Enhancement of spatial coherence by surface plasmons

Abstract

We report on a method to generate a stationary interference pattern from two independent optical sources, each illuminating a single slit in Young's interference experiment. The pattern arises as a result of the action of surface plasmons traveling between subwavelength slits milled in a metal film. with E the complex amplitude of the field, assumed here to be scalar; P 1 and P 2 denote the positions of the slits, a delay time, and the brackets a time average. For our purpose it is useful to employ the normalized mutual coherence function (the so-called complex degree of coherence), defined aswhere I͑P i ͒ is the averaged intensity at slit i. Under typical circumstances, the visibility V of the interference fringes near a point P in the far zone is equal to the modulus of the complex degree of coherence, i.e.,with equal to the time difference ͑P 1 P − P 2 P͒ / c, c being the speed of light in air. If one slit is illuminated by a light source radiating at frequency 1 while the other slit is illuminated by a separate source radiating at frequency 2 , it is easily seen that then ␥͑P 1 , P 2 , ͒ = 0. Under these illumination conditions the fringe visibility should thus be zero across the entire interference pattern for sufficiently long integration times. In this line of reasoning it is assumed that the radiative field emerging from a slit is simply, up to some factor, equal to the radiative field incident on that slit. When surface plasmons propagate between the two slits this assumption is no longer valid. 4,5 Consequently, a stationary interference pattern should be observed even if the frequencies of the lasers illuminating the individual slits are very different. Here we confirm this idea in an experiment where the two lasers run at frequencies differing by as much as 1.8 THz. Furthermore, we show that an interference pattern is also observed when only one slit is illuminated. When the polarization of the incident light is chosen such that no surface plasmons can be excited, the stationary interference pattern is observed to be absent.The experimental setup is shown in Fig. 1. Two separate lasers, a tunable narrowband Ti:sapphire laser and a semiconductor diode laser operating at 812 nm, each illuminate a single subwavelength slit in a 200 nm thick gold film. Each laser is focused to a spot of approximately 5 m FWHM. The two parallel slits, ϳ25 m apart, are 50 m long and 0.2 m wide. The gold film is evaporated on top of a 0.5 mm thick fused-quartz substrate with a 10 nm thick titanium adhesion layer between the gold and the quartz. A CCD camera is used to record the far-field pattern. The outputs of a fiber-coupled diode and a Ti:sapphire laser are individually focused on one of a pair of 200 nm wide slits, separated by Ϸ25 m, in a thin gold film. The light diffracted at the two parallel slits is imaged onto a CCD camera. A, attenuator; M, mirror; BS, beam splitter; /2, half-wave plate; P, polarizer; L, lens; S, gold sample. The inset shows the illumination of the double slit.

A microscopic view of the electromagnetic properties of sub-λ metallic surfaces

“…The later acts as a secondary source that produces fringes by interference with the light scattered by the first slit in the far field. Indeed the SPP multiple-scattering model [17][18][19][20][21][22][23][24] is conceptually powerful and represents a microscopic vision that helps in the design of compact plasmonic devices [25][26][27][28][29][30][31][32][33]. The question arises of how we model the slit doublet experiment in practice: how do we calculate the far-field response, for instance?…”

Near-field Characterization on Light Emanated from Subwavelength Plasmonic Double Slit of Finite Length

“…This experiment also inspired the development of the particle-wave duality [3], which became again a trigger for building up the concepts in quantum mechanics. Although this experiment was initially performed more than two centuries ago, even nowadays, intensive research activity on double slit interference is still in progress to explore some novel physical phenomena, including for instance, the light interference characteristics due to a coherence degree of two light sources [2,[4][5][6][7][8], analyses of geometrical effects leading to enhanced light transmission and propagation [9][10][11]. Recent significant attention to the surface plasmons [12] has been also related to the double slit interference.…”

“…Recent significant attention to the surface plasmons [12] has been also related to the double slit interference. The interference pattern produced by the double slit in a plasmonic metal film is quite different from the traditional ones due to the surface plasmon generation and localization near the metal surface at optical wavelengths [2,[4][5][6][7][13][14][15]. While the plasmonic double slits have also been utilized for novel subwavelength optics applications such as unidirectional surface plasmon excitation [16,17], optical beam manipulations [18,19], and sub-diffraction limited optical spot generation in the intermediate field region [20,21], the studies on the a subwavelength double slit are still inactive compared with those on the single slit structures.…”

Coherence-converting plasmonic hole arrays

“…Recently, through a series of investigations based on Young's double slit configuration, it has been found that surface plasmons play a central role in not only the interference and transmission of light but also the coherence properties of the transmitted light [10,11]. Specifically, the presence of surface plasmons in the double-slit system leads to the modulation of the spatial coherence of light that emerges from the slits, and the resulting coherence can be higher or lower than the coherence of the illuminating field.…”