Depending on the device parameters, operating wavelength, and incident angle, there can be a single resonance or multiple resonances. Using powerful electromagnetic design methods, the spectral bands of these resonant leaky-mode elements can be engineered for various photonic device applications. For example, it has already been shown that a single periodic layer with 1-D periodicity enables narrow line filters, polarizers, reflectors, and polarization-independent elements.26–28 An example of spectral response of a single-resonance device is shown in Fig. 2. For the computations, we use a computer code based on rigorous coupled-wave analysis (RCWA),29 which is a proficient algorithm to evaluate diffraction efficiency of periodic structures as further discussed in 30. In our numerical simulations, we use 11 harmonics and we verify good convergence by test simulations using 21 harmonics. For a simple waveguide-grating structure28 with , , , , , , and , a complete energy exchange occurs between transmitted and reflected waves and a sharp resonance reflectance peak occurs at the 487-nm wavelength when illuminated with normally incident transverse electric (TE: electric field vector being orthogonal to the plane of incidence) polarized light. On the contrary, a homogeneous layer with an effective refractive index of the grating layer does not exhibit such spectral signature, as evidenced by dashed lines in Fig. 2.