A radiation reflecting element (34), more particularly a so-called triple
prism or a retro-reflector, is provided with a photonic element on a surface serving
either for the passage of an electromagnetic energy beam (light) (40,43)
or on one of the surfaces (30-32) serving for the deflection of the beam.
The photonic element is characterized by the presence of a photonic band gap (5,7)
capable of being shifted e.g. by an electric signal. Thus, with an appropriate
selection of the wavelength of the light beam, the photonic element can be switched
from reflective to transmissive, thereby allowing a modulation of the light beam
with a high modulation depth.