We show that the envelope of the diffraction efficiency of a two-dimensional photonic crystal can exhibit spectral regions of very small diffraction efficiency (510 3), while in other regions, the diffraction efficiency is near unity. The experimental results on higher bands of hexagonal, silicon-based photonic crystals agree well with corresponding numerical calculations and highlight the prominent role of the surface termination, an aspect which cannot be described by the photonic band structure alone. We speculate about possible applications of such additional spectral filters in Raman and photoluminescence spectroscopy. © 2003 American Institute of Physics. Photonic crystals PCs 1,2 have recently attracted consid-erable attention because of their potential applications in telecommunication, as host for high finesse nanocavities, or as materials exhibiting negative refraction. Another interest-ing aspect, which was recently studied theoretically, is the use of two-dimensional PCs as diffraction gratings. 3,4 As an electromagnetic plane wave hits the surface of a PC, the impedance mismatch between air and the PC leads to both, reflection and diffraction at the periodically modulated sur-face. A two-dimensional 2D PC, illuminated perpendicular to the structure see Fig. 1, diffracts within the plane of incidence—as a conventional surface diffraction grating— while three-dimensional 3D PCs diffract off-plane as well. 5,6 Thus, 3D–PCs are not considered in what follows. For a 2D–PC, the envelope of the diffraction efficiency in Littrow geometry, i.e., backscattering geometry, follows the usual formula 3 m2s sin ,