Light trapping within the grooves of 1D diffraction gratings under monochromatic and sunlight illumination

TL;DR

This paper investigates light trapping in 1D diffraction grating grooves using the Rayleigh-Modal method, introducing an enhancement coefficient to quantify energy concentration, with results varying under monochromatic and sunlight illumination.

Contribution

It introduces the enhancement coefficient to quantify electromagnetic energy concentration within grating grooves and analyzes its dependence on wavelength and groove spacing.

Findings

Enhancement coefficient can reach several decades at specific wavelengths.

Under sunlight, the enhancement coefficient is about 20%.

Optimal energy trapping occurs when optical spacing exceeds 500 nm.

Abstract

The Rayleigh-Modal method is used to calculate the electromagnetic field within the grooves of a perfectly conducting, rectangular-shaped 1D diffraction grating. An \emph{enhancement coefficient} ($\eta$) is introduced in order to quantify such an energy concentration. Accordingly, $\eta >$1 means that the amount of electromagnetic energy present within the grooves is larger than that one will have, over the same volume, if the diffraction grating is replaced by a perfectly reflecting mirror. The results in this paper show that $\eta$ can be as large as several decades at certain, often narrow, ranges of wavelengths. However, it reduces to approximately 20% under sunlight illumination. In this latter case, such values are achieved when the \textit{optical spacing} between the grooves $dn$ is greater than 500 nm, where $d$ is the groove spacing and $n$ is the refractive index of the substance within the grooves. For $dn$ smaller than 500 nm the enhancement coefficient turns negligibly small.