Scanning Near-field Optical Microscopy Images Research Articles

Scanning near-field optical microscopy system based on frequency-modulation atomic force microscopy using a piezoelectric cantilever

We developed a scanning near-field optical microscopy (SNOM) system based on frequency-modulation atomic force microscopy (FM-AFM), using a microfabricated force-sensing cantilever with a lead zirconate titanate (PZT) thin film as an integrated deflection sensor. In the system, an evanescent field is generated on the sample surface by total internal reflection, and the light transmitted through the optically transparent tip is collected by a photomultiplier while the gap distance between the tip and the sample surface is regulated by the FM detection method. We obtained a SNOM image of a sample with the nanometer-scale resolution of polystyrene particles dispersed in a polyvinyl alcohol thin film deposited onto a glass substrate with the developed SNOM/AFM system, under a completely dark condition.

Independently analyzing different surface plasmon polariton modes on silver nanowire.

In this paper, surface plasmon polariton (SPP) modes on silver nanowire (AgNW), with different field symmetric, are studied by different near field methods, respectively. In the experiment, the excitation and detection of SPPs are performed by two probes of near field scanning optical microscope (NSOM) simultaneously, which realizes the study of SPPs in complete near field. By controlling the experimental conditions, two of the fundamental SPP modes are detected separately and their intensity distributions on AgNW are given by the NSOM images. In the discussion, creeping wave (CW) is introduced to analyze the experimental results, which improves the coincidence of the experimental results and the theoretical calculations. A detailed characterization of SPPs modes in near field, which gives a further insight into optical properties of AgNW, will benefit integrated optical circuits.

Opto-mechanical probe for combining atomic force microscopy and optical near-field surface analysis.

We have developed a new easy-to-use probe that can be used to combine atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). We show that, using this device, the evanescent field, obtained by total internal reflection conditions in a prism, can be visualized by approaching the surface with the scanning tip. Furthermore, we were able to obtain simultaneous AFM and SNOM images of a standard test grating in air and in liquid. The lateral resolution in AFM and SNOM mode was estimated to be 45 and 160 nm, respectively. This new probe overcomes a number of limitations that commercial probes have, while yielding the same resolution.

Microscopic distribution of extended defects and blockage of threading dislocations by stacking faults in semipolar (11¯01) GaN revealed from spatially resolved luminescence

Spatial distribution of extended defects in semipolar (11¯01)-oriented GaN layers grown on patterned (001) Si substrates with striped grooves of varying width was investigated by optical means only using near-field scanning optical microscopy (NSOM) and cathodoluminescence (CL). A high density of basal and prismatic stacking faults was observed in the c− wings, and the threading dislocations in c+ wings, which appear as dark patterns in the NSOM and CL images, were found to bend toward the surface during the initial stages of growth. In the case when growing c+ front of GaN made contact with the SiO2 masking layer during growth, stacking faults were found to form also in the c+ wings. These additional stacking faults effectively blocked propagation of dislocations along the c+ direction, resulting in high quality stripes virtually free of defects. As revealed by optical means only without the need for any structural investigation, such control over the threading dislocation density using select growth geometries is potentially advantageous for improving semipolar (11¯01)GaN.

Scanning plasmonic microscopy by image reconstruction from the Fourier space

We demonstrate a simple scheme for high-resolution imaging of nanoplasmonic structures that basically removes most of the resolution limiting allowed light usually transmitted to the far field. This is achieved by implementing a Fourier lens in a near-field scanning optical microscope (NSOM) operating in the leakage-radiation microscopy (LRM) mode. The method consists of reconstructing optical images solely from the plasmonic 'forbidden' light collected in the Fourier space. It is demonstrated by using a point-like nanodiamond-based tip that illuminates a thin gold film patterned with a sub-wavelength annular slit. The reconstructed image of the slit shows a spatial resolution enhanced by a factor ~/= 4 compared to NSOM images acquired directly in the real space.

Extinction Ratio in Apertureless Reflection-Mode Magneto-Optical Scanning Near-Field Optical Microscopy

We developed a reflection-mode scanning near-field optical microscopy (SNOM) with an apertureless probe with a high special resolution and a high extinction ratio. Patterned Chromium film with a checker pattern deposited on a quartz substrate was measured. In SNOM images, clear contrast between Chromium and quartz was obtained, and the spatial resolution was determined to be ~ 30 nm. A polarization property of scattered light from the near field was also investigated for both p- and s-polarized incidence. The extinction ratios for both polarization were found to be larger than 100.

