The role of tip plasmons in near-field Raman microscopy

Enhancing the magneto-optical Kerr effect through the use of a plasmonic antenna

We employ an extended finite-element model as a design tool capable of incorporating the interaction between plasmonic antennas and magneto-optical effects, specifically the magneto-optical Kerr effect (MOKE). We first test our model in the absence of an antenna and show that for a semi-infinite thin-film, good agreement is obtained between our finite-element model and analytical calculations. The addition of a plasmonic antenna is shown to yield a wavelength dependent enhancement of the MOKE. The antenna geometry and its separation from the magnetic material are found to impact the strength of the observed MOKE signal, as well as the antenna's resonance wavelength. Through optimization of these parameters we achieved a MOKE enhancement of more than 100 when compared to a magnetic film alone. These initial results show that our modeling methodology offers a tool to guide the future fabrication of hybrid plasmonic magneto-optical devices and plasmonic antennas for magneto-optical sensing.

Signal enhancement in nano-Raman spectroscopy by gold caps on silicon nanowires obtained by vapour-liquid-solid growth

Silicon nanowires grown by the vapour-liquid-solid growth mechanism with gold as the catalyst show gold caps approximately 50-400 nm in diameter with an almost ideal hemispherical shape atop a silicon column. These gold caps are extremely well suited for exploiting the tip or surface enhanced Raman scattering effects since they assume the right size on the nanometre scale and a reproducible, almost ideal hemispherical shape. On attaching a nanowire with a gold cap to an atomic force microscopy (AFM) tip, the signal enhancement by the gold nanoparticle can be used to spatially resolve a Raman signal. Applications of this novel nanowire based technical tip enhanced Raman scattering solution are widespread and lie in the fields of biomedical and life sciences as well as security (e.g. detection of bacteria and explosives) and in the field of solid state research, e.g. in silicon technology where the material composition, doping, crystal orientation and lattice strain can be probed by Raman spectroscopy. A prerequisite for obtaining this spatial resolution in nano-Raman spectroscopy is the attachment of a nanowire with a gold cap to an AFM tip. This attachment by welding a nanowire in a scanning electron microscope to an AFM tip is demonstrated in this paper.

Local-field enhancement in an optical force metallic nanotrap: application to single-molecule spectroscopy

We study the local-field enhancement in a nanocavity created by optical nanomanipulation. Recently we showed that a metallic probe can modify the optical force experienced by a metallic particle and generate a material selective trapping potential. We show that the same configuration used for optical forces can be used to control both in magnitude and tune the local-field enhancement around the particle at resonance. The spatial resolution and material selectivity of this technique, allied to its capability to manipulate particles at the nanometric level, may offer a new and versatile way to achieve surface-enhanced Raman scattering spectroscopy at the single-molecule level.

Finite Element Simulations of Tip-Enhanced Raman and Fluorescence Spectroscopy

Finite element electromagnetic simulations of scanning probe microscopy tips and substrates are presented. The enhancement of the scattered light intensity is found to be as high as 10(12) for a 20 nm radius gold tip, and tip-substrate separation of 1 nm. Molecular resolution imaging (< 1 nm) is achievable, even with a relatively large radius tip (20 nm). We also make predictions for imaging in aqueous environments, noting a sizable red shift of the spectral peaks. Finally, we discuss signal levels, and predict that high-speed Raman mapping should be possible with gold substrates and a small tip-substrate separation (< 4 nm).

Field-enhanced scanning near-field optical microscopy

This manuscript reviews the principles and recent advances of scanning near-field optical microscopy based on tip-induced field enhancement. These scanning microscopes utilize minute probes to locally enhance an electromagnetic field through a complex interplay between surface plasmon excitation and localization of electric charges by geometrical singularities. The necessary conditions leading to an electromagnetic enhancement will be reviewed, as well as the means to characterize it. A brief account of the theoretical framework will be given, together with applications of the technique ranging from chemical imaging to nanolithography.

Electromagnetic Coupling in Near-Field Scattering by Small Homogeneous and Heterogeneous Nanoaggregates

Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role played by particle-particle interactions and a deeper insight of the influence of the incident field wavelength. This is particularly true for scanning near-field optical microscopy (SNOM), where the mechanism by which some components of the evanescent illuminating field are transformed into propagating field components that carry information about the sample is at the core of the image formation and where the role played by the interactions between sample and tip remains a still open problem. In this perspective, we investigate numerically the optical behavior of small aggregates of spherical nanoparticles, taking into account the electromagnetic coupling between all particles and the apertureless tip. The tip is modeled as a sphere made of different materials characterized by appropriate dielectric functions. We find that the tip material affects both qualitatively and quantitatively the SNOM images; more important, from the analysis of the calculated scattering cross section, the resonance plasmon location of the whole (aggregate + tip) system undergoes detectable changes, if the tip is constituted of the same material of the sample, as the tip is situated in different positions. This modification of the plasmon frequencies induces a nontrivial variation of the near-field intensity as a function of the tip position and the resulting SNOM image can be distorted with respect to the actual shape of the sample. No simple arguments can be used to relate the value of the local field on the tip surface to the scattering cross section value; depending on the tip material, the comparison between these two measurements can help to clarify the role of basic interactions in the scattering mechanism.

