Measurements of dichroic bow-tie antenna arrays with integrated cold-electron bolometers using YBCO oscillators.

We consider properties of dichroic antenna arrays on a silicon substrate with integrated cold-electron bolometers to detect radiation at frequencies of 210 and 240 GHz. This frequency range is widely used in cosmic microwave background experiments in space, balloon, and ground-based missions such as BICEP Array, LSPE, LiteBIRD, QUBIC, Simons Observatory, and AliCPT. As a direct radiation detector, we use cold-electron bolometers, which have high sensitivity and a wide operating frequency range, as well as immunity to spurious cosmic rays. Their other advantages are the compact size of the order of a few micrometers and the effect of direct electron cooling, which can improve sensitivity in typical closed-loop cycle 3He cryostats for space applications. We study a novel concept of cold-electron bolometers with two SIN tunnel junctions and one SN contact. The amplitude-frequency characteristics measured with YBCO Josephson Junction oscillators show narrow peaks at 205 GHz for the 210 GHz array and at 225 GHz for the 240 GHz array; the separation of these two frequency bands is clearly visible. The noise equivalent power level at an operating point in the current bias mode is 5 × 10-16 W/√Hz.

A simple method to measure the temperature and levitation height of devices rotating at cryogenic temperatures.

We describe a simple system to measure the temperature and levitation height of levitating cryogenic devices in rotation. Devices of this kind are the half-wave-plates rotating on superconducting magnetic bearings used in several cryogenic polarimeters for the cosmic microwave background. The temperature measurement is important to monitor the radiative background and potential systematic effects in the polarimeter. In our implementation, the temperature sensor is a thermistor, physically mounted on the rotating device. The sensor is biased with an AC current, which is transferred from the stationary electronics to the rotating device via capacitive coupling. The levitation height sensor is a network of capacitors, similar to the one used for the capacitive coupling of the thermistor. We describe the optimization of the readout system and its performance, which has been tested on a room-temperature prototype. We show that this system reaches an accuracy better than 3% for the measurement of the thermistor resistance and an accuracy of ∼10 µm for the measurement of its levitation height.

A clamp and release system for superconducting magnetic bearings.

We describe a simple and reliable clamp/release system for the rotor of large superconducting magnetic bearings. The system was developed for a cryogenic half-wave plate spinner for cosmic microwave background polarimetry but can have other applications. The main features of this system are its large rotor mass compliance (∼10 kg); zero power dissipation while holding the rotor and zero power dissipation when the rotor is released; fast (∼40 ms), balanced release and clamp actions, and low power dissipation (∼30 J) on the cold stage during each operation; low cost; and high reliability over a very large number of operation cycles.

Multi-octave metamaterial reflective half-wave plate for millimeter and sub-millimeter wave applications.

The quasi-optical modulation of linear polarization at millimeter and sub-millimeter wavelengths can be achieved by using rotating half-wave plates (HWPs) in front of polarization-sensitive detectors. Large operational bandwidths are required when the same device is meant to work simultaneously across different frequency bands. Previous realizations of half-wave plates, ranging from birefringent multi-plates to mesh-based devices, have achieved bandwidths of the order of 100%. Here we present the design and experimental characterization of a reflective HWP able to work across bandwidths of the order of 150%. The working principle of the novel device is completely different from any previous realization, and it is based on the different phase-shift experienced by two orthogonal polarizations reflecting, respectively, off an electric conductor and an artificial magnetic conductor.

Common-mode rejection in Martin-Puplett spectrometers for astronomical observations at millimeter wavelengths.

The Martin-Puplett interferometer (MPI) is a differential Fourier transform spectrometer that measures the difference between spectral brightness at two input ports. This unique feature makes the MPI an optimal zero instrument, able to detect small brightness gradients embedded in a large common background. In this paper, we experimentally investigate the common-mode rejection achievable in the MPI at millimeter wavelengths, and discuss the use of the instrument to measure the spectrum of cosmic microwave background anisotropy.