摘要： Topologically protected surface states in photonic systems, with unique properties such as unidirectional propagation and light con?nement insensitive to thermal or fabrication imperfections, have attracted immense attention over the past ten years. We introduce the notion of weak protection where a topological invariant predicts the existence of surface states in the photonic bulk gap, and strong protection which instead corresponds to the number of glueing points of a surface band with the bulk edges. While strong protection is desired for many key applications such as topologically protected loss-free cavities to boost non-linear interactions, it requires optical reciprocity breaking in two dimensions. We here provide a group theoretical recipe to design weakly protected states on purely dielectric metasurfaces, and genuinely strongly protected states in fully connected 3D photonic crystals. Both design principles can be readily realized by current direct manufacturing techniques, such as electron beam lithography and direct laser writing, respectively.