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.
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.
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.
KEYWORDS: Bolometers, Sensors, Polarization, Polarimetry, Silicon, Space observatories, Mirrors, Magnetism, Semiconductors, Picture Archiving and Communication System
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.
CEA has a long history of customizing optoelectronic components for space and astronomy applications. Based on this expertise, we are undertaking the development of cooled silicon bolometers for millimetre-wave (mm-wave) polarization detection in the next generation of space astronomy missions such as SPICA. Silicon bolometer technology has been demonstrated successfully in space conditions through the Herschel mission. There are many benefits of this technology such as the use of a simple and low-power read-out circuit that can be integrated below the detector array in an above-IC (Integrated Circuit) integration scheme. The advanced integration in a large array and the fabrication process based on microelectronics techniques are key challenges for these developments. This work presents the early results on the design, the fabrication and the first characterization of an innovative pixel for mm-wave polarization detection. The aim is to have an adapted absorption around λ=1.5 mm. These bolometers are composed of an absorbing layer and a thermometer, which are thermally insulated from the substrate. To increase the sensitivity, these detectors are working at very low temperature typically between 50 and 100 mK. The suspended thermometer is made of silicon implanted with Phosphorus and Boron species, and we optimized the design to have a high sensitivity with a 3D Variable Range Hopping conduction (Efros law) and a low 1/f noise at low temperature. The heat capacity of the bolometer is optimized by using a meander shape of the thermometer together with superconducting Ti/TiN thin films for the electromagnetic wave absorption. This sensor is implemented on a standard SOI substrate. Measurements of test structures at room temperature, and first results at very low temperature have been performed to evaluate the electrical performances of the fabricated detectors. The mechanical behaviour of released structures, including pixels with a pitch of 1200μm and 600μm, is presented and discussed.
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