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

## TL;DR

This paper investigates how thin dielectric slabs with rectangular nano-pits can significantly enhance local light fields through resonances, providing analytical insights and demonstrating potential for broad spectral and ultra-thin applications.

## Contribution

It introduces a quasi-exact modal expansion method to analytically interpret light enhancement phenomena in nano-pitted dielectric slabs, advancing understanding of resonant behaviors and design rules.

## Key findings

- High-quality Fabry-Perot resonances lead to significant local light amplification.
- Multi-cavity and 2D gratings can produce broad spectral surface enhancements.
- Ultrathin dielectric membranes can achieve electric field enhancements over 10^6.

## Abstract

We closely study the local amplifications of visible light on a thin dielectric slab presenting a sub-wavelength array of small, rectangular, bottom-closed holes. The high-quality Fabry-Perot resonances of eigen modes which vertically oscillate, and their corresponding near-field maps, especially inside the voids, are numerically quantified with RCWA and analytically interpreted through a quasi-exact modal expansion. This last method gives explicit opto-geometrical rules allowing 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 ($>10^6$) that largely extends in the surrounding space.