Image dipole approach and polarization effects in scanning near-field optical microscopy

A coupled-dipole approach is proposed in order to study the coupling between the probe tip and the rough sample in SNOM. In the present model both the optical probe tip and the sample protrusions are represented by polarizable dipole spheres. The induced polarization effects on the sample surface can be replaced by the image dipoles in the circumstance of quasi-static electromagnetic field approximation. Applying the radiation theory of the dipole, we have established a set of self-consistent equations to describe the field distribution at the sites of the probe tip and the sample protrusions. The results are completely the same as those obtained by means of the dyadic electromagnetic propagator formalism and also the derivation procedure is relatively simple. This method permits us to analyze the physical mechanisms of the interaction between the probe tip and the rough surface in SNOM intuitively. Based on this approach, we further discuss the influence of polarization of the incident light on the imaging quality. The calculating result shows that the shape and the contrast of the images of the sample are both sensitive to the field polarization, and the z-polarized mode is proved to give better resolution in SNOM.     

Contact scanning near-field optical microscopy

A new method of scanning in near-field optical microscopy, which makes it possible to operate in contact with the experimental sample, is proposed and implemented. This method permits the practical utilization of the idea of using the dipole-dipole resonance transfer of excitation energy from the active element of the microscope to the sample for achieving a fundamental improvement in the resolution of near-field optical microscopy. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 245–250 (25 February 1998)     

Characterization of strong electromagnetic field confinement on gold nanostructures by apertureless scanning near-field optical microscopy

We report on the detection of the optical near field of a 1D gold particle array by using an apertureless scanning near-field optical microscope. The strong near-field confinement measured above the grating proves unambiguously the near-field origin of the detected optical signal. Comparing the experiment with theory leads us to assign the optical near field to the first diffracted order of the grating, which is evanescent.     

Cloaking apertureless near-field scanning optical microscopy tips

We numerically demonstrate that properly designed plasmonic covers can be used to enhance the performance of near-field scanning optical microscopy (NSOM) systems based on the employment of apertureless metallic tip probes. The covering material, exhibiting a near-zero value of the real permittivity at the working frequency, is designed in such a way to dramatically reduce the undesired scattering due to the strongly plasmonic behavior of the tip. Though the light scattering by the tip end is necessary for the correct operation of NSOMs, the additional scattering due to the whole probe affects the signal-to-noise ratio and thus the resolution of the acquired image. By covering the whole probe but not the very tip, we show that unwanted scattering can be effectively reduced. A realistic setup, working at mid-IR frequencies and employing silicon carbide covers, has been designed and simulated to confirm the effectiveness of the proposed approach.     

Signal detection techniques for scattering-type scanning near-field optical microscopy

Scattering-type scanning near-field optical microscopy (s-SNOM) has been playing more and more important roles in investigating electromagnetic properties of various materials and structures on the nanoscale. In this technique, a sharp tip is employed as the near-field antenna to measure the sample's properties with a high spatial resolution. As the scattered near-field signal from the tip is extremely weak and contaminated by strong background noise, the effective detection, and subsequent extraction of the near-field information from the detected signals is the key issue for s-SNOM. In this review, we give a systematic explanation of the underlying mechanisms of s-SNOM, and summarize and interpret major signal detection techniques involved, including experimental setups, theories for signal analysis and processing, and exposition of advantages and disadvantages of such techniques. By this, we hope to provide a practical guide and a go-to source of detailed information for those interested in and/or working on s-SNOM.     

Polarization-dependent contrast of near-field and far-field-propagating signal intensities in C-mode scanning near-field optical microscopy/photon scanning tunneling microscopy

A recent paper by Nayaet al. (Opt. Commun.124 (1996) 9) presented high-resolution imaging results obtained in the sub-100-nm range with a collection-mode near-field optical microscope. The images exhibit apparent polarization dependence. A simple modeling and calculation based on the experiment, using a semi-microscopic and perturbative approach, showed that the far-field-propagating signal intensity converted from the near-field can qualitatively explain the polarization dependence of the experiment if the taper angle of the probe tip is taken into account.     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.     

