Anti-reflection coating with mullite and Duroid for large-diameter cryogenic sapphire and alumina optics.

We developed a broadband two-layer anti-reflection (AR) coating for use on a sapphire half-wave plate (HWP) and an alumina infrared (IR) filter for the cosmic microwave background (CMB) polarimetry. Measuring the faint CMB B-mode signals requires maximizing the number of photons reaching the detectors and minimizing spurious polarization due to reflection with an off-axis incident angle. Sapphire and alumina have high refractive indices of 3.1 and are highly reflective without an AR coating. This paper presents the design, fabrication, quality control, and measured performance of an AR coating using thermally sprayed mullite and Duroid 5880LZ. This technology enables large optical elements with diameters of 600 mm. We also present a thermography-based nondestructive quality control technique, which is key to assuring good adhesion and preventing delamination when thermal cycling. We demonstrate the average reflectance of about 2.6% (0.9%) for two observing bands centered at 90/150 (220/280) GHz. At room temperature, the average transmittance of a 105 mm square test sample at 220/280 GHz is 83%, and it will increase to 90% at 100 K, attributed to reduced absorption losses. Therefore, our developed layering technique has proved effective for 220/280 GHz applications, particularly in addressing dielectric loss concerns. This AR coating technology has been deployed in the cryogenic HWP and IR filters of the Simons Array and the Simons observatory experiments and applies to future experiments such as CMB-S4.

Broadband plasma spray anti-reflection coating technology for millimeter-wave astrophysics.

We present a broadband plasma spray anti-reflection (AR) coating technology for millimeter-wave astrophysics experiments with large-format, cryogenic optics. By plasma spraying alumina- and silica-based powders, we have produced coatings of tunable index of refraction and thickness, low loss, and coefficient of thermal expansion matched to alumina substrates. We demonstrate two-layer AR coatings on alumina with reflection below 5% over 82% and 69% fractional bandwidths for 90/150 and 220/280 GHz passband designs, respectively, and band-averaged absorption loss reduced to ∼1% at 100 K for both AR coatings. We describe the design, tolerance, fabrication process, and optical measurements of these AR coatings.

The Simons Observatory: modeling optical systematics in the Large Aperture Telescope.

We present geometrical and physical optics simulation results for the Simons Observatory Large Aperture Telescope. This work was developed as part of the general design process for the telescope, allowing us to evaluate the impact of various design choices on performance metrics and potential systematic effects. The primary goal of the simulations was to evaluate the final design of the reflectors and the cold optics that are now being built. We describe nonsequential ray tracing used to inform the design of the cold optics, including absorbers internal to each optics tube. We discuss ray tracing simulations of the telescope structure that allow us to determine geometries that minimize detector loading and mitigate spurious near-field effects that have not been resolved by the internal baffling. We also describe physical optics simulations, performed over a range of frequencies and field locations, that produce estimates of monochromatic far-field beam patterns, which in turn are used to gauge general optical performance. Finally, we describe simulations that shed light on beam sidelobes from panel gap diffraction.

Cryogenic infrared filter made of alumina for use at millimeter wavelength.

We propose a high-thermal-conductivity infrared filter using alumina for millimeter-wave detection systems. We constructed a prototype two-layer antireflection-coated alumina filter with a diameter of 100 mm and a thickness of 2 mm and characterized its thermal and optical properties. The transmittance of this filter at 95 and 150 GHz is 97% and 95%, respectively, while the estimated 3 dB cut-off frequency is at 450 GHz. The high thermal conductivity of alumina minimizes thermal gradients. We measure a differential temperature of only 0.21 K between the center and the edge of the filter when it is mounted on a thermal anchor of 77 K. We also constructed a thermal model based on the prototype filter and analyzed the scalability of the filter diameter. We conclude that the temperature increase at the center of the alumina IR filter is less than 6 K, even with a large diameter of 500 mm, when the temperature at the edge of the filter is 50 K. This is suitable for an application to a large-throughput next-generation cosmic-microwave-background polarization experiment such as POLARBEAR-2.

Understanding the Phase of Responsivity and Noise Sources in Frequency-Domain Multiplexed Readout of Transition Edge Sensor Bolometers.

