Terahertz-driven phase transition applied as a room-temperature terahertz detector

There are few demonstrated examples of phase transitions that may be driven directly by terahertz-frequency electric fields, and those that are known require field strengths exceeding 1 MVcm$^{-1}$. Here we report a room-temperature phase transition driven by a weak ($\ll 1$ Vcm$^{-1}$), continuous-wave terahertz electric field. The system consists of caesium vapour under continuous optical excitation to a high-lying Rydberg state, which is resonantly coupled to a nearby level by the terahertz electric field. We use a simple model to understand the underlying physical behaviour, and we demonstrate two protocols to exploit the phase transition as a narrowband terahertz detector: the first with a fast (20 $\mu$s) nonlinear response to nano-Watts of incident radiation, and the second with a linearised response and effective noise equivalent power (NEP) $\leq 1$ pWHz$^{-1/2}$. The work opens the door to a new class of terahertz devices controlled with low field intensities and operating around room temperature.