Sub-wavelength diffraction losses in a silicon nano-patterned membrane reflector

Silicon nano-patterned membrane reflectors - Si-MR (2-dimensional photonic-crystal slabs) hold many promising applications in photonic integrated circuits and can be used as ultra-compact mirrors [1] with controlled spectral response [2]. Built entirely from dielectric materials, such membrane reflectors can have very low losses and therefore can deliver near-100% reflectivity both in narrow and wide-band configurations [2]. The technical factors which lead to optical losses in Si-MR and thus limit their reflectivity are largely due to imperfections in the fabrication process. The fundamental limitations of the reflectivity are due to their photonic crystal structure and the associated sub-wavelength diffraction phenomenon. Here we show that the main beam reflected by the Si-MR is accompanied by a wide-angle diffraction field originating from the lattice structure of the sub-wavelength scale on the reflector.