Plasmonic Nanofocusing Spectroscopy
New Optical Microscopes Enable Spectroscopy with Few Nanometer Resolution
- Fig. 1: Plasmonic nanofocusing spectroscopy (PNS) (a) A broadband laser is coupled to surface plasmon polariton waves on a gold nanocone. This creates a spatially isolated, point-like light source at the taper apex, which is used to excite and probe optical near fields of the sample. (b) Scanning electron microscope image of the conical gold taper and its apex. (c) Light scattering from the taper showing the bright light spot at the taper apex.
- Fig. 2: (a) Topography and (b) light scattering image of a single, 40 nm x 10 nm-sized gold nanorod recorded at a wavelength of 800 nm. (c) Cross section through the optical image showing a spatial resolution of 5nm.
- Fig. 3: Control of optical near-field coupling on the nanometer scale. (a) Distance-dependent light scattering spectra recorded along the left dashed line in panel (b). (b) Vertical cross-section through the near-fields around a single nanoparticle concentrated at the two apices. (c) Same as (a) but taken along the right dashed line in (b). With decreasing distance, the coupling between tip apex and particle resonance alters the light scattering spectra. (d) A look into the experimental setup.
Ideally, we would not only want to probe the structure of nanoparticles but also the dynamics of electrons and nuclei in such particles, for example, to follow the harvesting of light in photosynthesis, the light-induced transfer of charge in photovoltaics, or the ultrafast switching of spins in magnetic nanosystems. These are formidable and unresolved research questions requiring spectroscopic studies with nanometer-sized spatial and femtosecond temporal resolution. One possible approach for providing this information relies on sensing the dynamics of optical near fields emitted by the moving electrons and ions in these systems.
Conceptually, one of the techniques that can in principle reach such a combined space-time imaging of optical near fields is near-field scanning optical microscopy (SNOM).
It replaces the focused light spot of a glass objective with light that is scattered from the apex of a sharp metallic or dielectric conical taper. This reduces the light spot to a size given by the apex diameter of typically 10 nm or even below. In combination with broadband coherent laser sources, SNOM can provide true nanometer scale spatial resolution imaging of coherent optical properties in nanostructured matter. While in principle, fluorescence-based near- and far-field techniques (STED, STORM, PALM, …) can also yield nanometer resolution by monitoring incoherent optical excitations, optical fields and their coherent dynamics can only be probed by resonant linear or nonlinear light scattering techniques.
A major challenge for currently established resonant scattering SNOM techniques lies in the proper suppression of background fields. Usually, the near field at the taper apex is created by illumination with a diffraction-limited light spot. Since the size of the taper apex is much smaller than the spot size, the scattered light is dominated by a strong far-field background and the near-field scattering makes only a small contribution to the total detected signal. Most current near-field microscopes therefore use advanced background suppression and interferometric detection techniques to filter out the weak near-field signal. This, however, limits the sensitivity of the measurement technique and makes the image acquisition and interpretation complicated. To fully unfold the potential of this technique, it therefore appears desirable to eliminate this unwanted scattering background and to create a freestanding and spatially isolated broadband coherent light source.
Some 15 years ago, the Russian-American physicist Mark Stockman proposed to consider a new type of metallic superlens, a simple gold cone with a sharply pointed tip . Electromagnetic waves that are bound to the surface of the cone, so called surface plasmon polaritons, would be focused all the way to the very apex of the tip, to a spot size that is limited only by the apex diameter, ideally even down to the size of a single atom. A sketch of an experimental implementation of this idea is depicted in figure 1(a). A grating coupler, placed on the shaft of the gold taper, is illuminated by a broadband white-light laser source and used to launch evanescent surface plasmon polariton waves. Their focusing by the conical taper creates a bright and spatially isolated light spot at its very apex. Conventional scanning probe techniques can then be used to raster scan the light spot across the surface of a sample. This light spot creates optical near fields in the nanostructures that are placed on the surface. This near-field light is scattered into the far field by the sharp gold tip and is spectrally resolved in a monochromator followed by detection with a CCD camera.