Near-Field Scanning Optical Microscopy Enables Direct Observation of Moiré Effects at the Nanometer Scale

This work reports probing the Moiré effect directly at the nanometer scale via near-field scanning optical microscopy (NSOM). Periodic metal nanostructures of Au and Cu have been produced sequentially using particle lithography, and the overlapped regions serve as Moiré patterns at nanometer scale. The Moiré effect in these regions can be directly visualized from NSOM images, from which periodicity and structural details are accurately determined. In addition, the near-field Moiré effect was found to be very sensitive to structural changes, such as lateral displacement and/or rotations of the two basic arrays with respect to each other. Further, nanostructures of Cu exhibited higher photon transmission than Au from NSOM images. Collectively, NSOM enables direct visualization of the Moiré effect at nanoscale levels, from optical read out, and without enhancements or modification of the structures. The results demonstrate the feasibility to extend applications of the Moiré effect-based techniques to nanometer levels.

Relaxation of single polymer chain in binary molecular weight blends observed by scanning near-field optical microscopy

The conformational relaxation of single polymer chains in uniaxially elongated poly(methyl methacrylate) (PMMA) films were examined by direct observation using scanning near-field optical microscopy (SNOM) and excitation polarization modulation microscopy (EPMM). The relaxation behavior of the individual PMMA chains was investigated in the binary blend systems consisting of high and low molecular weight components. The whole chain dimension and the segmental orientation were evaluated for high molecular weight long chains from SNOM and EPMM images, respectively. At the early stage of the stress relaxation process, not only the segmental orientation but also the whole chain dimension of the long chain decreased faster in matrices with short chains. This indicates that the relaxation at the whole chain scale is accelerated by disentanglement, which is caused by the motion of the short surrounding chains. The fast relaxation in the whole chain dimension cannot be explained by the constraint release picture in the theoretical models, which assumes that the disentanglement only activates the local motion of the long chain.

Relaxation of Single Polymer Chain in Poly(methyl methacrylate) Films under Uniaxial Extension Observed by Scanning Near-Field Optical Microscopy

Single chain conformations during the stress relaxation process in uniaxially stretched poly(methyl methacrylate) (PMMA) films were observed in real space by scanning near-field optical microscopy (SNOM). The extension ratio at the molecular level was directly evaluated from the SNOM images and compared with the macroscopic stress relaxation. At the early stage of the relaxation process, the whole single chain retained the stretched conformation in spite of the decrease in stress. This suggests that the fast relaxation in stress is caused by the local conformational change at the length scale much smaller than the entire chain length. The experimental data were successfully explained by the contraction of the primitive chain contour in the Doi–Edwards model. Furthermore, the distribution function of the chain dimension was found to retain its shape during the relaxation process, indicating that the conformational relaxation of polymer chain is rather homogeneous in the well-entangled system.

Mechanisms of cell–cell adhesion identified by immunofluorescent labelling with quantum dots: A scanning near-field optical microscopy approach

Scanning near-field optical microscopy (SNOM) has been employed to simultaneously acquire high-resolution fluorescence images along with shear-force atomic force microscopy from cell membranes. Implementing such a technique overcomes the limits of optical diffraction found in standard fluorescence microscopy and also yields vital topographic information. The application of the technique to investigate cell-cell adhesion has revealed the interactions of filopodia and their functional relationship in establishing adherens junctions. This has been achieved via the selective tagging of the cell adhesion protein, E-cadherin, by immunofluorescence labelling. Two labelling routes were explored; Alexa Fluor 488 and semiconductor quantum dots. The quantum dots demonstrated significantly enhanced photostability and high quantum yield making them a versatile alternative to the conventional organic fluorophores often used in such a study. Analysis of individual cells revealed that E-cadherin is predominantly located along the cell periphery but is also found to extend throughout their filopodia. We have demonstrated that with a fully optimised sample preparation methodology, quantum dot labelling in conjunction with SNOM imaging can be successfully applied to interrogate biomolecular localisation within delicate cellular membranes.

Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.

