Abstract
A detailed study of light absorption by silver gratings having two-dimensional periodicity is presented for structures constructed either of channels or of holes with subwavelength dimensions. Rigorous numerical modelling shows a systematic difference between the two structures: hole (cavity) gratings can strongly absorb light provided the cavity is sufficiently deep, when compared to the wavelength, whereas very thin channel gratings can induce total absorption. A detailed analysis is given in the limit when the period tends towards zero, and an explanation of the differences in behavior is presented using the properties of effective optical index of the metamaterial layer that substitutes the periodical structure in the limit when the period tend to zero.
Highlights
Enhanced light absorption by periodic structures has attracted the attention of scientists and engineers since the first observation of grating anomalies by R
As already observed in [7,8,9], the reflection properties of subwavelength two-dimensional grating depend significantly on the form of the structure. When it consists of pillars separated by channels, there exists a fundamental mode that propagates in vertical direction due to the field interaction through the channels, even for narrow channels, with width less than 5% of the light wavelength
Due to the existence of this mode, the grating structure behaves like a lossy high-index anisotropic dielectric, with characteristic Fabry-Perot resonances observed as a function of its thickness
Summary
Enhanced light absorption by periodic structures has attracted the attention of scientists and engineers since the first observation of grating anomalies by R. A typical example is the study of the similarities and differences between inductive (with continuous perforated metal layer in the grating region) and capacitive grids (where the metal inclusions are separated from each other inside the grating) [10] Another problem is the choice of homogetization procedure for structures having very small periods when compared with the wavelength of light, structures known as metamaterials. While channel gratings of just several nm depth can absorb light totally, as already shown in [7, 8], the inverted geometry requires much deeper modulation values to obtain similar performance This difference persists even when the period is reduced to just several nanometers (1/100 of light wavelength). A very good agreement is observed between the rigorous electromagnetic modeling and he effective index approach
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