Near-field and confocal surface-enhanced resonance Raman spectroscopy at cryogenic temperatures

For laser spectroscopy at variable temperatures with high spatial resolution a combined scanning near‐field optical and confocal microscope was developed. Rhodamine 6G (R6G) dye molecules dispersed on silver nano‐particles or nano‐clusters were investigated. For optical excitation of the molecules, either an aperture probe or a focused laser spot in confocal arrangement were employed. Raman spectra in the wavenumber range between 300 cm^?1 and 3000 cm^?1 at room temperatures down to 8.5 K were recorded. Many of the observed Raman lines can be associated with the structure of the adsorbed molecule. Intensity fluctuations in spectral sequences were observed down to 77 K and are indicative of single molecule sensitivity.     

Photo-initiated energy transfer in nanostructured complexes observed by near-field optical microscopy

We report an apertureless near‐field optical study on nanostructured objects formed by J‐aggregates adsorbed on silver (Ag) nanoparticles. Near‐field images reveal that the enhanced near‐field from the dressed particle's (DP) resonantly excited plasmon oscillation is efficiently absorbed by the J‐aggregates. The sensitivity of the near‐field images recorded at the harmonics of the probe vibration frequency suggests that the DP is releasing part of the absorbed energy radiatively upon interaction with the probe. The role of the probe in providing this new radiative relaxation channel is further confirmed as fluorescence from the J‐aggregates on the particle is detected on the particle location only. We based the interpretation of our results on the near‐field optical response from a bare Ag particle excited at the plasmon resonance as well as on far‐field emission and transient absorption experiments.     

Near-field imaging of surface plasmon on gold nano-dots fabricated by scanning probe lithography

We obtained scanning near‐field optical microscopy images to study the excitation of surface plasmons on metallic dots fabricated using scanning probe lithography. Gold nano‐dots were fabricated by applying electric voltages to conducting probes installed in an atomic force microscope using the mechanism of field‐induced diffusion and nano‐oxidation plus Au‐coating. High spatial resolution of scanning near‐field optical microscopy revealed a ‘bifold’ pattern of surface plasmon mode on fabricated Au dots in the polarization direction of incident light. We found that scanning near‐field optical microscopy imaging combined with scanning probe lithography is able to provide a systematic study of surface plasmon excitation on nano‐metallic structures.     

Near-field Raman spectroscopy using a sharp metal tip

Near‐field Raman spectroscopy with a spatial resolution of 20 nm is demonstrated by raster scanning a sharp metal tip over the sample surface. The method is used to image vibrational modes of single‐walled carbon nanotubes. By combining optical and topographical signals rendered by the single‐walled carbon nanotubes, we can separate near‐field and far‐field contributions and quantify the observed Raman enhancement factors.     

Apertureless near-field Raman spectroscopy

We report on the tip‐enhanced Raman spectra of C60 obtained on a custom‐built apertureless scanning near‐field optical microscope. A commercial atomic force microscope tip coated with 100 nm thickness of gold was used to enhance locally the Raman signal and permit topographic and spectral information to be acquired simultaneously. We present preliminary data which demonstrate the tip enhancement effect using C60 as a test sample.     

Experimental statistics of near-field intensity distributions at nanostructured surfaces

Scanning near‐field optical microscopy is a technique in which the resolution is primarily determined by the size of a probe and not by the wavelength of illumination as in classical (far‐field) microscopy. However, the relationship between a sample and its near‐field optical image is usually rather complex. Typical factors responsible, at least partially, for such a complexity are the conditions of illumination and detection, sample characteristics (e.g. roughness and dielectric constant) and optical properties of the probe. Theoretical and experimental works conducted to improve our understanding of the relation between the object and the image have been reported ( Greffet & Carminati, 1997 ). Recently, with the help of a photon scanning tunnelling microscope we have carried out an extensive study of the resultant near‐field intensity distributions due to the elastic (in the plane) scattering of surface plasmon polaritons (SPPs) at metal film surfaces. We have also directly observed (in similar experimental conditions) localized dipolar excitations in silver colloid fractals ( Bozhevolnyi et al., 1998 ). In both cases, the studied phenomena are intimately related to the regime of multiple light scattering, in which the interference effects are rather complicated and therefore a proper interpretation of them was far from being trivial. Thus, even though a certain understanding of many features inherent to the subwavelength light interference phenomena was gained ( Bozhevolnyi & Coello, 1998 ; Bozhevolnyi et al., 1998 ; Coello & Bozhevolnyi, 1999 ), it is clear from the outcome of the investigations that more systematic studies in this context are still needed. A different and more powerful approach may be a statistical study of the recorded near‐field intensity distributions. In this work, we report what we believe to be the first results on experimental statistics of near‐field optical images exhibiting localized optical excitations (related to the regime of multiple scattering of light). We investigated optical images obtained with SPPs excited at different light wavelengths and scattered at different film surfaces, and with different polarizations and wavelengths of light scattered by silver colloid fractal structures. We have found significant differences in statistics between near‐field intensity distributions taken at rough and smooth metal film surfaces and fractal structures. Finally, our predictions seem to be in agreement with theoretical studies reported by other authors ( Sanchez‐Gil & Garcia‐Ramos, 1998 ).     

