Light-field enhancement on thin and ultra-thin high-index dielectric slabs with rectangular nano-pits

We closely study the local amplifications of visible light on a thin gallium-phosphide slab presenting a sub-wavelength array of small, rectangular, bottom-closed holes. The high-quality Fabry–Perot resonances of eigenmodes which vertically oscillate, and their corresponding near-field maps, especially inside the voids, are numerically quantified with rigorous coupled-wave analysis and analytically interpreted through a quasi-exact modal expansion. This last method gives explicit opto-geometrical rules allowing one to finely understand the general trends in 1D and 2D. In more advanced examples, we show that multi-cavity and/or slightly thicker two-dimensional gratings may generate anomalously frequency-susceptible surfaces over a broad spectral range. Also, dielectric membranes a few nanometers thick only can catch light, with tremendous enhancements of the electric field intensity (>106) that largely extends in the surrounding space.