Polarization dependent near-field speckle of random gold films

The near-field speckle of random gold nanostructures is experimentally and theoretically investigated. The topography and the intensity distribution, under polarized illumination, are obtained with an aperture scanning near-field optical microscope. The sharpness of the experimental images entitles us to perform a fine statistical analysis based on the autocorrelation function (ACF). The intensity ACF clearly exhibits an anisotropic behavior and subwavelength oscillations, depending on the incident polarization. A comparison with a numerical simulation exhibits the influence of the surface topography on the anisotropic near-field speckle.

Figure 1

Spectrophotometric measurements of a semicontinuous gold film near the percolation threshold (the metallic surface coverage is 59%): (◻) transmission, (▴) absorption and (○) Reflection spectra.

Figure 2

(Color online) AFM map, obtained in tapping mode, of a semicontinuous gold film. The metallic surface coverage is 59%.

Figure 3

(Color online) (a) Autocorrelation function of the surface profile for a granular gold film. (b) The polar correlation curve $C_S\left(R\right)$ fitted by a Gaussian-like function (dashed line).

Figure 4

(Color online) (a) Topographic image of a gold film near the percolation threshold. (b) SNOM image at $\lambda =514.5\mathrm{nm}$ ($400\times 400$ data points, the incident linear polarization is indicated by the arrow). (c) Corresponding probability density function (PDF).

Figure 5

(a) Three-dimensional mapping of the near-field normalized intensity ACF calculated from SNOM data. The direction of the incident polarization is indicated by an arrow. [(b) and (c)] Correlation curves $C_I(\Delta x,0)$ and $C_I(0,\Delta y)$, respectively.

Figure 6

(Color online) (a) Near-field topographic image of a semicontinuous gold film obtained by SNOM and corresponding optical images: (b) simulated image and (c) experimental intensity distribution.

Figure 7

(Color online) (a) Normalized intensity ACF calculated from the simulated intensity distribution of (b). The incident field is shown by an arrow. (b) one-dimensional (1D) curves plotted in the parallel (solid line) and in the perpendicular (dashed line) directions of the incident field.

Figure 8

(a) Near-field intensity ACF simulated with a Gaussian correlated surface. The incident field polarization is indicated by an arrow. (b) The 1D curves are plotted in the parallel (solid line) and in the perpendicular (dashed line) directions of the incident field.

Figure 9

The correlation curves are plotted (a) in the perpendicular direction to the incident field $C_I(\Delta x,y=0)$ and (b) in the parallel direction to the incident field $C_I(x=0,\Delta y)$. The graphs are plotted for $z=1\mathrm{nm}$ (◼), $z=10\mathrm{nm}$ (+), and $z=100\mathrm{nm}$ (▴).

Figure 10

Anisotropic factor $\kappa$ as a function of the observation distance $z$.