Mode Scanning Near-field Optical Microscope Research Articles

True Tapping Mode Scanning Near-Field Optical Microscopy with Bent Glass Fiber Probes.

In scanning near-field optical microscopy, the most popular probes are made of sharpened glass fiber attached to a quartz tuning fork (TF) and exploiting the shear force-based feedback. The use of tapping mode feedback could be preferable. Such an approach can be realized, for example, using bent fiber probes. Detailed analysis of fiber vibration modes shows that realization of truly tapping mode of the probe dithering requires an extreme caution. In case of using the second resonance mode, probes vibrate mostly in shear force mode unless the bending radius is rather small (ca. 0.3 mm) and the probe's tip is short. Otherwise, the shear force character of the dithering persists. Probes having these characteristics were prepared by irradiation of a tapered etched glass fiber with a CW CO2 laser. These probes were attached to the TF in double resonance conditions which enables achieving significant quality factor (4000–6000) of the TF + probe system (Cherkun et al., 2006). We also show that, to achieve a truly tapping character, dithering, short, and not exceeding 3 mm lengths of a freestanding part of bent fiber probe beam should also be used in the case of nonresonant excitation.

Plasmonic properties of a nanoporous gold film investigated by far-field and near-field optical techniques

We report a study of the plasmonic properties of a 20-nm-thick nanoporous Au film by far-field and near-field optical techniques. The film is prepared sequentially by deposition of gold and copper, thermal annealing, and chemical etching, and has randomly distributed nanopores with sizes ranging between 20 and 350 nm. The absorbance of the nanoporous Au film is much higher than that of a plain Au film and can be attributed to the conversion of incident light into surface plasmon polaritons (SPPs). In addition, a broad peak appears at around 630 nm in the scattering spectrum and serves as evidence of hole plasmon resonance. From transmission mode near-field scanning optical microscopy measurements, two types of local field enhancement are observed. One has a small spatial extent of around 200 nm and the other has a large spatial extent of around 1 μm. The two types of enhancement correspond to strong and weak SPP localizations, respectively.

Particles nanomanipulation by the enhanced evanescent field through a near-field scanning optical microscopy probe

A near-field scanning optical microscopy (NSOM) probe and a polarized semiconductor laser (808 nm, cw) were applied to push the trapping resolution down to 120 nm on near-field optical manipulation. A multi-circular shape with a minimum size of 400 nm consisting of 120 nm polystyrene particles can be obtained. They are at a resolution of d ( d: NSOM probe tip diameter) and λ/7 ( λ: laser wavelength), respectively. It is proved that sample concentration and laser power can affect feature size of trapping patterns. In this paper, the effect of trapping forces acted on a nanoparticle along three axis directions on trapping positions is studied, and different trapping positions are generated: the aperture edge in polarization direction and center surface of the probe tip. The result indicates that the single mode NSOM fiber probe is able to trap nanoparticles in a circular shape with lower laser intensity than that required by conventional optical tweezers. The simulated trapping positions around the probe tip based on the conservation law of momentum are found to agree well with experimental results.

Prototype of Illumination-Collection Mode Scanning Near-Field Optical Microscopy and Raman Spectroscopy with Gold Inner-Coated Aperture-Less Pyramidal Probe

We have prototyped illumination-collection mode scanning near-field optical microscopy (SNOM) and near-field Raman spectroscopy (NFRS) with gold inner-covered aperture-less pyramidal probe in order to study the possibility to detect optical images, and Raman spectrum and Raman peak shift for stress distribution in Si device with high resolution of about 10 nm.

Intrinsic detection of scattering phase with near-field scanning optical microscope

We show that the interferometric interaction between the tip and the sample is inherently measured by a near-field scanning optical microscope (NSOM) operating in reflection mode. This is demonstrated by measuring the phase of the sample reflectivity on a standard multilayer system with variable thickness. The demonstrated intrinsic sensitivity to the phase has implications in the interpretation of images collected by using reflection mode NSOM.

