Scattering of electrically excited surface plasmon polaritons by gold nanoparticles studied by optical interferometry with a scanning tunneling microscope

Tao Wang, Benoît Rogez, Geneviève Comtet, Eric Le Moal, Wafa Abidi, Hynd Remita, Gérald Dujardin, and Elizabeth Boer-Duchemin

Phys. Rev. B 92, 045438 – Published 31 July 2015

Abstract

We study the scattering of electrically excited surface plasmon polaritons (SPP) from individual nanostructures. The tunneling electrons from a scanning tunneling microscope (STM) are used to excite an out-going, circular SPP wave on a thin (50-nm) gold film on which isolated gold nanoparticles (NPs) have been deposited. Interaction of the excited SPPs with the NPs leads to both in-plane (SPP-to-SPP) and out-of-plane (SPP-to-photon) scattering. We use SPP leakage radiation microscopy to monitor the interference between the incident and in-plane scattered SPP waves in the image plane. By changing the location of the STM tip, the distance of the pointlike SPP source to the scatterers can be varied at will, which constitutes a key advantage over other existing techniques. As well, the out-of-plane scattered radiation interferes with the direct light emission from the STM tip in the back focal plane (Fourier plane). This confirms the mutual coherence of the light and SPP emission resulting from the inelastic tunneling of an electron in the STM junction. We use this effect to demonstrate that SPP-to-photon scattering at NPs is highly directional.

Received 9 March 2015
Revised 17 April 2015

DOI:https://doi.org/10.1103/PhysRevB.92.045438

Authors & Affiliations

Tao Wang^1,*, Benoît Rogez¹, Geneviève Comtet¹, Eric Le Moal^1,†, Wafa Abidi², Hynd Remita², Gérald Dujardin¹, and Elizabeth Boer-Duchemin¹

¹Institut des Sciences Moléculaires d'Orsay, CNRS–Université Paris-Sud (UMR 8214), Orsay, France
²Laboratoire de Chimie Physique, CNRS–Université Paris-Sud (UMR 8000), Orsay, France

^*Present address: Department of Chemistry, NUS 3 Science Drive 3, Singapore 117543.
^†Corresponding author: eric.le-moal@u-psud.fr

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Issue

Vol. 92, Iss. 4 — 15 July 2015

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Images

Figure 1
(a) Schematic of the experiment: gold nanoparticles (NPs) are dispersed on a thin gold film deposited on a glass coverslip. Inelastic electron tunneling from the STM tip to the gold film electrically excites surface plasmon polaritons (SPPs) that propagate isotropically away from the tunnel junction. SPP-to-SPP scattering at the gold NPs is imaged by leakage radiation microscopy (LRM), using an inverted optical microscope coupled to the STM. (b) Darkfield optical microscopy image of gold NPs dispersed on the gold film. This image is obtained using an air objective of numerical aperture $NA = 0.75$ upon illumination of the sample with a collimated white light beam at grazing incidence. (c) Atomic force microscopy (AFM) image of the same area as in (b). (d) Gray-scale optical LRM image of the same area during STM-SPP excitation, obtained using an oil-immersion objective of $NA = 1.45$ . The location of the STM tip on the gold film is indicated by the red arrow. (e) Same as in (d) but obtained using an air objective of $NA = 0.75$ . Such an objective cannot collect the SPP leakage radiation.
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Figure 2
In-plane SPP-to-SPP scattering by a single NP studied in real space. [(a) and (b)] Gray-scale optical LRM images of SPP scattering after STM-SPP excitation on a gold film for two different tip-NP separation distances $d$ (acquired using an oil-immersion objective of $NA = 1.45$ ). The location of the tip and NP are indicated by a red dot and a yellow arrow, respectively. The scattering angle $2 Ψ$ defines the angular aperture of the hyperbolic dark “shadow” that is seen after the NP. Image acquisition time is 120 s and the full broadband emission spectrum is used. The rings seen in these experimental images are due to the point spread function of the diffraction-limited microscope optics [42]. (c) Experimental and theoretical dependence of the scattering angle $2 Ψ$ on the tip-NP separation distance $d$ . (d) Theoretical (nearfield) distribution of the SPP electric field intensity on the gold film for $λ_{0} = 650$ nm, $d = 3.9 μ m$ , and $arg (α) = \frac{π}{2}$ . Theoretical data in (c) and (d) are calculated within the simple scalar model of a pointlike isotropic scatterer of effective polarizability $α$ [21].
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Figure 3
Out-of-plane SPP-to-photon scattering by a single NP studied in Fourier space. [(a) and (b)] Gray-scale optical LRM images measured in Fourier space for two different tip-NP distances (exposure time: 120 s, bandpass filter at $λ_{0} = 650$ nm). The red dashed arrow indicates the tip-to-NP direction. [(c) and (d)] Schematics of two possible scenarios that may explain the interference fringes observed in Fourier space (see the text for parameter definitions). [(e) and (f)] Experimental and theoretical dependence of (e) the fringe period, (f) maximal intensity (black squares), and visibility (magenta dots) on the tip-NP separation $d$ . The distance dependence of $I_{\max}$ validates scenario 1 and rules out scenario 2.
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