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips

The fabrication of silicon cantilever‐based scanning near‐field optical microscope probes with fully aluminium‐coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic‐ and contact‐mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone‐angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near‐field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near‐field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.     

Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50–100 nm by scanning near-field optical microscopy

Scanning near‐field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T‐tip) show a distribution of dark and bright spots at distances in the order of 25–50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near‐field optical microscopy images using a dipole serving as a light‐emitting scanning near‐field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50–100 nm in metal nanostructures.     

Non-optical bimorph-based tapping-mode force sensing method for scanning near-field optical microscopy

A non‐optical bimorph‐based tapping‐mode force sensing method for tip–sample distance control in scanning near‐field optical microscopy is developed. Tapping‐mode force sensing is accomplished by use of a suitable piezoelectric bimorph cantilever, attaching an optical fibre tip to the extremity of the cantilever free end and fixing the guiding portion of the fibre to a stationary part near the tip to decouple it from the cantilever. This method is mainly characterized by the use of a bimorph, which carries out simultaneous excitation and detection of mechanical vibration at its resonance frequency owing to piezoelectric and anti‐piezoelectric effects, resulting in simplicity, compactness, ease of implementation and lack of parasitic optical background. In conjugation with a commercially available SPM controller, tapping‐mode images of various samples, such as gratings, human breast adenocarcinoma cells, red blood cells and a close‐packed layer of 220‐nm polystyrene spheres, have been obtained. Furthermore, topographic and near‐field optical images of a layer of polystyrene spheres have also been taken simultaneously. The results suggest that the tapping‐mode set‐up described here is reliable and sensitive, and shows promise for biological applications.     

Current-sensing scanning near-field optical microscopy using a metal probe for nanometre-scale observation of electrochromic films

A novel technique for scanning near‐field optical microscopy capable of point‐contact current‐sensing was developed in order to investigate the nanometre‐scale optical and electrical properties of electrochromic materials. An apertureless bent‐metal probe was fabricated in order to detect optical and current signals at a local point on the electrochromic films. The near‐field optical properties could be observed using the local field enhancement effect generated at the edge of the metal probe under p‐polarized laser illumination. With regard to electrical properties, current signal could be detected with the metal probe connected to a high‐sensitive current amplifier. Using the current‐sensing scanning near‐field optical microscopy, the surface topography, optical and current images of coloured WO₃ thin films were observed simultaneously. Furthermore, nanometre‐scale electrochromic modification of local bleaching could be performed using the current‐sensing scanning near‐field optical microscopy. The current‐sensing scanning near‐field optical microscopy has potential use in various fields of nanometre‐scale optoelectronics.     

Near-field imaging of organic nanofibres

The optical near‐field of orientated nanofibres of para‐hexaphenyl was investigated by a combination of localized far‐field ultraviolet excitation and scanning near‐field fluorescence detection. Morphological inhomogeneities of the nanofibres together with selective incoupling into the near‐field probe resulted in a detection sensitivity was dependent on individual nanoaggregates. Strongest near‐field intensity and radiation into the far‐field were observed at the crossing points of nanofibres. The near‐field observation of a distance‐dependent damping of the waveguiding along the nanofibres allowed us to determine the imaginary part of the dielectric function of the nanofibres, in quantitative agreement with far‐field optical data.     

Microfabrication of a combined AFM-SNOM sensor

The objective of this work is to fabricate a scanning probe sensor that combines the well-established method for atomic force microscopy, employing a micro-machined Si cantilever and integrated tip, with a probe for the optical near field. A photosensitive pn-junction is integrated into the tip for that purpose and an Al coating is applied to the tip. It comprises an aperture of 50-70 nm in diameter at the apex of the tip in order to spatially limit the interaction of the tip to the optical near field of the sample. Characterization of the tip and first results of simultaneously recorded force and photon images are presented.     

Scanning probe microscopies applied to the study of the domain wall in a ferroelectric crystal

Scanning near‐field optical microscopy is capable of measuring the topography and optical signals at the same time. This fact makes this technique a valuable tool in the study of materials at nanometric scale and, in particular, of ferroelectric materials, as it permits the study of their domains structure without the need of chemical etching and, therefore, not damaging the surface (as will be demonstrated later). We have measured the scanning near‐field optical microscopy transmission, as well as the topography, of an RbTiOPO₄ single crystalline slab, which exhibits two different of macroscopic ferroelectric domains. A chemical selective etching has been performed to distinguish between them, obtaining areas with a noticeable roughness (C⁻ domain) in comparison with the original flat aspect of the other ones (C⁺ domain). The effects of the selective chemical etching have been quantified in topographic images obtained by means of our fibre tip probe, and have been compared to topographic images obtained by Atomic Force Microscopy, with a better resolution. The near‐field optical transmission images recorded have been obtained under different excitation wavelengths. These images are modulated by the light scattering due to the grains at the rough surface, which depends on the excitation wavelength used. In addition, they show a significant optical contrast due to the variations of the dielectric constant on the proximity of the ferroelectric domain wall.     

