Modified Titanium TES Architectures Target Loop-Gain Noise in BICEP Array BA4 Receiver

The team etched and deposited variant Ti TES geometries with altered aspect ratios, bilayer thicknesses and normal-metal bars to reduce the dimensionless parameter alpha that governs transition steepness. Time-division SQUID readout tests at multiple bath temperatures quantified loop gain, noise spectra and saturation power, directly comparing each architecture against the baseline devices that exhibited unexpectedly high loop gain in existing BICEP receivers. Results show several geometries lowered alpha by 30-50 percent while preserving required responsivity and Tc uniformity across 100-mm wafers.

High loop gain in prior Ti TES arrays produced out-of-band noise that aliases into the CMB science band under TDM multiplexing, limiting polarization sensitivity at the frequencies where inflationary signals are expected to peak. The new wafer-level engineering therefore addresses a critical systematic that could otherwise degrade BA's ability to tighten constraints on the tensor-to-scalar ratio below current limits. The work also reveals that simple geometric changes outperform earlier attempts that relied solely on bias-power tuning.

A remaining limitation is that all data come from small prototype arrays tested in laboratory cryostats rather than full-scale focal planes integrated with the BA4 optics and atmosphere; on-sky validation during the 2026-2027 austral summer will be required to confirm that the noise reduction survives telescope-level systematics. Full-wafer yield statistics and long-term stability under thermal cycling are still being quantified before the final mask set is frozen.