Effect of Sample and Substrate Electric Properties on the Electric Field Enhancement at the Apex of SPM Nanotips

Finite element (FE) models were built to define the optimal experimental conditions for tip-enhanced Raman spectroscopy (TERS) of thin samples. TERS experimental conditions were mimicked by including in the FE models dielectric or metallic substrates with thin dielectric samples and by considering the wavelength dependence of the dielectric properties for the metallic materials. Electromagnetic coupling between the substrate/sample and the SPM tips led to dramatic changes of both the spatial distribution and magnitude of the scattered electric field which depended on the substrate dielectric permittivity and excitation wavelength. Raman scattering as high as 10(8) with a spatial resolution of approximately 8 nm was estimated for gold SPM tips and gold substrate when excitation is performed at 532 nm (near-resonance wavelength). For dielectric samples (approximately 4 nm thick), the enhancement of Raman scattering intensity is estimated at approximately 10(5); this does not depend significantly on the sample dielectric permittivity for dielectric samples. These results suggest that TERS experimental conditions should be estimated and optimized for every individual application considering the geometric factors and electric properties of the materials involved. Such optimizations could enlarge the range of applications for TERS to samples eliciting weaker intrinsic Raman scattering, such as biological samples.

Plasmon resonances on metal tips: Understanding tip-enhanced Raman scattering

Calculations of the electric-field enhancements in the vicinity of an illuminated silver tip, modeled using a Drude dielectric response, have been performed using the finite difference time domain method. Tip-induced field enhancements, of application in "apertureless" Raman scanning near-field optical microscopy (SNOM), result from the resonant excitation of plasmons on the metal tip. The sharpness of the plasmon resonance spectrum and the highly localized nature of these modes impose conditions to better exploit tip plasmons in tip-enhanced apertureless SNOM. The effect of tip-to-substrate separation and polarization on the resolution and enhancement are analyzed, with emphasis on the different field components parallel and perpendicular to the substrate.

Resonant excitation of tip plasmons for tip-enhanced Raman SNOM

The conditions necessary for the optimisation of tip-enhanced scanning near-field optical microscopy have been determined. The Raman scattering efficiency can be enormously increased by enhancements in the local field amplitude, such as that which can occur in the vicinity of a metallic nanostructure. The field enhancement in the vicinity of a silver tip is investigated theoretically here using the finite difference time domain method. Field enhancements from electron oscillations on the tip are shown to display strong maxima at resonant illumination wavelengths and the nature of these enhancements at the substrate surface beneath the tip, both on and off resonance, has been calculated. The enhancement of the Raman signal on the surface decreases exponentially as the tip-substrate separation is increased and a peak Raman enhancement of 10(7) is theoretically achievable at a tip-surface separation of 2 nm. The resolution is also strongly related to the distance between the tip and the substrate surface narrowing to <7 nm, significantly smaller than the radius of curvature of the end of the tip.

Near-field microscopy: throwing light on the nanoworld

Optical microscopy with nanoscale resolution, beyond that which is possible with conventional diffraction-limited microscopy, may be achieved by scanning a nanoantenna in close proximity to a sample surface. This review will first aim to provide an overview of the basic principles of this technique of scanning near-field optical microscopy (SNOM), before moving on to consider the most widely implemented form of this microscopy, in which the sample is illuminated through a small aperture held less than 10 nm from the sample surface for optical imaging with a resolution of ca. 50 nm. As an example of the application of this microscopy, the results of SNOM measurements of light-emitting polymer nanostructures are presented. In particular, SNOM enables the unambiguous identification of the different phases present in the nanostructures, through the local analysis of the fluorescence from the polymers. The exciting new possibilities for high-resolution optical microscopy and spectroscopy promised by apertureless SNOM techniques are also considered. Apertureless SNOM may involve local scattering of light from a sample surface by a tip, local enhancement of an optical signal by a metal tip, or the use of a fluorescent molecule or nanoparticle attached to a tip as a local optical probe of a surface. These new optical nanoprobes offer the promise of optical microscopy with true nanometre spatial resolution.