SPP tomography: A simple wide‐field nanoscope

We explore the wide‐field optical nanoimaging capabilities of the surface plasmon polariton (SPP) tomography technique. We show that nanofeatures with lateral dimensions smaller than λ/20 can be observed in the surface emission (SE) images of plasmonic crystals with a period of 300 nm. Moreover, as a proof‐of‐concept, we demonstrate that SPP tomography permits to resolve two single objects with a center‐to‐center separation of 200 nm and edge‐to‐edge separation as small as λ/7. We present a comprehensive discussion about the nanoimaging capabilities of the SPP tomography technique. In contrast to other optical subwavelength resolution techniques, in our approach for imaging nanosize features, enhanced evanescent waves are coupled to the far‐field via leakage radiation associated with SPPs excited by near‐field fluorescence; therefore wide‐field images, which are not out‐of‐plane diffraction‐limited, are formed directly in the microscope's camera. We also discuss additional imaging processing capabilities associated with the fact that SPP tomography SE images are formed by the microscope lenses through an analog tomography process. SCANNING 35: 246‐252, 2013. © 2012 Wiley Periodicals, Inc.     

Magnetic characterization of microscopic particles by MO-SNOM

This paper reports on the development of a magneto‐optical scanning near‐field optical microscope and the experimental near‐field study of the domain structure for a model magnetic particle of 16 × 16 µm² of a Co_70.4Fe_4.6Si₁₅B₁₀ amorphous thin film, deposited on a silicon substrate. We present the topographic, optical and magneto‐optical differential susceptibility (MODS) images of the particle. Imaging by using the local MODS reveals the domain structure. These images are also used for positioning the tip in order to acquire local hysteresis loops, with submicrometre spatial resolution.     

Near-field observation of carrier diffusion in GaAs quantum structures under high magnetic fields

Photoluminescence from a two‐dimensional electron‐gas system in GaAs single hetero‐structures was investigated using a scanning near‐field optical microscope (SNOM) operated at cryogenic temperatures under high magnetic fields. The local intensity of the luminescence increased 600‐fold that at 0 T as the magnetic field was increased up to 6 T. The enhancement depended on the spatial resolution of the SNOM. These characteristics are explained by the suppression of the diffusion of photocarriers caused by the Lorentz force in magnetic fields.     

Harmonic demodulation and minimum enhancement factors in field‐enhanced near‐field optical microscopy

Field‐enhanced scanning optical microscopy relies on the design and fabrication of plasmonic probes which had to provide optical and chemical contrast at the nanoscale. In order to do so, the scattering containing the near‐field information recorded in a field‐enhanced scanning optical microscopy experiment, has to surpass the background light, always present due to multiple interferences between the macroscopic probe and sample. In this work, we show that when the probe–sample distance is modulated with very low amplitude, the higher the harmonic demodulation is, the better the ratio between the near‐field signal and the interferometric background results. The choice of working at a given n harmonic is dictated by the experiment when the signal at the n + 1 harmonic goes below the experimental noise. We demonstrate that the optical contrast comes from the nth derivative of the near‐field scattering, amplified by the interferometric background. By modelling the far and near field we calculate the probe–sample approach curves, which fit very well the experimental ones. After taking a great amount of experimental data for different probes and samples, we conclude with a table of the minimum enhancement factors needed to have optical contrast with field‐enhanced scanning optical microscopy.     

On the influence of lipid‐induced optical anisotropy for the bioimaging of exo‐ or endocytosis with interference microscopic imaging

Some implementations of interference microscopy imaging use digital holographic measurements of complex scattered fields to reconstruct three‐dimensional refractive index maps of weakly scattering, semi‐transparent objects, frequently encountered in biological investigations. Reconstruction occurs through application of the object scattering potential which assumes an isotropic refractive index throughout the object. Here, we demonstrate that this assumption can in some circumstances be invalid for biological imaging due to the presence of lipid‐induced optical anisotropy. We show that the nanoscale organization of lipids in the observation of cellular endocytosis with polarized light induces a significant change in far‐field scattering. We obtain this result by presenting a general solution to Maxwell's equations describing light scattering of core–shell particles near an isotropic substrate covered with an anisotropic thin film. This solution is based on an extension of the Bobbert–Vlieger solution for particle scattering near a substrate delivering an exact solution to the scattering problem in the near field as well as far field. By applying this solution to study light scattering by a lipid vesicle near a lipid bilayer, whereby the lipids are represented through a biaxial optical model, we conclude through ellipsometry concepts that effective amounts of lipid‐induced optical anisotropy significantly alter far‐field optical scattering in respect to an equivalent optical model that neglects the presence of optical anisotropy.