Analysis of light scattering from human breast tissue using a custom dual‐optical scanning near‐field optical microscope

In this paper we introduce a custom scanning near-field optical microscope (SNOM) that simultaneously collects reflection and transmission near-field images along with topography. This dual-optical SNOM uses a bent probe, which allows for axial reflection imaging, accurate surface scanning, and easy identification of topographic artifacts. Using this novel dual-optical SNOM, we image desiccated and non-desiccated human breast epithelial tissue. By comparing the simultaneous SNOM images, we isolate the effects of tissue morphology and variations in refractive indices on the forward- and back-scattering of light from the tissue. We find that the reduction in back-scattering from tissue, relative to the glass slide, is caused by dense packing of the scattering sites in the cytoplasm (morphology) in the desiccated tissue and a thin-film of water adhering to the glass slide (refractive index) in the non-desiccated tissue sample. Our work demonstrates the potential of our customized dual-optical SNOM system for label-free tissue diagnostics.

Polarization-dependent extraction properties of bare fiber probes

Despite their modest spatial resolution, uncoated tapered fiber probes are now widely used by the nano-optics community for mapping, with scanning near-field optical microscopy (SNOM), the nonradiative fields at the surface of optical and plasmonic microstructures and nanostructures. Given the significant complexity of the vectorial optical phenomena associated with subwavelength structures, the correct interpretation of SNOM acquisitions requires a complete and accurate understanding of the intrinsic image-formation procedure. In this theoretical study, we show that the SNOM imaging process with uncoated tapered fiber probes is highly polarization dependent and that the dominant effect is, surprisingly, the choice of optical fiber from which the tapered probe was fabricated. We demonstrate that although a tapered monomode fiber is unable to collect the component of the vector electric field parallel to the tip axis, a tapered multimode fiber can successfully collect all the three field components. However, we show that the signal from the longitudinal field component is collected only 10% as efficiently as the signal from the two transverse field components.

SNOM fluorescence techniques applied to cytoskeleton study in cell cultures

Introduction Scanning near field optical microscope (SNOM) is a tool which overcomes the diffraction limit of conventional optical microscopy and allows optical imaging with a resolution of less than 100 nm without having to go through any special sample preparation. Moreover one of the best advantages of SNOM is to produce topographic, transmission and reflection images simultaneously [1]. Several studies have been carried out by SNOM in the last years on biological specimens, but the fluorescence SNOM technique has been rarely applied on cell culture investigations. Here we set up a TriA-SNOM with a specific fluorescence equipment to develop the performance of this probe microscope and identify cytoskeleton elements in cell cultures, mainly primary rat podocytes cultures. Materials and methods The microscope used were a TriA-SNOM (APE Research, Trieste, Italy) equipped with a specific fluorescence set-up that allowed to obtain different SNOM images of the samples simultaneously: topography, SNOM fluorescence in reflection and SNOM-fluorescence in transmission. Cell cultures: Primary culture of rat podocytes were treated by immunofluorescence techniques with TRITC-phalloidin to visualize cytoskeletal elements such as actin filaments. Actin filaments were analyzed by SNOM-fluorescence and compared with conventional fluorescence. Results SNOMfluorescence, unlike all other techniques, may reconstruct a fluorescence mapping of the sample, and obtain a direct measure in height of the fluorescence structures. The major technical problem in this kind of microscopy is to reduce of the optical background noise. We actually obtained a SNOM-fluorescence set-up to adequately visualize the specimens, producing images with low background noise and high resolution. SNOM moreover allowed to discriminate the superficial actin filaments from the deep filaments by reflection system. Conclusions SNOM with fluorescence technique may be suitable method for detection of specific molecules in cell cultures, providing an overlapping of the fluorescence image on the 3D mapping of the sample. These studies open further possibilities for application of SNOM to biological materials, especially in cells observed in liquid environment.

Light scattering of interacting gold nanorods

AbstractThe optical field intensity of light scattering from nanorods of gold has been imaged at distances that are intermediate between the near‐field and far‐field regimes using a near‐field scanning optical microscope (NSOM). For scattering from isolated nanorods the Fraunhofer diffractive behaviour is modified slightly by the dipolar nature of metal nanoantannae as would be expected at these imaging distances. However, when the nanorods are brought into close proximity, interactions between the nanorods alter the scattering behaviour substantially creating large field intensities between the structures. By sampling the field with the near‐field microscope tip scanned at different heights, detailed maps of the scattering profile can be generated.The NSOM image of far‐field scattered light from an isolated gold nanorod. The nanorod was imaged at a distance of roughly 8 µm above the support substrate using a scanning near‐field microscope operated at constant height mode.magnified image

Near-Field Scanning Optical Microscopy Measurements of Fluorescent Molecular Probes Binding to Insulin Amyloid Fibrils