Experimental Confirmation of Miniature Spiral Plasmonic Lens as a Circular Polarization Analyzer

A spiral plasmonic lens can focus circular polarization of a given handedness while simultaneously defocus the circular polarization of the opposite chirality, which may be used as a miniature circular polarization analyzer. In this letter, we experimentally investigated the plasmonic focusing properties of the spiral lens using a collection mode near-field scanning optical microscope. A single Archimedes' spiral slot with a single turn was etched through gold thin film as a spiral plasmonic lens. The plasmonic field at the focus of a spiral lens strongly depends on the spin of the incident photon. Circular polarization extinction ratio better than 50 is obtainable with a device size as small as only 4 times of surface plasmon wavelength.

Direct near-field optical imaging of plasmonic resonances in metal nanoparticle pairs

In this paper we investigate the near-field optical behavior of plasmon coupling in gold nanoparticle pairs. In particular, by performing series measurements through a fiber-collection mode near-field scanning optical microscope (NSOM), we directly observed the localized electromagnetic (EM) field distribution between two nanospheres is sensitively depended on the incident polarization and interparticle distance. The qualitative near-field observation and quantitative analysis facilitate more understanding of localized hot spots in surface-enhanced Raman scattering (SERS), and nano-applications in selectively controlling the spatial distribution of localized surface plasmon (SP) modes on a fabricated nanostructure by adjusting the polarization direction.

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.

Evanescent fields—Direct measurement, modeling, and application

The evanescent field surrounding an exposed planar waveguide in silica is accurately measured using scanning near field optical microscopy (SNOM) and compared to models of the field distribution. Distortions in the field due to edge effects and the proximity of the mode to the surface are all detected. The characterized field is use to quantitatively explore the difference in collection efficiency between contact mode SNOM and intermittent contact mode SNOM. A strong correlation between tip oscillation amplitude and detection efficiency is determined.

Realization of femtosecond-resolved near-field optical system and its application

By combining femtosecond laser with the scanning near-field optical microscope (SNOM), we have successfully realized the femtosecond-resolved near-field optical system. Through high frequency acousto-optic modulation and difference frequency lock-in detection, the signal-to-noise ratio was greatly improved and the background signals of the pump and probe light were removed. Therefore, we can measure a femtosecond-resolved weak transmission change in the collection mode SNOM. Spatial resolution of 80nm and temporal resolution better than 200fs were achieved simultaneously. Using this system, we studied the relaxation processes of hot electrons in gold nanostrucures and the difference in relaxation dynamics was observed at different sample positions.

Active bimorph-based tapping-mode distance control for scanning near-field optical microscopy of biological samples in liquid

Commonly used shear force scanning near-field optical microscopy (SNOM) of soft biological samples is more critical to implement in aqueous environment than in air. A tapping-mode distance control based on a rectangular piezoelectric bimorph cantilever attached vertically by a straight fiber tip as force sensor for SNOM is introduced. The bimorph lever serving as both the probe dither and the force responder operates in flexural mode with a spring constant k=3.7×103N∕m. The sensitivity of the sensor is enhanced through the increase in lever’s quality factor (Q) and the usage of a higher eigenmode. Experimental results reveal that the describe sensor can operate in liquid with an effective Q up to 103 at its second eigenfrequecy f2=18.8kHz. High sensitivity of the sensor is demonstrated by imaging soft biological samples. Near-field optical resolution of better than 100nm on red blood cells in water is obtained. Compared to the existing tapping mode SNOM setups, our approach is compact, sensitive, lacking in parasitic optical background, and easy to practice in liquid.

Theoretical Analyses on the Resolution of Collection Mode Scanning Near-Field Optical Microscopy

Numerical simulations have been carried out in the framework of waveguide theory to model collection mode scanning near-field optical microscopy (SNOM). The theoretical model includes the optical fiber end and describes the metal coated aperture on the probe tip. The developed formalism goes beyond the existing Bethe-Bouwkamp theories for electromagnetic transmission of subwavelength apertures. The finite coating and optical fiber end are now taken into account. The new features enable us to simulate the near-field probes that are widely used in the collection mode SNOM. The emphases of the numerical analyses have been mainly on the resolution mechanism of the microscopy. Influence on the resolution from important parameters of the probe tips, such as the size of the apertures and the probe-sample distance, is extensively studied. The resolution dependence has been analyzed in the light of the near-field coupling efficiency of the probe tip. An optimum tip size has been found which is balanced between the significant signal transmission and the resolution of the device.

