Broad-band polarization-independent absorption of electromagnetic waves by an overdense plasma

Surface plasmon-polaritons can be efficiently excited on a plasma-vacuum interface by an electromagnetic wave when a subwavelength diffraction grating is placed in front of the plasma boundary. The excitation efficiency depends strongly on the wave frequency (or plasma density, when the frequency is fixed) and polarization. We show both experimentally and theoretically that this sensitivity can be essentially suppressed. A nonzero angle of incidence and an axially symmetric diffraction grating ensure near-total absorption of the incident wave in a broad range of wave frequencies (or plasma densities, when the frequency is fixed). Direct detection of surface plasmon-polaritons has been achieved for the first time using a miniature antenna embedded in the plasma. A new absorption mechanism which is not associated with surface plasma wave excitation is revealed.