Hot-electron nanobolometers (nano-HEB) made from superconducting Ti have shown a remarkable sensitivity for detection of submillimeter radiation (NEP = 3×10^-19 W/Hz^1/2 @ λ = 480 µm). This makes them a strong candidate for the future applications in spectral imaging arrays on telescopes with cryogenically cooled mirrors. The application that will benefit the most is a galactic line survey where a moderate resolution (λ/δλ ~ 1000) is required. To take advantage of the ultralow background conditions, a detector with an NEP = 10^-19 – 10^‑20 W/Hz^1/2 is sought in the 25-400 µm waverange. Although the superconducting nano-HEB is very close to meeting this sensitivity goal, more work is needed in order to achieve a large (~ 10³) array of such detectors. The challenge here is the necessity to multiplex a large number of readout amplifiers (SQUIDs) and also to make the system immune to the SQUID back action and the crosstalk.
More recently, we conceptualized a normal-metal nano-HEB where the Johnson noise in the sensor is used as the measure of an increase of the electron temperature due to the absorbed radiation. The noise thermometry readout is heavily dependent on the availability of a broadband and low-noise (quantum noise limited) amplifier for operation at ~ 10 GHz. The theoretical sensitivity limit set by the amplifier is NEP = 3×10^-20 W/Hz^1/2. The big advantage of the normal metal nano-HEB is the absence of any bias lines or SQUIDs and the enormous dynamic range.
I will present a comparison of the array architectures for superconducting and non-superconducting versions of the nano-HEB discussing their pros and cons for detection of both cw radiation and single far-IR photons. The latter mode of operation is desirable for λ < 300 µm where the background photons arrive at a rate ~ 100 s^-1.

This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

Imaging Terahertz applications require large arrays of sensitive Terahertz detectors. Superconducting transition-edge bolometers are among the most sensitive ones. They use low-frequency biasing and SQUID-based readout which makes multiplexing of large arrays difficult and expensive. Moreover, it is important to minimize the number of biasing and readout lines to reduce heat load. A usual solution of the mentioned problems is Frequency-Division Multiplexing (FDM) of an array of high sensitive detectors.
We present a novel detection technique based on microwave bias and readout of an antenna-coupled superconducting Nb nanobolometer operating close to Tc and embedded in a quarter-wave high-Q superconducting resonator. This approach is suitable for FDM readout of large arrays using single broadband low-noise HEMT amplifiers. The high Q-factor of superconducting resonators ensures that the nanobolometer is well isolated from external electrical noise and interference at all frequencies except close to resonance. Since the operation temperature is close to the transition temperature of the nanobolometer, we called this detector RFTES. We have fabricated and studied experimentally antenna-coupled superconducting nanobolometers made of ultra-thin niobium. To bias the superconducting Nb nanobolometer, the RF power at resonator frequency was applied via a transmission line which is weakly coupled to the resonator. The THz antenna of RFTES was placed in the focus of an immersion lens inside a He⁴ cryostat equipped with a blackbody radiation source and a semiconductor RF amplifer. We have measured and analyzed the optical response of the RFTES to THz radiation. Operation principles, theoretical estimations of the noise and details of the experimental measurements of the optical sensitivity and noise will be presented and discussed.

The work supported by grant 12-02-01352-а from RFBR, Increase Competitiveness Program of NUST «MISiS» (К2-2014-025) and contract 11.G34.31.0062 from the Ministry of Education and Science of the Russian Federation; grants 05K13VK4, 13N12025 from German Federal Ministry of Education and Research (BMBF), project 284456 from European Comission

