Scanning near-field optical microscope using an atomic force microscope cantilever with integrated photodiode

Scanning near-field optical microscopy (SNOM) is a method to obtain information about the optical properties of a sample at a lateral resolution below the diffraction limit of far-field microscopy. In SNOM, a light source of a dimension which is small compared to the wavelength of light and which is held at a small distance from the sample is scanned across the surface of the sample. The modulation by the sample of the light emitted from the source is recorded as a signal. As a general rule one may say that the size of the source and the distance to the sample limit the resolution of SNOM. A radiating self-emitting point dipole may be regarded as an idealized SNOM source. With such a source the resolution of SNOM imaging is expected to be limited by the distance of this dipole to the surface of the object [1]. It is difficult to design a light-emitting SNOM probe corresponding to a dipole at a distance of less than 10 nm from the object and it is therefore difficult to conceive SNOM imaging beyond a resolution of 10 nm. There have been, however, occasional reports of near-field optical imaging at a resolution in the range of 1-10 nm [2, 3]. In SNOM-images recorded with the tetrahedral tip (T-tip) a resolution in the range of 1-10 nm was obtained reproducibly on samples consisting of small grains of silver of a size of the order of 2-10 nm embedded in a flat surface of gold [3, 4]. An example of an image is shown in Fig. 1. In a different experiment we investigated a surface-embedded latex bead projection pattern [5] consisting of a flat surface of a polymer into which gold patches of a triangular shape of a size of about 50 nm and a thickness of 20 nm were embedded [6].

A cantilever-based probe is introduced for use in scanning near-field optical microscopy (SNOM) combined with scanning atomic-force microscopy (AFM). The probes consist of silicon cantilevers with integrated 25-mum-high fused-silica tips. The probes are batch fabricated by microfabrication technology. Transmission electron microscopy reveals that the transparent quartz tips are completely covered with an opaque aluminum layer before the SNOM measurement. Static and dynamic AFM imaging was performed. SNOM imaging in transmission mode of single fluorescent molecules shows an optical resolution better than 32 nm.

We report illumination-mode near-field optical microscopy images of individual 80–115-nm-diameter Au particles recorded with metal-coated fiber probes. It is found that the images are strongly influenced by the metal-coating thickness. This dependence is consistent with theoretical models, which are in good agreement with the experimental images. Probes with thick coatings (∼λ/2) produce images consisting of three extrema, owing to a resonancelike polarization of the probe end. Probes with thinner coatings generally produce simpler images. However, in some cases the images contain wavelike structures that are due to interference between direct radiation from the tip and propagating tip fields scattered by the particles.

The assay of DNA biosensor-based nucleic acid recognition using microfabrication technology provides for high sensitivity, good surface coverage and reproducibility. We have achieved efficient immobilization and hybridization of nonlabeled DNA using cyclic voltammetry (CV), square wave voltammetry (SWV) and scanning near-field optical microscopy (SNOM) techniques. The increased electrochemical response observed following the immobilization of biotinlyated ssDNA probe suggests that nucleic acid is a somewhat better medium for electronic transfer. We demonstrated the high coverage of immobilized FITC-labeled biotinylated DNA probe on a streptavidin-modified surface using SNOM imaging. SNOM imaging of FITC-labeled complementary DNA also exhibited fluorescent light spots of hybridization distributed throughout. No fluorescent light was observed with the hybridization of non-complementary DNA.

The concept of high resolution imaging of a sample on a metal substrate by scanning near field optical microscopy (SNOM) with the tetrahedral tip is used for imaging of domains of oriented dye molecules on a metal substrate at a resolution in the order of 10 nm. The metal substrate is important for obtaining a high resolution in the SNOM images and at the same time it is expected to provide a mechanism for an increased contrast in SNOM imaging by scanning probe enhanced elastic scattering of molecules in the gap between a metal tip and a metal surface.

Microfabricated tips, usually used for atomic force microscopy (AFM), can be modified and used in a scanning nearfield optical microscope. Initial images show a resolution of about 80 nm (λ/8) demonstrating the quality and usefulness of these tips. They are mechanically stable, potentially commercially available, and have high light transmissibility. Furthermore, they can be easily combined with an atomic force microscope to give simultaneous nearfield optical and AFM images.

We report on atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) investigations of single crystals of the spin crossover complex {Fe(pyrazine)[Pt(CN)4]} across the first-order thermal spin transition. We demonstrate for the first time that the change in spin state can be probed with sub-micrometer spatial resolution through various topographic features extracted from AFM data. This original approach based on surface topography analysis should be easy to implement to any phase change material exhibiting sizeable electron-lattice coupling. In addition, AFM images revealed specific topographic features in the crystals, which were correlated with the spatiotemporal evolution of the transition observed by far-field and near-field optical microscopies.

