Near-Field Analysis of Surface Waves Launched at Nanoslit Apertures

With the aim of analyzing the properties of the waves that are scattered by nanoslits on metallic surfaces, we provide a direct observation of the near-field in a slit-doublet experiment at optical wavelengths. We show that two distinct waves are involved: a surface plasmon polariton and another wave with a free-space character. From the recorded data, we have extracted the amplitudes and phases of these waves, their damping characteristic lengths and their relative weights as a function of the separation distance from the slit. The analysis is fully supported by a quantitative agreement with vector-theory computational results.

Figure 1

Near-field fringe $xz$ pattern in a slit-doublet experiment. (a) Experimental data. (b) Computational results. The displayed image shows $(|E_z{|}^2+|E_x{|}^2{)}^2$ calculated with a fully vectorial method for the actual parameters measured on the sample and for $2d=6.24\mu \mathrm{m}$. The intensity of the images is both saturated at the same level for the sake of clarity.

Figure 2

$yx$-scanned near-field images obtained at the air-gold interface of the slit doublet shown in the scanning electron microscope (SEM) photograph (c). The same laser power is used for TE (a) and TM (b) polarizations.

Figure 3

(a) $xy$-scanned near-field image for $2d=10.44\mu \mathrm{m}$ under TM polarization. (b) SEM picture of the slit doublet. (c) Associated data (black dots) obtained by averaging 10 adjacent line scans from (a). The fitted curve obtained from Eqs. (1, 2) is shown with the solid red line. The vertical arrows indicate the slit positions. The inset shows the fitted curve obtained for a pure SPP pattern fit in the central part of the interference pattern, $|x|<2\mu \mathrm{m}$.

Figure 4

REC-field main characteristics and comparison with fully vectorial computational results. (a) and (b) Real part of the electric-field $z$ component scattered by an isolated nanoslit on an interface as a function of the separation distance $x_S$ from the slit center : SPP (black-dashed line), REC field (red-thick line), total field (blue line). (a) Extracted with Eq. (3) from the experimental data. (b) Computational results displayed with the same scale. (c) Calculated REC-field components as a function of $x_S$: $\mathrm{Re}(H_C)$ in blue, $\mathrm{Re}(E_{z,C})$ in red-dashed and $\mathrm{Re}(E_{x,C})$ in black. (d) Relative fraction ${\alpha }_{\mathrm{SP}}$ of the intensity carried by the SPP mode. The red-dashed curve corresponds to computational results. The black-dotted and blue-solid curves are obtained from Eq. (3) with the optimized fitted parameters for $2d=6.24$ and $10.44\mu \mathrm{m}$, respectively.