On-chip spectroscopic solutions for polarimetric bolometer arrays in submillimeter astronomy

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.