Calorimetric measurement of absolute terahertz power at the sub-microwatt level

We developed a calorimeter for determining absolute terahertz (THz) power. The calorimeter is based on a DC substitution method using an isothermal temperature-control technique. A neutral-density optical filter glass was used as a volume absorber, and its THz absorption was evaluated by a time-domain spectrometer. Highly sensitive measurement of the absolute THz power was experimentally achieved in the range from the submicrowatt to microwatt level at room temperature. Power measurement of a continuous-wave THz source using a photomixer was demonstrated. This calorimeter is expected to demonstrate the traceability of THz power at a submicrowatt level.