In this paper, we present the development and experimental characterization of a cryogenic Fabry-Perot spectrometer, proposed for on-chip integration with submillimeter wave detectors. The Fabry-Perot interferometer design was chosen for its compactness, compatibility with polarimetric imaging, and versatile spectral resolution capabilities. By using dielectric Bragg mirrors, the presented Fabry-Perot mitigates the absorption losses typical of conventional metallic mesh mirrors, thereby enhancing transmission efficiency while keeping high finesse. We discuss the theoretical framework and performance considerations of a Fabry-Perot interferometer based on silicon Bragg reflectors and provide an analysis of spectral responses obtained from cryogenic optical measurements. These results demonstrate the potential of Bragg-mirror Fabry-Perot spectrometers as a viable approach for future medium and high-resolution, multi-functional submillimeter imaging detection systems for ground and space-based observatories.

Following the discoveries of the Herschel space telescope and the numerous observations in the submillimeter range of the interstellar medium (ISM), we are developing two cryogenic spectrometer concepts that can be integrated on a detector focal plane to understand the evolution of the various phases of the ISM. In this talk, we present the principle and measured performance of those two multi-wavelength Fabry-Perot. The first concept is an adjustable Fabry-Perot cavity with dielectric Bragg mirrors. This prototype, equipped with a piezoelectric mechanism is used to demonstrate the concept of a scanning Bragg mirror Fabry-Perot, enabling the spectrum around a line to be finely scanned without loss. The second concept is a stationary array of Fabry-Perot made entirely of silicon. In this spectrometer, the cavity is made of microstructured silicon. By adjusting the size of the sub-wavelength structures, we change the optical index of the cavity and therefore the transmitted wavelength.

Silicon bolometers with in-pixel polarimetric capabilities and above-IC integration on a CMOS read-out integrated circuit were developed in the frame of the SPICA mission for the observation of galaxies and star-forming regions at sub-millimeter waves. This paper highlights the fabrication and integration of these cooled (50 mK) detectors and their main experimental performances. Polarization-sensitive detection with a high responsivity of similar to 5.1011 V/W and state-of-the-art Noise Equivalent Power of similar to 5 aW/root Hz at 100 mu m wavelength are demonstrated.

Characterization of the magnetic fields at different scales in the Universe is a new frontier for submillimeter astronomy. Polarimetric measurements between 50 and 500 µm are the golden path for this research. We develop, in the prospect of space observatories, all-silicon 50 mK bolometer arrays with polarimetric capabilities in the pixel. Here, we present the first results of the new detectors: performances of thermal sensors, optical absorption and polarimetry.

We are studying and developing high performance spectroscopy solutions in silicon technology in the framework of the instrumental developments around the B-BOP bolometer arrays [1] and in the continuity of the former developments of the Herschel space telescope instruments and in particular its PACS instrument [2]. The integration of multi-wavelength spectroscopic capabilities close to the array of a cooled bolometer would make it possible to detect the interstellar medium (ISM) continuum by scanning and to trace its evolution in particular by the detection of characteristic lines such as the cooling line [CII] at 158µm. In this paper, we present the first concept under development and the preliminary results obtained in our cryogenic optical test bench. This solution is a tunable cavity Fabry-Perot (FP) with silicon mirrors driven by a cryogenic piezoelectric mechanism with a nanometric step. Each mirror is a dielectric Bragg structure, a stack of quarter-wave layers of silicon and air providing a high reflectivity without metal losses. This solution allows extremely accurate scanning around a targeted wavelength. The first prototype is a relatively bulky proof of concept and future optimizations will allow it to be adapted to a focal plane array.

We develop polarimetric detector arrays for submillimeter Space astronomy. The technology is based on the all-silicon bolometer scheme established for the Herschel/PACS photometer. Each pixel detects two orthogonal polarization directions with an efficiency greater than 95%. Every other pixel in the array is rotated by 45° to retrieve the linear Stokes parameter of the incident light inside the Airy disc of the telescope optics. We reduced the temperature to 50 mK to improve sensitivity and introduced new readout schemes based on single or double Wheatstone bridge configurations for fully differential DC outputs. The detectors are grown directly on a CMOS circuit wafer to ensure the largest bandwidth with the highest response.

