Surface Plasmons Modulate the Spatial Coherence of Light in Young’s Interference Experiment

It is shown how surface plasmons that travel between the slits in Young’s interference experiment can change the state of spatial coherence of the field that is radiated by the two apertures. Surprisingly, the coherence can both be increased and decreased, depending on the slit separation distance. This results in a modulation of the visibility of the interference fringes. Since many properties of a light field—such as its spectrum, polarization, and directionality—may change on propagation and are dependent on the spatial coherence of the source, our results suggest that the use of surface plasmons provides a new way to alter or even tailor the statistical properties of a light field.

Figure 1

Illustrating the geometry.

Figure 2

Plots of: (a) fringe visibility $\mathcal{V}$, (b) absolute value of the spectral degree of coherence $|{\mu }_{12}(\omega )|$, as a function of ${\mu }_{12}^{(\mathrm{inc})}(\omega )$, the spectral degree of coherence of the incident field. Markers (*, ×, +) indicate analytic results, while the unfilled shapes (○, □, ◇) indicate numeric results. In this case the slit width $w=200\mathrm{nm}$. In all examples the wavelength $\lambda =600\mathrm{nm}$, and the thickness of the gold film $t=200\mathrm{nm}$. The refractive index was taken to be $n=0.21+i3.27$ and hence $k_{\mathrm{sp}}=(1.099+i0.00721)\times {10}^7{\mathrm{m}}^{-1}$.

Figure 3

The absolute value of the spectral degree of coherence $|{\mu }_{12}(\omega )|$ as a function of ${\mu }_{12}^{(\mathrm{inc})}(\omega )$, for slit widths: (a) $w=100\mathrm{nm}$, (b) $w=50\mathrm{nm}$. For the sake of clarity, only numeric results are shown.