Strong Electron Self-Cooling in the Cold-Electron Bolometers Designed for CMB Measurements

Paper in proceeding, 2018

We have realized cold-electron bolometers (CEB) with direct electron self-cooling of the nanoabsorber by SIN (Superconductor-Insulator-Normal metal) tunnel junctions. This electron self-cooling acts as a strong negative electrothermal feedback, improving noise and dynamic properties. Due to this cooling the photon-noise-limited operation of CEBs was realized in array of bolometers developed for the 345 GHz channel of the OLIMPO Balloon Telescope in the power range from 10 pW to 20 pW at phonon temperature T ph =310 mK. The negative electrothermal feedback in CEB is analogous to TES but instead of artificial heating we use cooling of the absorber. The high efficiency of the electron self-cooling to T e =100 mK without power load and to Te=160 mK under power load is achieved by:-a very small volume of the nanoabsorber (0.02 μm 3 ) and a large area of the SIN tunnel junctions,-effective removal of hot quasiparticles by arranging double stock at both sides of the junctions and close position of the normal metal traps,-self-protection of the 2D array of CEBs against interferences by dividing them between N series CEBs (for voltage interferences) and M parallel CEBs (for current interferences),-suppression of Andreev reflection by a thin layer of Fe in the AlFe absorber. As a result even under high power load the CEBs are working at electron temperature T e less than T ph . To our knowledge, there is no analogue in the bolometers technology in the world for bolometers working at electron temperature colder than phonon temperature.