Cosmic microwave background (CMB) experiments have deployed focal planes with O ( 10 4 ) transition edge sensor (TES) bolometers cooled to sub-Kelvin temperatures by multiplexing the readout of many TES channels onto a single pair of wires. Digital Frequency-domain Multiplexing (DfMux) is a multiplexing technique used in many CMB polarization experiments, such as the Simons Array, SPT-3 G, and EBEX. The DfMux system studied here uses LC filters with resonant frequencies ranging from 1.5 to 4.5 MHz connected to an array of TESs. Each detector has an amplitude-modulated carrier tone at the resonant frequency of its accompanying LC resonator. The signal is recovered via quadrature demodulation where the in-phase (I) component of the demodulated current is in phase with the complex admittance of the circuit and the quadrature (Q) component is orthogonal to I. Observed excess current noise in the Q component is consistent with fluctuations in the resonant frequency. This noise has been shown to be non-orthogonal to the phase of the detector's responsivity. We present a detailed analysis of the phase of responsivity of the TES and noise sources in our DfMux readout system. Further, we investigate how modifications to the TES operating resistance and bias frequency can affect the phase of noise relative to the phase of the detector responsivity, using data from Simons Array to evaluate our predictions. We find that both the phase of responsivity and phase of noise are functions of the two tuning parameters, which can be purposefully selected to maximize signal-to-noise (SNR) ratio.

Epoxy-based broadband antireflection coating for millimeter-wave optics.

We have developed epoxy-based, broadband antireflection coatings for millimeter-wave astrophysics experiments with cryogenic optics. By using multiple-layer coatings where each layer steps in dielectric constant, we achieved low reflection over a wide bandwidth. We suppressed the reflection from an alumina disk to 10% over fractional bandwidths of 92% and 104% using two-layer and three-layer coatings, respectively. The dielectric constants of epoxies were tuned between 2.06 and 7.44 by mixing three types of epoxy and doping with strontium titanate powder required for the high dielectric mixtures. At 140 K, the band-integrated absorption loss in the coatings was suppressed to less than 1% for the two-layer coating, and below 10% for the three-layer coating.

Successful same day discharge after immediate post-mastectomy alloplastic breast reconstruction: A single tertiary centre retrospective audit.

BACKGROUND: Immediate Post-Mastectomy Alloplastic Breast Reconstruction (IPMABR) traditionally requires a post-operative overnight stay. Recent initiatives have identified same day discharge as a safe option. METHODS: A retrospective audit of all cases at a tertiary breast cancer centre was performed. Patients received surgery at a day care facility or regional hospital (RH). Unplanned admission was defined as any patient unable to be discharged or any patient returning to the emergency room within the first 24 h. Planned admission cases had a history of BMI >40, obstructive sleep apnoea or previous anaesthetic complications. Data were collected on planned same day discharge, unplanned admission and planned admission cases. Factors differentiating the groups were identified and variables predicting unplanned admission were determined. RESULTS: A total of 785 patients received IPMABR over a 5-year period of which 743 had satisfactory data sets for review. Greater than 96% of patients receiving care at the day care facility were successfully discharged. The success rate for same day discharge at the RH was 65%. We determined that the greatest variables determining successful planned discharge were shorter surgical time (67 min; SD 6 min; p<.01), shorter PACU time (130 min; SD 21 min; p<.01) and surgical institution (p<.01). This difference between institutions was significant when all other variables (age, co-morbidities, unilateral/bilateral and BMI) were controlled, indicating a strong institutional bias. There was no difference between groups in complication rates (infection, dehiscence, seroma and haematoma). CONCLUSION: Same day discharge following IPMABR is safe and greatly reduces resource use. It is imperative that members of the perioperative team understand the validity and benefits of the programme to ensure success and reduce unplanned admissions.

Numerical optimization of integrating cavities for diffraction-limited millimeter-wave bolometer arrays.

Far-infrared to millimeter-wave bolometers designed to make astronomical observations are typically encased in integrating cavities at the termination of feedhorns or Winston cones. This photometer combination maximizes absorption of radiation, enables the absorber area to be minimized, and controls the directivity of absorption, thereby reducing susceptibility to stray light. In the next decade, arrays of hundreds of silicon nitride micromesh bolometers with planar architectures will be used in ground-based, suborbital, and orbital platforms for astronomy. The optimization of integrating cavity designs is required for achieving the highest possible sensitivity for these arrays. We report numerical simulations of the electromagnetic fields in integrating cavities with an infinite plane-parallel geometry formed by a solid reflecting backshort and the back surface of a feedhorn array block. Performance of this architecture for the bolometer array camera (Bolocam) for cosmology at a frequency of 214 GHz is investigated. We explore the sensitivity of absorption efficiency to absorber impedance and backshort location and the magnitude of leakage from cavities. The simulations are compared with experimental data from a room-temperature scale model and with the performance of Bolocam at a temperature of 300 mK. The main results of the simulations for Bolocam-type cavities are that (1) monochromatic absorptions as high as 95% are achievable with <1% cross talk between neighboring cavities, (2) the optimum absorber impedances are 400 ohms/sq, but with a broad maximum from approximately 150 to approximately 700 ohms/sq, and (3) maximum absorption is achieved with absorber diameters > or = 1.5 lambda. Good general agreement between the simulations and the experiments was found.