Controlling Strong Electromagnetic Fields at Subwavelength Scales

Abstract

We investigate the optical response of two sub-wavelength grooves on a metallic screen, separated by a sub-wavelength distance. We show that the Fabry-Perot-like mode, already observed in onedimensional periodic gratings and known for a single slit, splits into two resonances in our system : a symmetrical mode with a small Q-factor, and an antisymmetric one which leads to a much stronger light enhancement. This behavior results from the near-field coupling of the grooves. Moreover, the use of a second incident wave allows to control the localization of the photons in the groove of our choice, depending on the phase difference between the two incident waves. The system exactly acts as a sub-wavelength optical switch operated from far-field.PACS numbers: 71.36+c,73.20.Mf,78.66.Bz Surface Enhancement Raman Scattering (SERS) still remains a mystery in a large part, even though it is now accepted that the excitation of localized electromagnetic modes of irregular metallic surfaces is involved in its basic mechanism [1,2]. Optical excitation of such modes can indeed lead to important concentration of electromagnetic energy in volumes (cavities) much smaller than λ 3 where λ is the excitation wavelength, as it is the case for SERS active surfaces. These specific places of very strong electromagnetic fields localization are called "active sites" or "hot spots". However, the debate on the origin of these hot spots remains open, as well as the hope to control one day this phenomenon. The large interest raised by this fundamental physics is also increased by its wide potential applications in biochips, sensors, nano-antennae, optoelectronics or energy transport on nanostructured surfaces.In this letter, we consider a simple system which allows to produce and control the localization in space of such hot spot phenomenon. It only consists of two deep grooves on a plane metallic gold surface ( fig.1). The excited modes appear, for the chosen geometry, in the infrared region where we can consider the metal as being a good reflector. Under this condition, a reliable theoretical method, i.e the modal method using surface impedance boundary conditions, can be used [3]. This method has already demonstrated its ability to give a good qualitative and quantitative agreement with the measured reflectivity of metallic gratings [4,5,6]. The case of one groove only was considered a long time ago [7], while the transmission for one [8] and two slits [9] were only recently considered. In contrast with [9], the distance between our two grooves is small with respect to the incident wavelength. Very recently it also was shown [10] that sharp and deep resonances appear in the transmission response of gratings with more than one slit per period or in gold dipole antennas [11]. We here analyze the physical origin of this new kind of resonances for a two slit system. As we will see, this allows us to point out some very fundamental aspects of electromagnetic resonances on metallic surfaces, and to control the light localization by using a simp...

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