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

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 outgoing , 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.

Coherence Between Different Propagating Surface Plasmon Polariton Modes Excited by Quantum Mechanical Tunnel Junctions

“…[14] Since then both photon and SPP emission have been reported from various macroscopic MIM tunneling devices including scanning tunneling microscope (STM), [26] molecular junctions, [27] metallic break junctions, [28] nanowire-metal stripe junctions, [29] and metal-2D material hybrid systems. [30] In the past, modulation of the spatial coherence of light by plasmonic near-field has been demonstrated in Young's interference experiments, [31,32] in metal stripe waveguides supporting SPPs [9] and also in the context of inelastic tunneling, with STM light emission, [33,34] where the localized plasmonic response of the STM tip [35] plays a crucial role. For large area TJs, [18] the lack of correlation between inelastic tunneling events typically results in spatially incoherent and spectrally broad (lack of temporal coherence) photon or plasmon emission.…”

An electrically induced probe of the modes of a plasmonic multilayer stack

“…Combining our technique with polarimetry would lead to the characterization of the optical response of chiral nano and microstructures [70]. Beyond optical mode characterization, our technique may be used as an interferometric tool to determine the phase in the optical response of nanostructures, as introduced in [42]. It is also a valuable tool for the testing and design of integrated plasmonic microstructures in view of their integration in optoelectronic microdevices, as proposed in [44].…”

“…Over the last decade, the combination of ambient STM with optical microscopy has been increasingly applied to experiments in nano-optics [28][29][30][31][33][34][35][36][37][38][39][40][41][42][43][44][45]. Most recently, this technique has been used to study the propagation of surface plasmon polaritons (SPPs) in 2D plasmonic crystals [43] and the spectral response of plasmonic lenses [44]; however, the measurement of the dispersion relation of the optical modes in a plasmonic system using this technique has never been reported before.…”

Decay channels of gap plasmons in STM tunnel junctions

“…Recently, it was found that the gap mode can also excite the surface plasmon polaritons (SPPs) propagating along the metal surface 31,32 . This provides an efficient, local, electrical way of launching SPPs in optical structures, and its applications received considerable attention recently [33][34][35][36][37][38] . These results suggest that the electrical excited gap plasmon modes have several optical decay channels.…”