Large Angle Optical Access in a Sub-Kelvin Cryostat

J Low Temp Phys https://doi.org/10.1007/s10909-018-1940-1 Large Angle Optical Access in a Sub-Kelvin Cryostat S. Hähnle1 · J. Bueno1 · R. Huiting1 · S. J. C. Yates1 · J. J. A. Baselmans1 Received: 3 November 2017 / Accepted: 30 April 2018 © The Author(s) 2018 Abstract The development of lens-antenna-coupled aluminum-based microwave kinetic inductance detectors (MKIDs) and on-chip spectrometers needs a dedicated cryogenic setup to measure the beam patterns of the lens-antenna system over a large angular throughput and broad frequency range. This requires a careful design since the MKID has to be cooled to temperatures below 300 mK to operate effectively. We developed such a cryostat with a large opening angle θ = ± 37.8◦ and an optical access with a low-pass edge at 950 GHz. The system is based upon a commercial pulse tube cooled 3 K system with a 4 He–3 He sorption cooler to allow base temperatures below 300 mK. A careful study of the spectral and geometric throughput was performed to minimize thermal loading on the cold stage, allowing a base temperature of 265 mK. Radio-transparent multi-layer-insulation was employed as a recent development in filter technology to efficiently block near-infrared radiation. Keywords Cryostat · Lens-antenna · Kinetic inductance detectors · Cryogenic optics 1 Introduction Microwave kinetic inductance detectors (MKIDs) [1] become an increasingly attractive option for large-scale imaging instruments [2]. An interesting alternative to lumped element MKIDs [3] are lens-antenna-coupled MKIDs [4], which couple radiation using a coherent beam formed by the lens-antenna system. The advantage is that they are sensitive to radiation over a limited angular throughput given by the lens-antenna design. Therefore, they reject large angle radiation, which allows the use of a higher B S. Hähnle 1 SRON Netherlands Institute for Space Research, 3584 CA Utrecht, Netherlands 123 J Low Temp Phys temperature optics and Lyot stop. This results in cryogenically simple camera designs [5]. Imaging arrays of antenna-coupled MKIDs have fast beams to reduce the chip area for a given field of view: fast optics results in a spatially small airy pattern, smaller pixels and smaller arrays. The beam width of these antennas at the −10 dB taper is typically in the order of ± 20◦ , which makes measurements of the antennas full beam pattern, including side lobes, challenging to implement. However, it is critical to measure the beam shape to fully understand the detector performance [6]. In this paper, we present a cryostat that is designed to enable large angular throughput beam pattern measurements for antenna-coupled MKIDs. It fulfills the following requirements: – Sub-millimeter (sub-mm) wave access from the laboratory through a window with a large opening angle to the detector. – No reimaging optics in order to measure the unperturbed beam pattern. – Optical access to the detector over a spectral passband of up to 1 THZ. – Fast cooldown speed of the cryostat and easy assembly process motivated by the fast turnaround speed (1–2 days) necessary for an efficient iterative antenna development. – The cold stage temperature of less than 270 mK for operation of the detectors. 2 System Overview A cross section of the optical access for the cryostat is shown in Fig. 1. The cryogenic system consists of standard commercially available components. A pulse tube cooled cryostat (BlueFors Cryogenics) provides 0.9 W cooling power at 4.2 K, where a sorption cooler (CRC-7B-002, Chase Research Cryogenics) is mounted that can reach temperatures down to 240 mK. The sorption cooler is a two-stage, single shot system consisting of a 3 He cold head as the mounting point for experiments and a 4 He buffer head. The windows are in a cone-like configuration, providing a half opening angle of θ = 37.8◦ to the center of a detector chip mounted on the cold stage of the sorption cooler. The design of the geometry of these apertures is critical to the cooler performance and will be discussed in detail in the next section. Access to the cold stage is possible by removing only the vacuum can and heat shields, which allows for fast and efficient operation of the system. The detector assembly mounted on the cold stage can hold different chip dimensions although only an area of (10 × 10) mm2 is optically accessibly. Connection to room temperature electronics to read out the MKIDs is provided by means of a single pair of semirigid coax cables. From room temperature to the 3 K stage, 2.19-mm-diameter CuNi coax cables are used, while 0.86-mm-diameter and 15-mm-long NbTi cables (COAX CO., LTD.) provide a lossless connection from 3 K to the detector on the cold stage. The thermal load on the cold stage due to the coax cables is only 16 nW, which is negligible compared to radiative loading from the optical access. The readout signal is amplified at the 3 K stage using a commercial low-noise amplifier [7]. 123 J Low Temp Phys Fig. 1 Cross section of the cryostat’s optical access, showing all windows including the filters. A 1-cmthick polyethylene window in the vacuum can is air-tight but transparent in the sub-mm regime. In addition, a single thermal shader is mounted directly behind the vacuum window. The apertures in the 50 K and 3 K radiation shields are made of gold-plated copper and serve as mounting point for the filter stack: 7 layers of RT-MLI on the outer side of the 50 K window, as well as reflective metal mesh filters on both the 50 K and 3 K window. The latter are oriented at angles with respect to each other and the detector to avoid standing waves. Both the 1 K radiation shield and the detector assembly are located in tight proximity to the 3 K window, with the 1 K shield mounted on the 4 He buffer head and the detector assembly directly on the 3 He cold head. The 1 K shield is aluminum and serves as an optional mounting point for additional filter. A cryophy magnetic shield is integrated in the detector assembly (Color figure online) We design an infrared passband from 0 to 950 GHz by a combination of commercial metal mesh filters (QMC Instruments Ltd.), gore tex sheets and a RT-MLI infrared blocking assembly [8]. The combination of the optimized geometry and filter stack allows us to reach a loading on the cold stage of 6 µW, while the total power entering the cryostat window is 17 W. The spectral filtering thus reduces the input power by 40 dB and the geometric baffling by another 30 dB. The performance of the cryostat exceeds the requirements for operation (see Table 1), reaching a base temperature of 265 mK on the 3 He cold head for over 32 h. A full cooldown from room temperature to base temperature takes about 14 h. In the next section, we will discuss the filter design and geometrical design in detail. 123 J Low Temp Phys Table 1 Operating temperatures T and cooling power Pcool for all stages of the cryostat in addition to the calculated radiative and conductive thermal (...truncated)