Submicron Structure in l- α-Dipalmitoylphosphatidylcholine Monolayers and Bilayers Probed with Confocal, Atomic Force, and Near-Field Microscopy

Electrogenerated chemiluminescence at electrodes with effective diameters down to 155 nm was used as a stable light source for near-field scanning optical microscopy imaging of an interdigitated array and a submicrometer size test substrate. Light was generated in a thin (approximately 500 microm) layer of an aqueous solution of 15 mM Ru(bpy)3(2+) and 100 mM tri-n-propylamine in a pH 7.5 buffer. The resolution obtained was compared to that found with a micrometer size electrode. The shear force from the tip attached to a quartz tuning fork was used to monitor and control the tip-to-substrate separation within the near field regime.

Detecting structures below a cover film at the nanoscale resolution is of essential importance. In this work, we explored factors affecting subsurface material contrast and structural visibility in scattering-type scanning near-field optical microscopy (s-SNOM). A kind of multilayered reference samples containing different buried structures was fabricated and applied for s-SNOM imaging. The dependence of near-field optical contrast on structure geometry, dimension and cover thickness was investigated. Results demonstrate that distinguishing the buried slit pattern is easier than the circular hole with the same critical dimension. The s-SNOM can sense material difference under a more than 100 nm thick polymethyl methacrylate layer and it has a subsurface spatial resolution better than 100 nm.

The scanning near-field optical microscopy imaging of specimens in liquid and of cultured cells in aqueous solutions is reported. A scanning near-field optical/atomic-force microscope (SNOM-AFM) was developed, in which the scanning of an optical-fiber probe cantilever over the specimen was controlled by noncontact mode AFM (dynamic mode AFM). This imaging mode reduces damage to the probe and soft specimens. The resonant frequency of the probe cantilever decreased 20% to ≊14 kHz and the Q factor decreased by a factor of 8 to ≊30 in water, compared with these values in air, which was sufficient to perform SNOM-AFM imaging in liquid.

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).

The scanning near-field optical microscope (SNOM) is well known as a tool for observing nano-structures. On that account, a SNOM to which a certain magneto-optical (MO) measurement technique is applied, or MO-SNOM, is likely to become a powerful tool for nanometric studies on magnetism. In our study, a transmission-illumination mode SNOM with a bent optical fiber probe, which operates as a cantilever in atomic force microscopy (AFM), was employed. Incorporation of the polarization modulation technique into this system has made it possible to resolve magnetic structures with a high sensitivity of up to 1 mrad and a resolution of approximately 1/5 times the wave-length of the light source (argon ion laser: 488 nm). The behavior of polarized light in the probes, however, prevents quantitative analysis of MO effects. This paper describes an investigation into the polarization properties of the bent optical fiber probes, based on their Stokes parameters. A method for acquiring pure signals dependent on ellipticity or rotation in MO imaging is presented.

Being a key player in intercellular communications, nanoscale extracellular vesicles (EVs) offer unique opportunities for both diagnostics and therapeutics. However, their cellular origin and functional identity remain elusive due to the high heterogeneity in their molecular and physical features. Here, for the first time, multiple EV parameters involving membrane protein composition, size and mechanical properties on single small EVs (sEVs) are simultaneously studied by combined fluorescence and atomic force microscopy. Furthermore, their correlation and heterogeneity in different cellular sources are investigated. The study, performed on sEVs derived from human embryonic kidney 293, cord blood mesenchymal stromal and human acute monocytic leukemia cell lines, identifies both common and cell line-specific sEV subpopulations bearing distinct distributions of the common tetraspanins (CD9, CD63, and CD81) and biophysical properties. Although the tetraspanin abundances of individual sEVs are independent of their sizes, the expression levels of CD9 and CD63 are strongly correlated. A sEV population co-expressing all the three tetraspanins in relatively high abundance, however, having average diameters of <100 nm and relatively low Young moduli, is also found in all cell lines. Such a multiparametric approach is expected to provide new insights regarding EV biology and functions, potentially deciphering unsolved questions in this field.

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Near-field images of Ag nanoparticles are studied using a near-field scanning optical microscopy (NSOM) operating at illumination mode with blue, green, and red probing lights. The obtained far-field intensity contrast between the nanoparticle and background strongly depends on the sizes of nanoparticles and the wavelength of probing light. Experimental NSOM images supported by theoretical 3-D finite-difference time-domain simulation demonstrate that the intensity contrast is enhanced at wavelength close to the localized surface plasmon resonance (LSPR) peak of the nanoparticle. The abilities to distinguish nanoparticles with different LSPR properties on the same substrate can lead to a material-specific NSOM imaging technique. </para>