bands spanning 280--850 GHz. With over 100,000 sensors under development, these KID arrays will soon form the largest submillimeter focal plane ever built. With fixed microwave tones probing amplitude and phase modulations in the KIDs due to incoming radiation, challenges arise in determining the optimal readout settings, especially under varying atmospheric loading. Realizing the science goals of FYST requires operating the detectors at optimal performance and determining accurate responsivities, which depend on readout tone placement and power. To address these challenges, we present laboratory measurements of sample pixels from the 280 GHz TiN and Al arrays using a blackbody cold load to simulate observing conditions. These measurements probe detector responsivity and noise across varying optical loading, tone power, and tone placement, providing the foundation to guide in situ calibration and operation of the $>$100,000 KIDs. We characterize detector sensitivity via the Noise Equivalent Power (NEP) as a function of readout tone power and placement, and measure the impact of detuning due to varying optical power on the NEP. Our test setup and methodology will inform the commissioning of Prime-Cam, in situ detector calibration procedures, the cadence of probe tone resetting, and potential design refinements for future arrays, supporting FYST's planned first light in 2026.

Figures (4)

• Figure 1: Experimental setup. (a) Three TiN (280 GHz) sample pixels. The central cross and plus shaped features are inductors, which double as absorbers, while the rectangular structures are capacitors. Together, these elements form the LC resonant circuit of each detector. Al detectors are not shown but have a similar LC structure. (b) TiN sample array mounted on the 100 mK stage inside the dilution refrigerator (DR). (c) Complete detector and cold load setup inside the DR. (d) Cold load assembly. (e) Bottom of the cold load including temperature sensors and heaters controlled by a Lakeshore 325.
• Figure 2: Example $|S_{21}|$ magnitude responses of KID resonators under multiple optical loading levels. The left plot shows a resonator from the TiN sample array at a readout power of $1$ dB from bifurcation at 4K, and the right plot shows a resonator from an Al array at a readout power of $1$ dB from bifurcation at 4K. Red stars mark the tone placements used during data acquisition. Increasing optical power shifts the resonator frequency to lower values and decreases the quality factor.
• Figure 3: Top: Fractional frequency shift ($\Delta f/f_0$) as a function of optical loading for a representative set of 280 GHz KIDs, used to extract optical responsivity. Readout powers are relative to bifurcation levels for resonators. TiN array bifuraction occurs at $+4$ dB at 4 K and Al array bifuraction occurs at $+1$ dB at 4 K. Middle Left: Noise Equivalent Power (NEP) for a TiN KID with the readout power at $1$ dB from bifurcation at 4K. Middle Right: NEP for an Al KID with $1$ dB from bifuraction at 4K. Bottom: NEP averaged at 100 Hz over 20 Hz and plotted versus estimated optical power for both TiN and Al 280 GHz detectors. Multiple tone power levels are shown, showing NEP degradations up to 200% due to suboptimal readout power settings (10 dB off). At lower optical powers, large readout powers bifurcate the resonators, leading to artificially large NEPs.
• Figure 4: Top: $|S_{21}|$ and phase response of a representative Al KID resonator showing tone placement (black dot) at 9 K and resulting detuning as the cold load temperature is varied from 8 K to 10 K, corresponding to the points located at the varying optical powers. Middle (TiN KID) and Bottom (Al KID): NEP (averaged around 100 Hz) as a function of estimated optical power during drift, for multiple tone power levels. The noise level increases by up to a factor of 2 with $\sim$0.2 pW optical loading changes.