In this paper, we discuss the characterization of boron-doped silicon superconducting thin films with a thickness of 70 nm made on silicon-on-insulator substrates by ion implantation and ultra-violet nanosecond laser annealing under nitrogen at atmospheric pressure. Two different ion-implanted doses of boron of 1 × 1016 and 2.5 × 1016 cm−2 at 3 keV were tested in the study. Single laser pulses with energy densities in the range of 0.3–1.1 J/cm2 were applied to activate the boron species in the silicon. A transition from partially (monocrystalline) to fully-melted (polycrystalline) silicon is observed when increasing the laser energy density. The critical temperature (Tc) and the upper critical magnetic field (Bc2) were measured for different samples. A maximum Tc of 100 mK was obtained in the monocrystalline phase of the silicon just before the transition into the polycrystalline phase. An obvious impact of the doping level and laser annealing energy density on the Tc values was observed. Different morphological and physical characterizations such as transmission electron microscopy, X-ray photoelectron spectroscopy and secondary ion mass spectrometry were performed and analyzed in order to compare the samples.

The thermometric behavior of lateral 50-nm Silicon-On-Insulator PiN diodes are studied from ambient to liquid-nitrogen temperature. Prototypes were manufactured and their experimental performances are compared to theoretical models. Thanks to a temperature sensitivity reaching 25%/K and a low noise level at 80 K, a minimum thermal resolution of 0.1 mK is obtained. Such diodes represent an attractive solution for high performance thermal sensing in bolometric detectors.

Detectors with a Minimum Detectable Power (MDP) under the picowatt range are required for Terahertz passive imaging. A cooled bolometer using a high-impedance surface associated with a metal dipole to ensure a broadband absorption centered at 425 GHz is presented as a solution. A lateral PiN diode is sensing temperature modifications in the bolometer operating at liquid-nitrogen temperature. Diode prototypes were fabricated on Silicon-On-Insulator 8 '' wafers with a 50 nm active silicon layer. Their experimental thermal resolution as low as 0.2 mK at 81 K leads to an estimated MDP of the future bolometer reaching 2.45 pW at 25 frames per second.

High-impedance surfaces (HIS) are investigated here for the above-IC integration of sub-millimeter-wave (350 µm here) detectors on CMOS electronics using standard thin film manufacturing processes and materials. We demonstrate theoretically and experimentally the efficient optical absorption of a resistive dipole array above an HIS at cryogenic temperature. More specifically, an absorption above 78% in linear polarization is obtained over a bandwidth of 100 µm around a center wavelength of 350 µm. These results confirm the promising performances of HIS structures in the perspective of future polarimetric bolometers in terms of absorption efficiency, bandwidth and cross-polarization discrimination.

One of the main goals of the canceled Space Infrared Telescope for Cosmology and Astrophysics (SPICA), was to reveal the evidence of the influence of magnetic field in the structuration of different astrophysical objects, as for example the filamentary structure of star-forming regions. For this purpose, “instrument-in-pixel” detector arrays were developed under ESA, CNES and FOCUS contracts, to propose sensitive, compact and easy to integrate detection solutions for a Space Observatory. Magnetic field influences the light emission or absorption of small grains and molecules imprinting its characteristics in the received electromagnetic message in terms of polarization, degree, angle and intensity. Each pixel of the developed detectors absorb the radiation through two orthogonal dipole networks. The detector array is organized like a chessboard with every other pixel having absorbers rotated by 45° in order to unveil simultaneously the linear Stokes parameters without any optical loss. A very large absorption efficiency is obtained, as usual since PACS detectors, by a backshort-under-grid scheme. To obtain the goal sensitivity of 1 attoW/√Hz, detectors are cooled to 50 mK and linked to an Above IC CMOS readout circuit. For each pixel, four interleaved spiral silicon sensors gather the absorber power. They are organized in a Wheatstone bridge configuration that allows fully differential outputs: total power and polarization unbalanced intensity.

