Absolute photonic band gaps in two-dimensional square and honeycomb lattices of circular crosssection rods can be increased by reducing the structure symmetry. The addition of a smaller diameter rod into the center of each lattice unit cell lifts band degeneracies to create significantly larger band gaps. Symmetry breaking is most effective at filling fractions near those which produce absolute band gaps for the original lattice. Rod diameter ratios in the range 0.1-0.2 yield the greatest improvement in absolute gap size. Crystal symmetry reduction opens up new ways for engineering photonic gaps.[S0031-9007 (96) The last few years have witnessed an ongoing search for periodic dielectric structures which give rise to a photonic band gap (PBG)-a region of the frequency spectrum where propagating modes are forbidden. These "photonic crystals" could alter radiation-matter interactions and thus improve the efficiency of optical devices by controlling spontaneous emission [1]. Applications of these crystals in semiconductor lasers and solar cells [1], and high-quality resonant cavities and filters [2] have been proposed. Although three-dimensional (3D) PBG crystals suggest the most interesting ideas for novel applications, two-dimensional (2D) structures could also find several important uses, as a result of their strong angular reflectivity properties over a wide frequency band. For example, 2D PBG crystals with absolute band gaps provide a large stop band for use as a feedback mirror in laser diodes [3]. Photonic gaps at visible to near-infrared (IR) wavelengths could have the widest impact in applications. As the band gap frequency is directly related to the size of the scattering elements comprising the lattice, a near-IR band gap requires features with dimensions in the submicron size regime. Fabricating 3D periodic structures in this regime poses an overwhelming challenge, despite progress in microfabrication technology. Perhaps for this reason attention has been drawn towards 2D lattice structures. The successful fabrication of 2D crystals with near-IR band gaps has been recently reported [4,5].

The size of absolute band gaps in two-dimensional photonic crystals is often limited by band degeneracies at the lattice symmetry points. By reducing the lattice symmetry, these degeneracies can be lifted to increase the size of existing photonic band gaps, or to create new gaps where none existed for the more symmetric structure. Specifically, symmetry reduction by the addition of different diameter rods into the unit cell of two-dimensional square lattices ͑Laue group 4mm͒ is explored. This approach is especially useful in opening absolute band gaps in structures of dielectric rods in air, which are more easily microfabricated than a crystal of air columns in a dielectric background. Symmetry reduction offers a rational approach for exploring and designing new photonic crystal structures. ͓S0163-1829͑97͒03736-3͔

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