The binding of two amyloid fibril stain dyes, thioflavin T (ThT) and its neutral analog 2-[4'-(dimethylamino)phenyl]-benzothiazole (BTA-2), are measured using near-field scanning optical microscopy (NSOM), which is able to image individual amyloid fibrils. Polarized NSOM images reveal that both dyes bind to the fibrils with the long axis of the molecule aligned parallel to the long axis of the fibrils. This indicates that the dyes bind along the surface of the beta-sheet within the grooves of the fibril that run parallel to the fibril axis. The similarity in the binding motifs of the two dyes shows that electrostatic interaction of the charged amine group on the ThT dye plays a minimal role in the affinity of the dyes for the amyloids. The polarized NSOM images confirm that the enhanced fluorescence of the ThT and BTA-2 result from binding of the monomeric dye rather than micelles or excimer species.

Affine deformation of single polymer chain in poly(methyl methacrylate) films under uniaxial extension observed by scanning near-field optical microscopy

The conformation of single poly(methyl methacrylate) chain in the uniaxially stretched film was observed by the combination of the fluorescence labeling technique and scanning near-field optical microscopy (SNOM). The dimension of the individual probe chain ( M w = 1.99 × 10 6) along the extension axis was evaluated from SNOM image. In the high molecular weight matrix ( M w = 1.89 × 10 6) the average extension ratio at the single chain level coincided with the macroscopic extension ratio. The distribution of the chain conformation was in good agreement with that of the freely jointed chain followed by affine deformation. On the other hand, the probe chain embedded in the low molecular weight matrix ( M w = 1.76 × 10 5) showed the smaller extension than that expected from the affine deformation. This suggests that the conformation of the probe chain is affected by the relaxation of the short surrounding chains through disentanglement.

Confining Standing Waves in Optical Corrals

Near-field scanning optical microscopy images of solid wall, circular, and elliptical microscale corrals show standing wave patterns confined inside the structures with a wavelength close to that of the incident light. The patterns inside the corrals can be tuned by changing the size and material of the walls, the wavelength of incident light, and polarization direction for elliptical corrals. Finite-difference time-domain calculations of the corral structures agree with the experimental observations and reveal that the electric and magnetic field intensities are out of phase inside the corral. A theoretical modal analysis indicates that the fields inside the corrals can be attributed to p- and s-polarized waveguide modes, and that the superposition of the propagating and evanescent modes can explain the phase differences between the fields. These experimental and theoretical results demonstrate that electromagnetic fields on a dielectric surface can be controlled in a predictable manner.

Rigorous modeling and physical interpretation of terahertz near-field imaging

Apertureless scanning near-field optical microscopy (SNOM) operating with terahertz (THz) laser pulses is a subject of great research interest. The Mie scattering theory is commonly used to explain the features of the optical waves produced by field interactions with SNOM tips and microstructures. However, since Mie scattering fails with SNOMs at submillimeter wavelengths, a rigorous model and analysis are desirable to assess the feasibility of the THz tip-enhanced scanning near-field techniques. In this paper, we present a numerical simulation of an apertureless SNOM imaging system in the THz band. A 2-dimensional model based on the finite element method (FEM) is investigated and discussed. The modeling results are in good agreement with the experimental data obtained for this system at 2 THz radiation [H.-T. Chen at al., Phys. Rev. Lett. 93, 267401 (2004)]. Additionally, a physical interpretation using the antenna theory is successfully confirmed by the simulation results.

Characteristics of GaN‐based light emitting diode grown on circular convex patterned sapphire substrate

AbstractWe report the characteristics of GaN‐based light‐emitting diode (LED) grown on a circular convex patterned sapphire substrate (CCPSS) and conventional LEDs on a non‐patterned sapphire substrate (PSS) by metal‐organic chemical vapor deposition. A near field scanning optical microscope (NSOM) and micro Raman spectroscopy were employed to investigate the crystalline properties. In NSOM image and spectra of LED on CCPSS, an improved crystalline quality with emission fluctuation feature on the trench region was obtained. However, stress relaxation in this trench region was not achieved due to the presence of voids, indicating a stress concentration at the coalescence region. Compared to conventional LED on non‐PSS, increased internal quantum efficiency (IQE) (41.8%) and reduced leakage currents (40 nA at ‐10 V) were observed in the LED fabricated on CCPSS. The light output power of the LED on CCPSS was approximately two times higher than that of a conventional LED. This enhancement of output power is contributed to increase of IQE and the effective suppression of leakage current by reduction of dislocation density, as well as enhanced light scattering via the circular convex facets. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)