A first experimental demonstration of this plasmonic nanofocusing concept was reported in 2007 . It created substantial excitement in the community and several groups, in particular those of Markus Raschke  and Enzo di Fabrizio , have since then used this technique for various applications, in particular for tip-enhanced Raman  and nonlinear optical spectroscopy [6-9]. Despite substantial efforts, however, a finite amount of background scattering prevented its direct use for resonant light scattering SNOM. A substantial technological advance was reached in 2012, when single-crystalline gold wires for preparing the conical tapers were introduced by wet-chemical etching . It turned out (fig. 1b) that this results in particularly smooth taper surfaces which are helpful in transporting plasmons over long distances with much reduced scattering losses during plasmonic nanofocusing. With this, it indeed became possible to reproducibly create bright and spatially isolated nanometer-sized light spots at the apex of those tapers as depicted in figure 1(c).
Based on these advances, recently a new microscope has been implemented that enabled us to report the first resonant light scattering spectra of individual nanoparticles recorded by plasmonic nanofocusing . In proof-of-principle experiments, a broadband supercontinuum laser was used as a light source to study the spectra of individual gold nanoparticles – small nanorods of 40 nm length and 10 nm diameter. Typical results of such a raster scan are shown in figure 2. The optical fields at the nanoparticle surface are imaged with a very high spatial resolution of better than 5 nanometers. The presence of the nanoparticle results in a drastic reduction in light scattering from the apex by more than 35% in the absence of any background suppression. This indeed demonstrates the creation of an isolated, dipolar light spot at the taper apex. The scattering signal is reduced since such small nanoparticles mainly transform the incident light into heat. Importantly, the scattering signal from the apex is so bright that now entire light scattering spectra can be recorded within only 20 milliseconds. Such spectra are reported in figure 3 and show the excitation of the localized surface plasmon resonance of the rod at an energy of about 1.52 eV. They have been recorded while scanning the tip at variable distance across the nanorod. The amplitude of the rod resonance in the spectra decreases substantially when moving the tip only a few nanometers away from the nanorod. This demonstrates the short extent of the optical near field that is surrounding the nanorod. It also shows how difficult it is to sense these short-ranged near fields. With our sharp scattering tips, we can, however, see sense these short-ranged fields with high accuracy. Not only the amplitude of the spectra increases but also the line shape changes substantially when approaching the tip to the nanoparticle. These line shape changes reflect the near-field coupling between tip and nanorod. By varying the distance, the near-field coupling between tip and sample is controlled – that is, one can exactly monitor and dictate how many times near-field photons are exchanged between tip and nanorod. Most interesting are the color shifts of the plasmon resonance that can be seen when approaching the tip to the sample. They arise from an unexpected vectorial coupling between rod and tip: The near fields of the rod are not only inducing oscillations of the free electrons in the gold tip in the direction along the taper axis but also in the direction perpendicular to it.
The results show that plasmonic nanofocusing is a conceptually intriguing and powerful technique for creating a unique, bright and broadband nanoscopic light source. It holds substantial potential for sensing local optical near fields, probing quantum mechanical couplings in hybrid nanosystems and – in combination with ultrafast light sources - imaging the flow of energy and charge on the nanoscale. To this end, an automated fabrication of the sensitive nanofocusing tapers and their implementation in a commercial scanning probe microscope seems particularly desirable.
We wish to thank all members of the Ultrafast Nano-Optics group in Oldenburg for fruitful discussion. We are particularly indepted to Raimond Angermann, Simon F. Becker, Jens Brauer, Abbas Chimeh, Petra Groß, Anke Korte, Ralf Vogelgesang, Jinxin Zhan and Jinhui Zhong for their contributions to this work. We wish to thank the Deutsche Forschungsgemeinschaft (SPP1839, SPP1840) and the Studienstiftung des Deutschen Volkes for financial support.
Martin Esmann1 and Christoph Lienau2
1Center for Nanoscience and Nanotechnology (C2N), CNRS, Université Paris-Saclay, Universi-té Paris-Sud, Palaiseau, France
2Institute of Physics and Center of Interface Science, Carl von Ossietzky University, Olden-burg, Germany
Carl von Ossietzky University Oldenburg
Institute of Physics
Department for Ultrafast Nano-Optics
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