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Abstract
Lamellar gratings illuminated in conical (off-plane) mounting can achieve with suitable optogeometrical parameters (grating profile, angle of incidence and wavelength) a total absorption of light for any polarization provided there is only the zeroth propagating order. A detailed analysis shows that electromagnetic resonances are involved and their nature strongly depends on the polarization. When the incident electric field is parallel to the cross-section of the grating, the resonance is provoked by the excitation of surface plasmons. For the orthogonal polarization, total absorption occurs for deep gratings only, when the grooves behave like resonant optical cavities. It is possible to reduce the optimal grating height by filling the grooves with a high refractive index material.
Highlights
Reflection of light by metallic plane can be drastically reduced by structuring the metallic film at nanometer scales
If surface plasmons are involved in both polarizations with crossed gratings, full light absorption in bare metallic grating is due to two different electromagnetic resonances depending on the polarization
In conical mounting (φ = 90°), it has been clearly evidenced in the last part of the paper that surface plasmons are responsible of the strong absorption when the incident electric field is perpendicular to the grooves
Summary
Reflection of light by metallic plane can be drastically reduced by structuring the metallic film at nanometer scales. In the case of crossed gratings, surface plasmons are excited in both polarizations and, as a result, full light absorption is insensitive to the incident polarization [12]. In the case of non crossed gratings, excitation of surface plasmons are highly dependent on the polarization of the incident light, and it is no more evident that it may be possible to manufacture gratings able to fully absorb light in both polarizations. We start from a numerical optimization of light absorption in both polarizations with respect to the geometrical parameters of the grating, in the case of deep bare metallic grating. A thorough study of the physical origin of the full light absorption in each polarization is carried out It is the key point of the paper. We show that similar results can be obtained with moderate groove heights, and the two different mechanisms of absorbance depending on the incident polarization are clearly evidenced
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