Demonstration of high optical sensitivity in far-infrared hot-electron bolometer

We report on the measurement of a very low noise equivalent power of 3×10^-19 W/Hz^1/2 at 620 GHz in a superconducting antenna-coupled hot-electron bolometer. The sensing element is a micron-size titanium transition-edge sensor with NbTiN superconducting contacts fabricated on a sapphire substrate. The high sensitivity is due to the small device volume, low operating temperature, and weak electron-phonon coupling in titanium film. Measurements were done using a cryogenic black body emitter producing well-controlled femtowatt power levels. The achieved optical sensitivity is suitable for the low-background spectroscopy of molecular lines on next generation space telescopes.