Hot electron bolometer mixers are commonly used for high spectral resolution radio-astronomy for frequency above 1.2THz. Made from NbN film, HEB mixers offer low noise temperature as well as a gain bandwidth of 3-6GHz, nevertheless the astronomers demand a wider gain bandwidth which cannot be achieved using NbN. The high critical temperature and a picosecond electron-phonon interaction time of MgB2 film are expected to enhance the presently achieved gain bandwidth up to >10GHz. Moreover, MgB2 film could allow HEB mixers to increase the operating temperature to >15-20K where compact cryocoolers are available, opening the possibility for longer life time of space born THz observatories.
We demonstrated a DSB noise temperature as low as 600K (600GHz) for HEB on 10 nm MgB2 films with a Tc of 8.5K. More recent results have shown a DSB noise temperature of 700K and 1400K at 1.63THz and 2.55THz LO frequencies. For HEB with higher Tc (15K) we reported a gain bandwidth as wide as 3.4GHz and a constant noise temperature up to 10.5K in contrast to NbN HEB mixers where the noise temperature raised almost directly with the bath temperature.
Although MgB2 HEB mixers offer already competitive performances in terms of noise and gain bandwidth, however the latter can be further enhanced if high quality MgB2 films are used and if the IF HEB impedance is understood. In this work a systematic investigation of the MgB2 HEB IF output impedance for several devices at different bias conditions and LO frequencies as well as LO power will be presented. The measured HEB IF impedance will be modeled to extrapolate devices parameters which in the future can be used to estimate the MgB2 HEB mixers performances based on their dc parameters. The device modeling will also help to understand the physical mechanism involved in the MgB2 HEB mixers operation as sensitive THz detectors. We will also discuss in the possibilities to design a broadband matching network to optimize the mixer/LNA system.

The work at Chalmers University is done with a support from European Research Council (project Teramix).

A novel type of Resonant Cold-Electron Bolometer (RCEB) [1] with a “seashell” polarized antenna is proposed for multichroic receiving systems [2]. The slot polarized antennas of different frequencies are arranged in the form of a seashell that gives unique opportunities for independent tuning the antennas and bolometers with internal nanofilters at each frequency.
In this sensor, the frequency selection is realized by internal nanofilter organized by the kinetic inductance of an ultrasmall NbN superconducting nanostrip and the capacitance of a nanoscale SIN (Superconductor-Insulator-Normal) tunnel junction. The kinetic inductance is about 300 times smaller than the geometrical inductance of the same value. This internal resonance acts as a bandpass filter with a bandwidth of 3-50% needed for radioastronomy applications.
The RCEB can be effectively used to create multiband elements that are actual tasks in radioastronomy due to the benefit that comes from its ability to use co-located data, and problems with the dramatic increase of the size of the focal plane. Latest results on implementation of RCEB for COrE space mission will be informed.
Development of this system is done in scope of ESA project on creating new concepts of multifrequency pixels for a COrE space mission [2]. These multifrequency systems could be effectively used in new projects for detecting the primordial gravitational wave background. In particular, some uncertainty in interpretation of BICEP2 results on observation of gravitational waves has been just due to absence of measurements at two frequencies, 150 GHz and 95 GHz, planned originally for this experiment [3].
1. L. S. Kuzmin. “A Resonant Cold-Electron Bolometer With a Kinetic Inductance Nanofilter”,IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 4, pp 314-320, (2014).
2. The ESA Tender ESTEC ITT AO/1-7256/“Next Generation Sub-millimetre Wave Focal Plane Array Coupling Concepts”, February 2013.
3. BICEP2 II: EXPERIMENT AND THREE-YEAR DATA SET. BICEP2 COLLABORATION - P. ADE, et al., arXiv:1403.4302v1 [astro-ph.CO] 17 Mar 2014.

