“…Scattering theory in biperiodic structures has many important applications in micro-optics, where the biperiodic structures are also termed as 2D gratings or crossed gratings. We refer to Dobson and Friedman [27], Abboud [1], Bao [7], Bao, Dobson, and Cox [11], Bao and Dobson [10], Bao and Zhou [13], Li [33], Yachin and Yasumoto [49], and Chang, Li, Chu, and Opsal [20] for the existence, uniqueness, and numerical approximations of solutions to 2D grating problems. An introduction to grating problems can be found in Petit [41].…”