Three-dimensional modeling of amplitude-object imaging in scanning near-field optical microscopy

The imaging properties of the transmission-illumination mode of a scanning near-field optical microscope are investigated. Three-dimensional calculations of the power transmitted into classically allowed and forbidden regions for a nonsymmetrically positioned amplitude object are implemented by use of the finite-difference time-domain solution of Maxwell's equations. The evolution of the images with the distance from the object as well as the effect of the polarization of the illumination is shown. The computations show that for applications involving the imaging of an amplitude object, the use of the allowed light is preferred. Collection of light from both the allowed and the forbidden zones leads to degraded contrast and resolution.     

Numerical simulation of nanoparticle images in scanning near-field optical microscopy

The images of magnetic and nonmagnetic nanoparticles obtained by scanning near-field microscopy in the photon collection mode are numerically simulated. A theoretical approach that uses tensor electrodynamic Green’s functions to find the optical near field in a given observation scheme is considered. Typicalimages of nanoparticles with various shapes are obtained by numerical simulation. Subject to boundary conditions, the plane of polarization is shown to change at topographic features (edges and angles) of objects studied. This makes the observation of the magnetic structure of a nanoparticle with a magnetooptic method difficult. The near-field study of the magnetization distribution in homogeneous thin films appears to be more effective, since the rotation of plane of polarization is associated primarily with the magnetic properties of the sample in this case.     

Faraday-rotation imaging by near-field optical microscopy

Scanning near-field optical microscopy with polarization modulation (PM-SNOM) has been applied to image the surface of a yttrium-iron-garnet (YIG) film. Lock-in detection of the phase of the transmitted light directly gives the magnitude of the Faraday rotation angle.     

Photopatterning of a monomolecular dye film by means of scanning near-field optical microscopy

Patterned bleaching of a photolabile monomolecular dye film by means of scanning near-field optical microscopy (SNOM) is demonstrated. After exposure, the written patterns were verified by SNOM with fluorescence detection. The adsorption of appropriate material to such near-field optically created patterns seems now feasible. The ultimate resolution limit of a monomolecular dye film for patterned bleaching by SNOM is discussed on the basis of a simple model.     

Imaging of human meiotic chromosomes by scanning near-field optical microscopy (SNOM)

Centromeres and telomeres are key structures of mitotic and meiotic chromosomes. Especially telomeres develop particular structural properties at meiosis. Here, we investigated the feasibility of scanning near-field optical microscopy (SNOM) for light-microscopic imaging of meiotic telomeres in the sub-hundred nanometer resolution regime. SNOM was applied to visualise the synaptonemal complex (SC) and telomere proteins (TRF1, TRF2) after differential immuno-fluorescent labelling. We tested and compared two different preparation protocols for their applicability in a SNOM setting using micro-fabricated silicon nitride aperture tips. Protocol I consisted of differential labelling of meiotic chromosome cores (SC) by SCP3 immuno-fluorescence and telomeres by TRF1 or TRF2 immuno-fluorescence, while protocol II combined absorption labelling with alkaline phosphatase substrates of cores with fluorescent labelling of telomeres. The results obtained indicate that protocol I reveals a better visualisation of structural (topographic) details than protocol II. By means of SNOM, meiotic chromosome cores could be visualised at a resolution overtopping that of far-field light microscopy.     

Scanning near-field optical microscopy

Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.     

Apertureless scanning near-field optical microscopy: the need for probe-vibration modeling

In apertureless scanning near-field optical microscopy (ASNOM), the probe vibration is often used to increase the detected signal. The useful signal is detected at the probe-vibration frequency by a lock-in amplifier. By comparing two-dimensional numerical results with an experimental scan, we show numerically that, to explain or predict the detected signal, a realistic model of ASNOM should take into account the scan of the probe as well as the probe vibration and the material properties.     