Convenient near-field optical measurement and analysis of polystyrene spheres

The modification of a commercial tapping mode atomic force microscope (AFM) into a transmission mode near-field scanning optical microscope (NSOM) is presented and polystyrene spheres in the diameter of 100 nm are used in this experiment. The detection of near-field signals is based on photodiodes with lock-in technique, and resolutions of topography and near-field signals obtained are about 10 and 20 nm, respectively. Furthermore, it is discovered that the computer-simulated near-field energy distribution profile, obtained by scanning over polystyrene spheres under illumination mode, falls within 5% range as compared with experimental values. The near-field absorption coefficient can be determined by this way. This will be a useful theoretical model to analyze the near-field transmission effect from others.

Investigation of composition fluctuations in GaN:Mg using optical transmission spectroscopy, near-field scanning optical microscopy, and scanning Kelvin probe microscopy

Composition fluctuations in GaN:Mg have been investigated by optical transmission spectroscopy, near-field scanning optical microscopy (NSOM), and scanning Kelvin probe microscopy (SKPM). An Urbach tail is observed in the optical transmission spectrum of heavily Mg-doped GaN, with the magnitude of Urbach energy increasing with increasing compositional/potential fluctuations. From the relationship between the Urbach energy and the potential fluctuation magnitude, the spatial size of the compositional fluctuations in GaN:Mg (with the average Mg concentration of ∼4×1019cm−3) is calculated to be 220nm. The nanoscale compositional fluctuations are determined by NSOM and SKPM. A spatial variation of the transmission mode NSOM intensity is observed with a characteristic wavelength of ∼300nm, which is attributed to the inhomogeneous distribution of Mg. The normalized root-mean-square (rms) fluctuation of the transmission mode NSOM intensity for heavily Mg-doped GaN is a factor of 5 greater than that for undoped GaN. Similar microstructure patterns are observed in the NSOM and surface potential images of GaN:Mg as probed by SKPM. The variation of local surface potential (on the order of 100mV) is attributed to the same compositional fluctuations of Mg, which changes the local surface work function. The local Mg concentrations are calculated from the SKPM image. For GaN:Mg with the Mg concentration of 4×1019cm−3, a rms value of 5.4×1019cm−3 of the compositional fluctuations has been observed.

Indium concentration influence on PL spatial inhomogeneity in InGaN single quantum well structures detected by original low-cost near-field probes

Spatial distribution of InGaN photoluminescence (PL) at room temperature has been examined by observation through an illumination-transmission mode scanning near-field optical microscope (SNOM). The near-field PL mapping revealed a disuniform pattern of sharp or blunt emission peaks from few hundreds of nanometer extension to the micrometer scale range. The peaks’ average amplitude size and distances from neighboring peaks have been measured in a large number of experiments for three different samples at dissimilar indium doses, and their results compared. A general trend has been found and the influence of indium concentration is shown in a final plot. We describe the experimental approach adopted and present full details on the results obtained. Data were gathered from more than 100 different measurements; low-cost optical SNOM nanoprobes were originally fabricated and we briefly explain the fabrication procedure.

Indium concentration influence on PL spatial inhomogeneity in InGaN single quantum well structures detected by original low-cost near-field probes

Spatial distribution of InGaN photoluminescence (PL) at room temperature has been examined by observation through an illumination-transmission mode scanning near-field optical microscope (SNOM). The near-field PL mapping revealed a disuniform pattern of sharp or blunt emission peaks from few hundreds of nanometer extension to the micrometer scale range. The peaks’ average amplitude size and distances from neighboring peaks have been measured in a large number of experiments for three different samples at dissimilar indium doses, and their results compared. A general trend has been found and the influence of indium concentration is shown in a final plot. We describe the experimental approach adopted and present full details on the results obtained. Data were gathered from more than 100 different measurements; low-cost optical SNOM nanoprobes were originally fabricated and we briefly explain the fabrication procedure.

