Modal method in deep metal-dielectric gratings: the decisive role of hidden modes

Abstract

The modal method is well adapted for the modeling of deep-groove, high-contrast gratings of short period, possibly involving metal parts. Yet problems remain in the case of the TM polarization in the presence of metal parts in the corrugations: whereas most of the diffraction features are explained by the interplay of an astonishingly small number of true propagating and low-order evanescent modes, the exact solution of the diffraction problem requires the contribution of two types of evanescent modes that are usually overlooked. We investigate the nature and the role of these modes and show that metal gratings can be treated exactly by the modal method.

High order integral equation method for diffraction gratings

“…Efficient numerical methods are needed to analyze the diffraction and scattering of light by these periodic structures. Existing numerical methods for diffraction gratings include general-purpose methods such as the finitedifference time-domain (FDTD) method and the finite element method (FEM) [3], and more special methods such as the analytic modal method [4][5][6][7], numerical modal methods [8][9][10][11][12][13][14][15][16][17][18], the boundary integral equation (BIE) methods [19][20][21][22][23][24][25][26][27][28], etc. Although FDTD and FEM are extremely versatile, they are typically less efficient than the special methods.…”

Generalized antireflection coatings for complex bulk metamaterials

“…Furthermore, multilayer structures allow the derivation of analytical formulas for their eigenmodes, and in several cases for their transmission and reflection properties as well. For our demonstration, we investigate a lossless configuration which has a single propagating mode with an antisymmetric profile and a negative mode index: or anomalous [22,26]. Fig.…”

High-efficiency wide-band metal-dielectric resonant grating for 20 fs pulse compression

“…Using Lyndin's optimisation code [14] to model the phenomenologically designed structure gives a final structure which achieves the highest diffraction efficiency over the requested spectral width which is here 200 nm centred at 800 nm wavelength. The optimisation code uses a standard multivariate search procedure where the core of the code is a direct problem analysis based on the "true-mode method" [15]. This method forms the electromagnetic field from a basis of the modes of the corrugation, as the physical actuality, instead of decomposing the corrugation in to Fourier harmonics.…”