Near-field surface plasmon effects on Au-double-slit diffraction for polychromatic light
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NANO EXPRESS
Open Access
Near-field surface plasmon effects on Au-doubleslit diffraction for polychromatic light Pin Han
Abstract The surface plasmon effects on near-field diffraction for polychromatic light are studied. An Au-double-slit is used as the model and Fresnel integral is employed to perform the theoretic analysis. The results are illustrated with numerical examples and they show that, compared with the normal double-slit, the plasmon effect changes the spectral shift from redshift to blueshift and also enhances the intensity peak. This effect can be used in optical data transmission or specific spectral selectors. Keywords: Surface plasmon; Au-double-slit; Polychromatic light; Near-field; Spectral switch
Background The study of surface plasmons has gained much attention since the discovery of optical transmission enhancement through subwavelength apertures in metal films [1], which can be explained with the excitation of surface plasmons by the incident optical field on a metal-dielectric interface. For a nanostructure metallic double-slit, these plasmon waves travel toward the slits and couple with the field directly transmitted through the slits. In this way, the spectra and even the spatial coherence can be modulated [2]. In the past, the spectral changes induced by normal aperture diffraction have been intensively studied, and an interesting phenomenon called ‘spectral switch’ was discovered [3]. Also, some related applications were suggested, such as lattice spectroscopy [4] or optical data transmission scheme [5]. Recently, the effect of surface plasmons with Au-double-slit for polychromatic light was studied in the far-field [6], which also showed the spectral switch and was controlled by an electro-optic setup. However, in order to enhance the signal intensity and to use this type of optical device in micro/nanoscale, it is worth investigating the plasmonic effects in the near-field (or the so-called Fresnel zone), which is the topic of this work. The results show that the behavior of near-field diffracted spectral intensity with plasmonic effect differs substantially from that without the effect.
Correspondence: [email protected] Graduate Institute of Precision Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan
Methods Consider an Au-double-slit with slit width 2b and silt distance 2d, as shown in Figure 1a, and a spatially fully coherent polychromatic field incident from the left, which is modeled as a Gaussian profile U ′ ðωÞ ¼ exp −½ðω−ωc Þ2 =2σ 2 ;
ð1Þ
where ωc is the center frequency and σ is the bandwidth. To excite surface plasmons along the gold-air interface, TM light polarized in x′ polarization as indicated in Figure 1b is used and the field coming out of each slit is U ′Au ðωÞ ¼ α þ αβexp ik sp ⋅ 2d ⋅ U ′ ðωÞ;
ð2Þ
where α is the fraction of field directly transmitted, αβ is the fraction converted into surface plasmons which travel to the other slit where they reappear as a free propagating field, and ksp is the surface p
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