Spectral response measurements at 70 mK of ultra-sensitive TES detectors in the 34–60 μm wavelength range

We have measured the spectral response of sensitive Transition Edge Sensors (NEP ~ 10^-18 W/√Hz) in the frequency range from 1 to 10 THz. The TES sensors have been developed for the SAFARI instrument on SPICA and operate at 70 mK. The sensor is placed in a horn-coupled integrating cavity. We have developed a system with a light pipe that couples radiation from a room temperature Fourier Transform Spectrometer to the device. The attenuation of the light pipe system is of order 60 dB and the coupled power to the device is a few femto-Watts. This system works well and we have analyzed the spectral response of a detector in the short wavelength band of SAFARI (34-60 μm).