Picowatt calorimeter for optical absorption spectroscopy.

An optical picowatt calorimeter at 4 K is demonstrated to measure absorption spectra from 330 nm to 1700 nm of optical samples. The minimum detectable absorbed power is 10 pW, resulting in absorption sensitivities of 0.3 ppm for 30 µW of incident power from a tunable source and 0.6 ppb for 15 mW laser excitation. Active temperature stabilization of main components of the cryogen-free cryostat and the use of paramagnetic temperature sensors with superconducting quantum interference device readout provided a low noise environment (700 nK temperature rms) and nK temperature resolution.