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.     

Polarization effects in apertureless scanning near-field optical microscopy: an experimental study

Strong electric-field enhancements at the apex of a tungsten tip illuminated by an external light source were recently predicted theoretically. We present an experimental study of the dependence of this effect on the polarization angle of the incident light. It is shown that the intensity of the light scattered by the tungsten tip of an apertureless scanning near-field optical microscope is 2 orders of magnitude higher when the incident light is p polarized than when it is s polarized. This experimental result is in good agreement with theoretical predictions and provides an easy way to test the quality of the tips.     

Recovering of the apertureless scanning near-field optical microscopy signal through a lock-in detection

To enhance the signal-to-noise ratio and to remove the spatially slowly varying signals, a lock-in amplifier is often used in apertureless scanning near-field optical microscopy (ASNOM). The detected signals are therefore the Fourier harmonics of the signal along a vertical vibration of the probe and depend drastically on the vibration amplitude. All the Fourier harmonics contain contributions of the ‘near field’ and the ‘far field’ that are mixed in the near-field zone (i.e. at very short distance). Therefore, physical interpretation of contrast in high-harmonics records may be questionable. A more accurate approach consists in using all the harmonics of lock-in demodulation to reconstruct the ASNOM signal above nanoparticles. We show that the reconstructed and the detected signals can strongly differ and that the reconstructed signal must be computed from the lock-in harmonics and the dc term in order to give a physical interpretation of the phenomenon. PACS 07.79.Fc; 42.30.Wb; 42.30.Va     

Coherent scattering phenomena in apertureless scanning near-field fluorescence microscopy

We study near-field fluorescence images of samples composed of aggregates of 100 nm dye-doped latex spheres. These images have been performed by a reflection Apertureless Scanning Near-field Optical Microscope (A-SNOM). We show that the near-field distribution in fluorescence A-SNOM images arises from coherent scattering phenomena between all spheres. This is a consequence of the coherent nature of the fluorescence emission of each single sphere. The results shown here are significant for all fluorescent samples characterised by a nonnegligible topography.     

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.     

Near-field and far-field modeling of scattered surface waves. Application to the apertureless scanning near-field optical microscopy

The detection of surface waves through scanning near-field optical microscopy (SNOM) is a promising technique for thermal measurements at very small scales. Recent studies have shown that electromagnetic waves, in the vicinity of a scattering structure such as an atomic force microscopy (AFM) tip, can be scattered from near to far-field and thus detected. In the present work, a model based on the finite difference time domain (FDTD) method and the near-field to far-field (NFTFF) transformation for electromagnetic waves propagation is presented. This model has been validated by studying the electromagnetic field of a dipole in vacuum and close to a dielectric substrate. Then simulations for a tetrahedral tip close to an interface are presented and discussed.     

Anisotropy contrast in phonon-enhanced apertureless near-field microscopy using a free-electron laser

We demonstrate the imaging of ferroelectric domains in BaTiO3, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.     

近场扫描干涉无孔径显微术

本文介绍了国外研究的一种新型近场扫描无孔径干涉显微镜的原理及结构。这种显微镜的分辨率可达到１ｎｍ，开辟了在亚纳米尺度成像和进行光谱分析的可能性，特别是在化学和生物学研究领域具有潜在的应用前景。     

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.     

Designs of apertureless probe with nano-slits for near-field light localization and concentration

This paper presents recent studies on nano-patterned plasmonic probes that can provide highly localized and enhanced light for the near-field scanning optical microscopy. The mechanism to realize such localized light source is introduced and numerically characterized in the near field. In addition, the attainable wideband operation of the plasmonic probe through the proper design is also discussed with particular attention to developing potential applications in the near-field scanning optical microscopy.     

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.     

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)     

Spot-size reduction in terahertz apertureless near-field imaging

We show measurements and calculations of the terahertz (THz) near field of a metal tip with a specially formed, semicircular apex that allows us to identify the separate contributions of the tip apex and shaft to the measured signal. We find that when the tip-crystal distance is not modulated the measured near-field signal is overwhelmed by contributions from the tip shaft, resulting in a relatively large THz spot size. When the tip-crystal distance is modulated, with subsequent lock-in detection at the modulation frequency, only the near-field distribution of the semicircular apex is observed, resulting in a much smaller THz spot size and thus improved spatial resolution.     

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.     

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.     

Observation of nanometer-scale optical property discrimination by use of a near-field scanning apertureless microscope

We examine fundamental issues related to discriminating structural and optical features in near-field scanning apertureless microscopy. We report a series of controlled experiments with nanosphere-sized standard spheres in which we observed significant differences in resolution and structure between an atomic-force microscope image and a simultaneously acquired near-field optical (NFO) image. Further, in experiments that employed a mix of dyed and undyed nanospheres we found that we can observe differences in the same NFO image for adjacent nanospheres. Therefore we conclude that near-field scanning apertureless microscopy not only meets the criteria for a NFO image but also is capable of measuring optical properties below the diffraction limit. The two-point resolution was at least 200 nm when we were detecting optical phase and 50 nm when we were detecting optical intensity. The edge response was typically 15 nm, and the minimum observable features were of the order of 3 nm.     

Boundary-value problems in near-field optical microscopy and optical size resonances

We have solved a boundary-value problem for a ball probe interacting with a flat dielectric surface in an external optical radiation field. This interaction gives rise to the optical size resonance at frequencies significantly different from the natural frequencies of two-level atoms both in the medium and in the probe with allowance for the local field corrections. These resonances depend significantly on the distance from the probe center to the surface, on the ball probe size, on the concentration of two-level atoms in the probe and in the medium, on the spectral line width, and on the atomic inversion. The field strengths inside and outside the ball probe and a semiinfinite dielectric medium have been calculated in the near-field and wave zones. It is shown that the proposed electrodynamic theory of optical near-field microscopy agrees with the results of experimental measurements.     

Sensitivity analysis of scanning near-field optical microscope probe

In this paper, we study the dynamic modes of a scanning near-field optical microscope (SNOM) which uses an optical fiber probe; and the sensitivity of flexural and axial vibration modes for the probe were derived and the closed-form expressions were obtained. According to the analysis, as expected each mode has a different sensitivity and the first mode is the most sensitive mode of flexural and axial vibration for the SNOM probe. The sensitivities of both flexural and axial modes are greater for a material surface that is compliant with the cantilever probe. As the contact stiffness increases, the high-order vibration modes are more sensitive than the lower-order modes. Furthermore, the axial contact stiffness has a significant effect on the sensitivity of the SNOM probe, and this should be noted when designing new cantilever probes.