Two technologies of all-silicon on-chip spectrometers based on the Fabry-Perot interferometer principle are studied and under development for a target wavelength of 158µm ([CII]). We are developing these spectroscopic capabilities with the objective of including them in polarimetric detector arrays cooled at 50mK. The first solution is a tunable cavity Fabry-Perot with silicon mirrors driven by cryogenic piezoelectric motors with a sub-micron step size. Each mirror is a dielectric Bragg structure made of quarter-wave layers of silicon and air providing a high reflectivity without metal losses. The theoretical performance of a Fabry-Perot resonator with such Bragg mirrors has been confirmed by measurement in a low temperature FTS: the finesse of this interferometer is more than twice that of a traditional Fabry-Perot. The second solution is a fixed Fabry-Perot array with a silicon microstructured cavity, which allows having different optical indices in different areas. The cavity is made of deep-etched silicon microstructures whose section is adapted to obtain the adequate optical index. Therefore, multiple wavelengths around 158µm, distributed on the array, are transmitted by this Fabry-Perot. The mirrors of this spectrometer are metallic capacitive grids designed to be highly reflective at the targeted wavelength, easy to manufacture with reduced metal losses. The simulations show high performances in resolution, close to the Bragg mirrors Fabry-Perot. The first prototypes of this solution have been manufactured by the CEA/LETI and will be soon measured in the cryogenic facilities in Saclay.

In this paper, we investigate two bolometer detectors with a pixel size of 500 µm and 1200 µm to address, respectively, the 0.6 mm and 1.5 mm wavelengths in order to study the temperature and polarization of cosmic microwave background (CMB). The pixels are polarization sensitive using Ti/TiN superconducting absorbers. They are deposited on suspended doped Silicon thermometers operating at low temperature, typically in the range 50–100 mK. A quarter-wavelength optical cavity formed between absorbers and metal reflector of these pixels is adapted for an absorption around 100 µm. In order to address the millimeter band, we have used a dielectric (Silicon) superstrate placed above the absorbers to shift the absorption band to larger wavelengths. In this paper, we have conducted finite-element (FEM) electromagnetic simulations to optimize the pixel design. The optical absorption measurements of the pixels were performed at room temperature using a terahertz time-domain spectrometer (THz-TDS) and at 300 mK with a Fourier-transform spectrometer (FTS). Finally, an estimation of pixels performances is discussed showing, an expected high responsivity of around 1011 V/W and a low noise equivalent power (NEP) of 10–18 W/Hz1/2.

A fast image deconvolution algorithm is used to demonstrate the resolution enhancement of video rate camera acquired Terahertz images. Our algorithm is based on variable splitting technique with the use of a family of sparsity inducing regularizers for the first time in an image deconvolution application, it is also suitable for practical applications in industry with computationally constrained conditions. The results of the proposed process provide substantial enhancement on the quality and resolution of THz images.

We present the B-BOP instrument, a polarimetric camera on board the future ESA-JAXA SPICA far-infrared space observatory. B-BOP will allow the study of the magnetic field in various astrophysical environments thanks to its unprecedented ability to measure the linear polarization of the submillimeter light. The maps produced by B-BOP will contain not only information on total power, but also on the degree and the angle of polarization, simultaneously in three spectral bands (70, 200 and 350 microns). The B-BOP detectors are ultra-sensitive silicon bolometers that are intrinsically sensitive to polarization. Their NEP is close to 10E-18 W/sqrt(Hz). We will present the optical and thermal architectures of the instrument, we will detail the bolometer design and we will show the expected performances of the instrument based on preliminary lab work.

