  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.
