The Rayleigh-Modal method is used to calculate the electromagnetic field within the grooves of a perfectly conducting, rectangular-shaped one-dimensional diffraction grating. An enhancement coefficient (η) is introduced in order to quantify such an energy concentration. Accordingly, 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 η 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 optical spacing between the grooves is greater than , where d is the groove spacing and n is the refractive index of the substance within the grooves. For smaller than the enhancement coefficient turns negligibly small.
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