A substrate-integrated discrete-lens antenna manufactured in standard printed-circuit-board (PCB) technology is demonstrated in H-band (225-325 GHz). The arrays composed of phase-shifting unit cells and a waveguide-fed planar focal source are designed on a single PCB stack with five metal layers and multiple low-loss dielectric substrates in a monolithic module of 20 x 20 x 4.52 mm(3). The linearly polarized unit cells are based on arc-shaped resonators placed between two perpendicular polarizing grids. They achieve eight transmission phase states with less than 1 dB average insertion loss and 27% 1 dB bandwidth. Two antenna prototypes with radiating apertures of 6.6 x 6.6 mm(2) and 10.56 x 10.56 mm(2) are designed, fabricated, and measured. They demonstrate linearly polarized pencil-beam radiation patterns with low sidelobes, low cross-polarization, an experimental gain of 20.6/23.1 dBi at 327/332 GHz, and 3 dB gain bandwidth of 26.3%/17.2%. The impact of manufacturing tolerances is detailed both at unit-cell level and antenna level.

The thermal sensitivity and Low Frequency Noise (LFN) of compensation doped Silicon-On-Insulator (SOI) resistors are studied experimentally, down to the cryogenic regime. A high compensation nominal ratio K = N A /N D of 0.82 is compared with uncompensated and partially compensated configurations, using Phosphorus and Boron as dopant species. The Temperature Coefficient of Resistance (TCR) reaches -2.7%/K at 80 K, for an effective compensation ratio close to 0.98 considering incomplete dopant ionization. The measurements reveal a low LFN with a nearly frequency-independent spectrum, far from the classical 1/f trend observed in uncompensated silicon. The mean value of the Hooge constant on the highly compensated silicon sample equals 3.15 × 10 -4 at 300 K and 4.50 × 10 -5 at 80 K. The normalized LFN at 80 K does not depend on the resistor length and thereby seems independent of the volume. Such high performance thermistors represent an attractive, mature and affordable solution for high performance thermal sensing.

The first observation of coherent elastic neutrino-nucleus scattering (CE nu NS), reported by the COHERENT Collaboration in 2017, paved the way for a new generation of experiments using reactor (nu) over bar (e) and aiming at precisely measuring this process. In this context, the BASKET (Bolometers At Sub-KeV Energy Thresholds) R&D project investigates the use of cryogenic detectors for a reactor CE nu NS experiment. This article reports on the first test of a Mo-doped lithium tungstate scintillating bolometer (empty set18 x 7 mm, 8 g), performed in an aboveground laboratory at CSNSM, Orsay (France). The detector bolometric performance (energy and time response, particle identification capabilities) and radiopurity have been studied and confirm the promising potential of lithium tungstate-based bolometric detectors for the measurement of CE nu NS at reactors.

This paper presents the experimental results of a 2-bit electronically reconfigurable unit-cell for transmitarrays at Ka-band. The proposed unit-cell architecture is based on a six-metal layers design and three dielectric substrates. Two patch antennas are printed respectively on the top and bottom layers of the stack-up to achieve an antenna-filter-antenna structure. To implement the desired 2-bit phase resolution, two p-i-n diodes are bonded on each patch. The unit-cell has been fabricated and characterized in a specific waveguide simulator. The measurement results are compared to the simulated ones and show minimum transmission loss in the range 1.5 - 2.3 dB. The 3-dB fractional bandwidth is in the range 10.1 - 12.1%.

This paper reports our results on the electrothermal modeling of cryogenic silicon bolometers with pixel pitches of 500 and 1200 µm designed for cosmic microwave background polarimetric observation in 0.6 mm and 1.5 mm bands. These detectors should provide a high responsivity, typically around 1011 V/W, and a very low noise equivalent power (NEP) of 10−18 W/Hz1/2 between 50 and 100 mK. They are based on doped silicon thermometers, which exhibit a nonohmic behavior described by the “hot electron model” (HEM) at very low temperature under high bias currents. We compare this model to the experimental characterization of these thermometers at cryogenic temperatures to confirm that the HEM is governing their electrical characteristics and their sensitivity at very low temperature. Finally, this model is used to derive the simulated responsivity and NEP performances of the pixels under weak and moderate optical power illumination.