  Plasmons in two-dimensional electron systems (2DES) feature ultra-strong
confinement and are expected to efficiently mediate the interactions between
light and charge carriers. Despite these expectations, the electromagnetic
detectors exploiting 2d plasmon resonance have been so far inferior to their
non-resonant counterparts. Here, we theoretically analyse the origin of these
failures, and suggest a proper niche for 2d plasmonics in electromagnetic wave
detection. We find that a confined 2DES supporting plasmon resonance has an
upper limit of absorption cross-section, which is identical to that of simple
metallic dipole antenna. Small size of plasmonic resonators implies their weak
dipole moments and impeded coupling to free-space radiation. Achieving the
'dipole limit' of absorption cross-section in isolated 2DES is possible either
at unrealistically long carrier momentum relaxation times, or at resonant
frequencies below units of terahertz. We further show that amendment of even
small metal contacts to 2DES promotes the coupling and reduces the fundamental
mode frequency. The contacted resonators can still have deep-subwavelength
size. They can be merged into compact arrays of detectors, where signals from
elements tuned to different frequencies are summed up. Such arrays may find
applications in multi-channel wireless communications, hyper-spectral imaging,
and energy harvesting.
