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.     

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)     

Review of near-field optical microscopy

This review has introduced a new near-field optical microscope (NOM)—atomic force microscope combined with photon scanning tunneling microscope (AF / PSTM). During scanning, AF/PSTM could get two optical images of refractive index image and transmissivity image, and two AFM images of topography image and phase image. A reflected near-field optical microscope (AF/RSNOM) has also been developed on AF/PSTM platform. The NOM has been reviewed in this paper and the comparison between AF/PSTM & RSNOM and the commercial A-SNOM & RNOM has also been discussed. The functions of AF/PSTM & RSNOM are much better than A-SNOM & RNOM.     

Metastructural systems of activated nanospheres and optical near-field resonances

The near-field interaction of two spherical nanoparticles containing dense ensembles of two-or multilevel atoms in an external field of optical low-intensity radiation is shown to result in the formation of resonances whose frequencies differ considerably from the transition frequencies in the spectrum of the interacting atoms. Optical near-field resonances are shown to play an important role in metastructural systems composed of activated nanospheres. The reflectance of a metastructural system of activated nanospheres oriented along a certain direction depends strongly on the polarization and the frequency of external radiation, as well as on the concentration of impurity atoms inside the nanospheres and on their sizes.     

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.     

Method of resonance near-field optical microscopy

We have solved the problem in which a thin metal wafer (probe) with a nanohole interacts with the flat surface of a metastructured film consisting of metal nanoparticles in an external optical radiation field. Nanoparticles are considered as two-level atomic systems. This interaction of the wafer-probe and the flat surface in the external optical radiation field gives rise to optical near-field resonance, the frequency of which differs significantly from the natural frequencies of two-level atoms in the medium and the probe. The fields inside and outside the probe and metastructured film are calculated in the near-field and far-field zones. The maximum resolution, which is achievable in the suggested scheme of near-field optical microscopy, can reach about 10 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 499–506, July–August, 2007.     

An optical near-field microscope based on optical dimensional resonances of interacting atoms

A concept was suggested for an optical near-field microscope based on optical dimensional resonances in the system of a needle tip atom + a sample atom. High sensitivity and spatial resolution of an order of 1 nm were shown to be characteristic of this microscope. Furthermore, the instrument is applicable to a wide range of studied samples.     

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.     

Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples

Opaque samples are imaged by Scanning Nearfield Optical Microscopy (SNOM) in reflection mode: A quartz glass fiber tip is used both to illuminate the sample and to collect light locally reflected from or emitted by the surface. The collected light is coupled out by a 2×2 fiber coupler and fed into a grating spectrometer for spectral analysis at each sampled point. The tip-sample distance is controlled by a shear-force feedback system. The simultaneous measurement of topography and optical signals allows an assessment of imaging artifacts, notably topography-induced intensity changes. It is demonstrated that an optical reflectance contrast not induced by topographic interference can be found on suitable samples. Local spectral analysis is shown in images of a photoluminescent layer.     

Integral equation method in near-field optical microscopy of scattering

Based on the integral equation method, the boundary problem of classical optics of finding the strength of the electromagnetic field scattered by a probe positioned in the near zone of a plane surface is solved in the context of application in near-field optical microscopy of scattering. Only the dipole component of the scattered field was taken into account in the calculations. The obtained theoretical expressions were compared to the known experimental data on scanning the surface of SiC. Good agreement between the experimental and the theoretical results was obtained outside the region of the resonant interaction of the probe of a near-field microscope and the surface under study.     

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.     

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.     

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.     

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.     

Resonant optical cavity used as a nonradiating source for optical near-field microscopy

Nonradiating sources are emitting devices whose geometry prohibits electromagnetic emission. First, we show that a ring cavity can be substituted for the usual prism in scanning tunneling optical microscopy. Second, we find that near-field detection is an ideal way to explore the behavior of such nonradiating sources.