Standing-wave spectrometry in silicon nano-waveguides using reflection-based near-field scanning optical microscopy

Utilizing reflection-based near-field scanning optical microscopy(NSOM) to image and analyze standing-wave patterns, we present a characterization technique potentially suitable for complex photonic integrated circuits. By raster scanning along the axis of a straight nano-waveguide in tapping mode and sweeping wavelength, detailed information of propagating waves in that waveguide has been extracted from analyses in both space and wavelength domains. Our technique needs no special steps for phase stabilization, thus allowing long-duration and environment-insensitive measurements. As a proof-of-concept test, in a silicon single-mode waveguide with a few of etched holes, the locations and reflection strengths of the inner defects have been quantified. The measurement uncertainty of the reflection amplitude is less than 25% at current stage. Our technique paves the way for non-destructively diagnosing photonic circuits on a chip with sub-wavelength spatial resolution and detailed information extraction.     

Chemical silver coating of fiber tips in near-field scanning optical microscopy

We report what is believed to be the first experimental demonstration of silver coating by a wet chemical process on tapered fiber tips used in near-field scanning optical microscopy. The process occurs at room temperature and pressure and takes only a few minutes to complete. Many tips can be coated simultaneously.     

Image enhancement in near-field scanning optical microscopy with laser-trapped metallic particles

A trapped-particle near-field scanning optical microscope is constructed by use of submicrometer- or micrometer-sized metallic particles (gold and silver) to increase scattering efficiency. The image contrast of the evanescent-wave interference pattern on the surface of a prism upon total internal reflection, obtained with trapped gold particles of diameter 0.1 and 2microm , is improved by a factor of approximately 2 and 1.5, respectively, compared with that obtained with trapped polystyrene particles of similar size. The use of a 2-microm gold particle leads to image contrast that is approximately three times as great as that obtained with a 0.1-microm gold particle, and interference patterns of a subwavelength period are obtained in both cases.     

Mesoscale structures in luminescent conjugated polymer thin films studied by near-field scanning optical microscopy

We report the studies of various conjugated polymer thin films with near-field scanning optical microscopy, NSOM. Firstly, it is shown that MEH-PPV thin film undergoes significant changes in film morphology upon thermal annealing. The once homogeneous morphology becomes inhomogeneous after annealing. Secondly, polarization near-field measurements reveal mesoscale polymer ordering in PPV thin film. The average domain size and the coefficient for linear dichroism were studied as a function of film thickness. Finally, phase separation in polymer blend film was directly observed by transmission NSOM. Time-resolved fluorescence spectra indicate that the phase domains are decomposed of different fractions of the two constituent polymers. The near-field optical microscopy was also used to write lithographic patterns with a resolution of 100 nm, exceeding the diffraction limit.     

High-sensitivity piezoelectric tube sensor for shear-force detection in scanning near-field optical microscopy

An easy-to-implement non-optical shear-force detection setup for tip–sample distance regulation in scanning near-field optical microscopy is demonstrated. The detection method is based on attaching the near-field probe to a piezoelectric tube resulting in excellent mechanical contact between tip and detector. The main advantages of the method are good signal-to-background contrast and thus potential for high sensitivity. The method is demonstrated by obtaining approach curves of silicon surfaces. The suitability for optical experiments is further shown by measuring the near-field intensity distribution of the emission of a semiconductor laser.     

The role of propagating and evanescent waves in scanning near-field optical microscopy

Theoretical study on the image formation in scanning near-field optical microscopy is carried out in the framework of the direct moment method. Information brought, respectively, by the propagating and evanescent components in the optical near field that is collected by a scanning fiber tip with a sub-wavelength aperture is numerically and systematically analyzed in the light of the resolution achieved by the microscope. The analyses reveal that resolutions beyond the diffraction limit can be achieved even in the absence of the evanescent waves. That is, it is incorrect or at least incomplete to believe that a microscope that collects only the propagating waves is limited by the diffraction. Our studies show that a scanning near-field optical microscope can achieve resolutions beyond the diffraction limit by collecting only the propagating waves.