Abstract: 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.