NOVELTY - The device has a periodic network (26) of patterns (28) interposed between a first medium (14) and a second medium (30) along predetermined direction, where the first medium is made of semiconductor material i.e. crystalline silicon, and refraction index (n2) of the patterns is lower than refraction index (n1) of the first medium and refraction index (n3) of the second medium is lower than or equal to the refraction index of the patterns. Adjustment wavelength of the network is lower than wavelength corresponding to a forbidden band of the semiconductor material of the first medium.    USE - Reflector device for a photovoltaic device (claimed). Can also be used for an infra-red detector and a device comprising a semiconductor absorbent material such as cadmium telluride, cadmium mercury telluride, indium antimonide, germanium or gallium phosphide.    ADVANTAGE - The refraction index of the patterns is lower than or equal to 0.5 times the refraction index of the first medium, so that the reflector can reflect effectively on a spectral wide range.    DETAILED DESCRIPTION - The network's front face comprises a layer made of amorphous silicon or III-V type semiconductor material. The patterns are made of silicon dioxide, aluminum tri-oxide, silicon mononitride, silicon tetrahedra nitride, zirconium dioxide, titanium dioxide, boron trioxide and polymer such polymethyl methacrylate of zinc oxide, tin oxide or indium tin oxide, or conducting transparent resin. The second medium comprises an encapsulation layer made of ethylene-vinyl acetate, polychlorinated biphenyls, epoxy resin or polymethyl methacrylate. The second medium is an air medium. INDEPENDENT CLAIMS are also included for the following:    (1) a photovoltaic device    (2) a method for manufacturing a photovoltaic device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional view of a photovoltaic device.    Refraction indexes of mediums (n1, n3)    Refraction index of patterns (n2)    Mediums (14, 30)    Periodic network (26)    Patterns (28)