  Terahertz signals hold significant potential for ultra-wideband communication
and high-resolution radar, necessitating miniaturized detectors capable of
multi-parameter detection of intensity, frequency, polarization, and phase.
Conventional detectors cannot meet these requirements. Here, we propose plasmon
polariton atomic cavities (PPAC) made from single-atom-thick graphene,
demonstrating the monolithic multifunctional miniaturized detector. With a
footprint one-tenth the incident wavelength, the detector offers benchmarking
intensity-, frequency-, and polarization-sensitive detection, rapid response,
and sub-diffraction spatial resolution, all operating at room temperature
across 0.22 to 4.24 THz. We present the monolithic detection applications for
free-space THz polarization-coded communication and stealth imaging of physical
properties. These results showcase the PPAC's unique ability to achieve strong
absorption and weak signal detection with a thickness of only 10^-5 of the
excitation wavelength, which is inaccessible with other approaches.
