Modulation of electromagnetic radiation is described in which an incident radiation beam is directed toward a surface of a composite material and at least partially reflects to form a reflected radiation beam. The composite material comprises an arrangement of electromagnetically reactive cells of small dimension relative to a wavelength of the incident radiation beam, and exhibits at least one of a negative effective permeability and a negative effective permittivity for at least one frequency. A modulation signal is applied to the composite material to cause a variation in at least one of the effective permeability and the effective permittivity, at least one characteristic of the reflected radiation beam being modulated according to the modulation signal.