Assessing the absolute neutrino mass scale is one of the major challenges in nowadays particle physics and astrophysics. A powerful tool to directly estimate the effective electron-neutrino mass consists in the calorimetric measurement of the energy released within a nuclear beta decay. The progresses made over the last few decades on low temperature detector technologies have permitted to design experiments with expected sensitivities on the neutrino mass below 1 eV with the calorimetric approach. Despite the remarkable performances in both energy (~ eV at keV) and time resolutions (~ 1 microsecond ) on the single channel, a large number of detectors working in parallel is required to reach a sub-eV sensitivity. Microwave frequency domain readout provides a powerful technique to read out large arrays of low temperature detectors, such as Transition Edge Sensors (TESs) or Microwave Kinetic Inductance Detectors (MKIDs). In this way a multiplex factor of the order of thousands can be achieved, limited only by the bandwidth of the available commercial fast digitizers. The microwave multiplexing system will be used to read out the TES array of the HOLMES experiment, made of 1000 163Ho implanted microcalorimeters. HOLMES is a new experiment that aims to measure the electron neutrino mass by means of the electron capture (EC) decay of 163Ho with an expected sensitivity below the eV. The measurements on the first HOLMES detectors with the microwave frequency readout are planned for mid 2015.
In this contribution the first measurements of the HOLMES TES detectors acquired with the microwave frequency multiplex will be presented. In parallel, the latest progresses made with MKID detectors designed in Milano will be reported as well.

On behalf of the HOLMES collaboration.

ATHENA is the next largest space Advanced Telescope for High ENergy Astrophysics that will operate Low Temperature Detectors at about 0.05K with an excellent spectral performance in the soft X-rays. In order to exploit the full sensitivity for faint and diffuse x-ray source it is necessary to minimize the background signals generated by the cosmic rays, i.e., high energy protons and light nuclei, that leave a sizable amounts of energy in the same spectral window of the astrophysics signals. A superconducting large area phonon mediated detector for GeV protons an nucleai will be placed at less than 1 millimeter from the X-ray at 0.05K and will act as anti-coincidence to disentangle the fake signal of cosmics. The detector is made of about 5 cm² micromachined silicon in which the fast a-thermal and the slow diffusive phonons coming from the primary event trasfer the released energy in a uniformly distribute array of TESs. The TESs are made of Iridium-gold films that are grown by pulsed laser deposition onto the silicon surface. Here we present the full caractherization of the last demostration models we have built, and in particular we shown that the signal is proportional to the TES area coverage and the effect of the superconducting aluminum phonon collectors that transfer to the TES the hot quasiparticles produced by the incoming a-thermal phonons.

This work has been done under the grant n. 2014-045-R.O "Progetto premiale 2012 -Rivelatori Criogenici Superconduttivi per Astrofisica Spaziale" of the Italian Space Agency (ASI).

Superconductor-normal metal-superconductor (SNS) junction is an interesting system in which electronic state of normal metal alters superconducting transport through the junction. Semiconductor has been mainly used as N because its electronic state can be easily controlled by non-destructive means such as gate voltage, magnetic field, and electro-magnetic wave irradiation. Recently graphene has come into use as N due to its peculiar electronic properties [1]. Here we report optical effect on proximity-induced superconducting state in graphene. Previously reported optical effects on semiconductor-based Josephson junction (JJ) have been explained in terms of modulation of carrier density (n_s) in semiconductor [2]. In graphene-based JJ, however, optically created non-equilibrium electronic state in graphene plays an important role to the transport via proximity-induced superconducting state.
Sample studied is a superconductor-monolayer graphene-superconductor junction consisting of Ti/Al as superconducting electrodes. Supercurrent flowing through the graphene channel is clearly observed. As has been common in graphene-based JJs, the critical current (I_c) can be altered through the modulation of n_s by the gate voltage. When the junction is illuminated by continuous-wave near infrared light at the wavelength of 1.31 µm, I_c decreases monotonically with increasing illumination power (P) although normal state resistance keeps almost constant value, which indicates that I_c can be controlled without modification in n_s. Estimated electron temperature in graphene as a function of P shows that the heating of electronic system by photoexcitation and its relaxation process are responsible for the reduction in I_c. Generation of non-equilibrium electronic state by optical excitation will provide a new method to control proximity-induced superconducting state in graphene.
[1] H. B. Heersche et al., Nature 446, 56 (2007).
[2] T. Schäpers et al., Appl. Phys. Lett. 75, 391 (1999).

A part of this study was supported by JSPS KAKENHI Grant Number 25790010. Sample fabrication process was supported by NIMS Nanofabrication Platform in Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.