Global uniqueness in determining polygonal periodic structures with a minimal number of incident plane waves
In this paper, we investigate the inverse problem of recovering a two dimensional perfectly reflecting diffraction grating from the scattered waves measured above the structure. Inspired by a novel idea developed by Bao et al (Trans. Am. Math. Soc. to appear) we present a complete characterization of the global uniqueness in determining polygonal periodic structures using a minimal number of incident plane waves. The idea in this paper combines the reflection principle for the Helmholtz equation and the dihedral group theory. We characterize all periodic polygonal structures that cannot be identified by one incident plane wave, including the resonance case where a Rayleigh frequency is allowed. Furthermore, we show that those unidentifiable gratings provide non uniqueness examples for appropriately chosen wave number and incident angles. We also indicate and fix a gap in the proof of the main theorem of Elschner and Yamamoto (2007 Z. Anal. Anwend. 26 165 77) and generalize the uniqueness results of that paper.