Near-field magneto-optical analysis in reflection mode SNOM

Observation of magnetic domains with in-plane magnetisations is demonstrated by scanning near-field optical microscopy (SNOM) in reflection mode. The longitudinal and transverse magneto-optical Kerr effects are employed as the contrast mechanisms; these are observed as either a change in the polarisation of the reflected light or reflectance, depending on magnetisation direction. SNOM images of Co and Ni thin films show magneto-optical contrast depending on polarisation of the incident and detected light. For the smooth cobalt thin films, the orientation for magnetic domains is estimated, based on the correlation between the contrasts in SNOM images obtained in different polarisation configurations and the directions of the magnetic vectors of the incident and reflected light. For the nickel films with pronounced topographic structures, the resulting near-field polarisation dependencies are more complicated, suggesting that the magneto-optical contrast in SNOM images are affected by the topographic cross-talk due to the depolarisation effects on surface topographic features.

Optical Fibre Mode Structures Measured by Collection ModeNear-Field Scanning Optical Microscopy

High order fibre mode structures are measured by a near-field scanningoptical microscope (NSOM) and are determined to be consistent with thestandard fibre LP patterns. The mode-field distortions ofhigh-order mode LP11 and LP21 structures by far-fieldmeasurement are analysed based on scalar diffraction theory, and the maximumdistortions may reach as high as 5.1% and 6.2%. This shows that themethod of using the NSOM is more accuracy than the far-field measurementmethod, especially for high-order mode structures of optical fibres.

Hydrophobic optical elements for near-field optical analysis (NOA) in liquid environment—a preliminary study

Near-field Scanning Optical Microscopy (NSOM) in liquid environment is expected to allow time resolved morphological mappings on cellular surfaces on the nanoscale level. Near-field Optical Analysis (NOA) via NSOM exploits the energy transfer from the tip of an optical element (tip diameter ≥20 nm), oscillating within the range of the characteristic length of the energy transfer (∼10 nm) in the near-field of the surface to be analysed. In NOA, a molecular assembly is monitored by visible light with a resolution far below the wavelength of visible light. Actually, NOA is successfully applied in mapping local optical contrasts, for instance in photonic crystals with dielectric periodicities on the nanoscale. NSOM could in principle be performed in two different modes: tapping mode, with tip-oscillations perpendicular, or shear force mode, with tip-oscillations parallel to the substrate. Both basic modes have specific advantages and disadvantages. In biological systems (e.g. in cell cultures), where scanning in liquids is prevalent, elongated optical elements non-invasively operated in the shear force modus could have some specific advantages when compared to contact modus systems. While tapping mode NSOM provides satisfactory nanoscale images even on solid surfaces covered with millimetres of liquids, the performance of shear force mode NSOM is presently largely confined to operations on dry samples. This is due to the inability of conventional shear force mode NSOM systems to provide sharp topographic images of sample surfaces substantially covered with liquids. By equipping a conventional NSOM system with hydrophobic optical elements, shear force mode based topographic images could be obtained on biological samples in dry as well as in aqueous environment, and with resolutions on the nanoscale level.

Analysis of the effects arising from the near-field optical microscopy of homogeneous dielectric slabs

The term “near-field optical microscopy” refers to a range of techniques capable of producing images of surfaces at a resolution far exceeding the Rayleigh criterion for conventional microscopes. In the illumination mode scanning near-field optical microscope, the sample is placed in the near field of an optical source formed from the aperture at the end of a fibre-optic taper. In normal operation, the aperture is raster scanned over the sample and, by recording the resultant far-field intensity transmitted through the sample as a function of the source coordinates, an image is assembled whose appearance derives from a non-trivial combination of the optical and topological composition of the sample. In this paper a numerical analysis of the near-field interaction of the source and sample is performed using a three-dimensional finite-difference time-domain method. A propagation technique is applied to the near-field values to form a far-field signal that is representative of the signal generated by a real instrument. In this way, the microscope output for a variety of very simple samples is simulated. It is shown that the magnitude of the transmitted signal can be increased both by decreasing the separation between source and sample and, in some circumstances, by increasing the sample permittivity. Simulated microscope output from samples containing topological variation is compared with that produced by samples containing a variation in refractive index and it is demonstrated that contrast inversion may occur at different scan heights in the latter case, leading to the possibility of distinguishing between the different contrast mechanisms experimentally.