First and second coherent light beams of the same wavelength are propagated in opposite directions to interact on a sub-wavelength thickness metallic metamaterial layer which is structured with a periodicity such that there is a resonance matched to the wavelength of the coherent beams. The first beam is then able to modulate the intensity of the second beam by modulating the phase and/or intensity of the first beam. The interference of the counter- propagating beams can eliminate or substantially reduce Joule loss of light energy in the metamaterial layer or, on the contrary, can lead to a near total absorption of light, depending on the mutual phase and/or intensity of the interacting beams. A modulation is thus provided without using a non-linear effect.