Controlling Strong Electromagnetic Fields at Subwavelength Scales

We investigate the optical response of two subwavelength grooves on a metallic screen, separated by a subwavelength distance. We show that the cavity, Fabry-Perot-like mode, already observed in one-dimensional periodic gratings and known for a single slit, splits into two resonances in our system: a symmetric mode with a small $Q$ factor, and an antisymmetric one which leads to a much stronger field enhancement. This behavior results from the near-field coupling of the grooves. Moreover, the use of a second incident wave allows control of the localization of the photons in the groove of our choice, depending on the phase difference between the two incident waves. The system acts exactly as a subwavelength optical switch operated from far field.

Figure 1

Geometrical configuration and parameters. Region ($I$) and ($II$) correspond, respectively, to the region above and below the metallic surface. At the end of the Letter, we consider the case where a second incident plane wave is sent. $\varphi$ is the phase difference between the two incident waves.

Figure 2

Enhancement factor $E{F}_1$ of the cavity centered at $x=-d/2$, calculated as a function of the wave number, for $\theta =0°$ (full line) and $\theta =30°$ (dotted line). Inset (a) represents $E{F}_2$ (cavity centered at $x=+d/2$) as a function of the incidence angle $\theta$, for $k=k_{-}$. Inset (b) gives the enhancements of both cavities, $E{F}_1$ and $E{F}_2$, respectively, calculated for 50°.

Figure 3

Enhancement factor $E{F}_1$, calculated for $\theta =20°$, showing the resonances ($+$) and ($-$) characterized by their eigenfrequencies ${\omega }_{\pm }$, located on both sides of the frequency resonance ${\omega }_0$ of a unique cavity, and their bandwidth ${\gamma }_{\pm }$ (a). The right part schematically represents the in-phase coupling of the ($+$) mode (b) and the antiphase coupling of the ($-$) mode (c) and their corresponding equivalent dipole.

Figure 4

Enhancements $E{F}_1$ and $E{F}_2$ represented as a function of the wave number, at $\theta =45°$, for two incoming waves. For $\varphi ={\varphi }_{(1,0)}$ the full line is $E{F}_2$ and the dotted line is $E{F}_1$. For $\varphi ={\varphi }_{(0,-1)}$ it is the opposite (a). Related maps of the electric field amplitude $E_x$ at $1490{\mathrm{cm}}^{-1}$ when the first cavity is lit (b) and when the second one is lit (c).