Generation and control of hot spots on commensurate metallic gratings

Abstract

We study the light localization on commensurate arrangements of deep metallic sub-wavelength grooves. We theoretically show that as the degree of commensuration tends to an irrational number new light localization states are produced. These have properties close to that reported for hot spots on disordered surfaces and are not permitted for simple period gratings. Existence of these new resonances is experimentally provided in the infra-red region by reflectivity measurements performed on two commensurate samples with respectively two and three slits per period. Manipulations of these hot spots which can be controlled from far-field could be used for high sensitivity spectroscopy applications.

Plasmon dispersion diagram and localization effects in a three-cavity commensurate grating

“…The far-field signatures of these modes are dips observed in the reflectivity measurements of p-polarized light. They have been recently evidenced experimentally for the first time, in the infra-red region [15] and in the microwave regime [21] for gratings with two or three slits per period. [22].…”

“…The three cavity resonances result from the non-periodic arrangement of the cavities within the period of the grating which modifies their near-field coupling. This coupling splits the cavity resonance, of an individual cavity or of a simple-period grating [23], into three resonances [15]. The striking point of these resonances is that two of them are spectrally very narrow i.e they are only weakly radiative and they have a large quality factor Q.…”

“…The period tends to infinity and the arrangement of the grooves within a period tends, in a selfsimilar manner, to an aperiodic sequence [18,19]. By this approach we can study the gradual apparition and modifications of light localization effects, starting from a simple-period grating to an almost aperiodic one [15]. We have previously theoretically predicted that critically wavelength-dependent hot spots could be generated and controlled from far-field considering such arrangements.…”

“…Figure 1a displays the SEM image of the grating which was fabricated using electron-beam lithography and a double lift-off technique described in ref. [15] and references therein. The grooves obtained are trapezoidal with a height h=0.59 µm and a width w=0.36 µm at the bottom and w=0.62 µm at the top.…”

“…In a previous paper we proposed an alternative manner to induce light localization phenomena by considering commensurate gratings of deep sub-wavelength metallic grooves. By doing so, we combined the effects of light trapping within active sites (the deep cavities) with the properties of self-similar arrangements (commensurate gratings) and this in a controlled and applicable manner since the theory, the engineering of the sample and the experiments can be achieved [15]. Our results showed that, as it occurs for electron localization in quasi-crystal for instance [16,17], new light localization phenomena arise from merging electromagnetic resonances of deep metallic grooves with one-dimensional commensurate structures.…”