Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips

The fabrication of silicon cantilever‐based scanning near‐field optical microscope probes with fully aluminium‐coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic‐ and contact‐mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone‐angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near‐field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near‐field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.     

Penetration pathways of fluorescent dyes in human hair fibres investigated by scanning near-field optical microscopy

Thin cross-sections of human hairs were investigated by scanning near-field optical microscopy (SNOM) and confocal laser scanning microscopy (CLSM) after penetration of a fluorescent dye. The same samples were measured with both techniques to compare the observed structures. The images obtained from the two methods show nearly identical structures representing pathways of the dye molecules in hairs. The SNOM images provide a higher resolution than the CLSM images. Therefore, SNOM is believed to be a suitable method for investigations at a resolution of 100 nm on penetration pathways of fluorescent dyes such as the cell membrane complex pathway in cross-sections of hairs.     

Fluorescence imaging with a laser trapping scanning near-field optical microscope

We investigated fluorescence imaging using a near-field scanning optical microscope which uses a laser-stabilized gold nanoparticle as a near-field probe. This microscope is suitable for observations of biological specimens in aqueous solutions because the probe particle is held by a noncontact force exerted by a laser beam. Theoretical calculations based on Mie scattering theory are presented to evaluate the near-field enhancement by a gold particle of 40 nm diameter. We also present fluorescence images of a single fluorescent bead and discuss the near-field contribution to the fluorescence image in this type of microscope.     

Current-sensing scanning near-field optical microscopy using a metal probe for nanometre-scale observation of electrochromic films

A novel technique for scanning near‐field optical microscopy capable of point‐contact current‐sensing was developed in order to investigate the nanometre‐scale optical and electrical properties of electrochromic materials. An apertureless bent‐metal probe was fabricated in order to detect optical and current signals at a local point on the electrochromic films. The near‐field optical properties could be observed using the local field enhancement effect generated at the edge of the metal probe under p‐polarized laser illumination. With regard to electrical properties, current signal could be detected with the metal probe connected to a high‐sensitive current amplifier. Using the current‐sensing scanning near‐field optical microscopy, the surface topography, optical and current images of coloured WO₃ thin films were observed simultaneously. Furthermore, nanometre‐scale electrochromic modification of local bleaching could be performed using the current‐sensing scanning near‐field optical microscopy. The current‐sensing scanning near‐field optical microscopy has potential use in various fields of nanometre‐scale optoelectronics.     

Femtosecond near-field scanning optical microscopy study of molecular thin films

A near-field scanning optical microscope has been combined with a two-colour time-resolved pump-probe measurement system. It has a noise-equivalent transmittance change of 5.0 × 10⁻⁵ for a probe pulse with an intensity of 30 nW. The system has been used for evaluating molecular thin films that have a domain structure, particularly for observing a gate action of the single domains. The results include key features to understand an origin of the domains and suggest that the film composition is uniform over a distance of several micrometres.     

Domain formation in thin lipid films probed with near-field scanning optical microscopy

High-resolution near-field scanning optical microscopy (NSOM) fluorescence and topographic images of l -α-dipalmitoylphosphatidylcholine (DPPC) monolayers doped with a fluorescent dye are presented. DPPC monolayers are deposited onto mica substrates from the air–water interface at several surface pressures using the Langmuir–Blodgett technique. Sub-diffraction limit phase domain structures are observed in both fluorescence and topographic NSOM images of the lipid films. The morphology of the resulting monolayers depends strongly on the surface pressure and composition of the subphase used in the film transfer. Mechanisms for lipid domain formation and growth are discussed.     

Tapping-mode tuning-fork near-field scanning optical microscopy of low power semiconductor lasers

The newly developed inverted tapping-mode tuning-fork near-field scanning optical microscopy (TMTF-NSOM) is used to study the local near-field optical properties of strained AlGaInP/Ga_0.4In_0.6P low power visible multiquantum-well laser diodes. In contrast to shear-force mode NSOM, TMTF-NSOM provides the function to acquire the evanescent wave intensity ratio | I (2ω)|/| I (ω)| image, from which the evanescent wave decay coefficient q can be evaluated for a known tapping amplitude. Moreover, we probe the near-field stimulated emission spectrum, which gives the free-space laser light wavelength λ_o and the index of refraction n _r of the laser diode resonant cavity. Once q , λ_o, and n _r are all measured, we can determine the angle of incidence θ_o of the dominant totally internally reflected waves incident on the front mirror facet of the resonator. Determination of such an angle is very important in modelling the stability of the laser diode resonator.     

A compact sensor head for simultaneous scanning force and near-field optical microscopy

A compact sensor head based on scanning force microscopy (SFM) using cantilever probes has been developed. The idea is to replace the microscope objective of a conventional optical microscope by this compact module and turn the optical microscope into a scanning force and near-field optical microscope with subwavelength resolution. We describe our concept and present initial results showing images of the object’s optical properties and surface topography recorded simultaneously.     

Spectroscopy with scanning near-field optical microscopy using photon tunnelling mode

We have demonstrated Raman spectroscopy using scanning near-field optical microscopy (SNOM). Photon tunnelling mode was employed, in which the sample is illuminated using an attenuated total reflection (ATR) configuration and the evanescent wave perturbed by the sample is picked up by a sharpened optical fibre probe. By this experimental arrangement Raman scattering from the optical fibre probe was greatly reduced, therefore we were able to excite the sample using more intense laser light compared to the illumination mode SNOM. Raman spectra of copper phthalocyanine (CuPc) were obtained in the off-resonance condition and without using surface-enhanced Raman scattering (SERS).     

The tetrahedral tip as a probe for scanning near-field optical microscopy at 30 nm resolution

The tetrahedral tip is introduced as a new type of a probe for scanning near-field optical microscopy (SNOM). Probe fabrication, its integration into a scheme of an inverted photon scanning tunnelling microscope and imaging at 30 nm resolution are shown. A purely optical signal is used for feedback control of the distance of the scanning tip to the sample, thus avoiding a convolution of the SNOM image with other simultaneous imaging modes such as force microscopy. The advantages of this probe seem to be a very high efficiency and its potential for SNOM at high lateral resolution below 30 nm.     

Imaging 'intact' myofibrils with a near-field scanning optical microscope

Fluorescently labelled myofibrils were imaged in physiological salt solution by near-field scanning optical microscopy and shear-force microscopy. These myofibrils were imaged in vitro , naturally adhering to glass while retaining their ability to contract. The Z-line protein structure of the myofibrils was antibody labelled and easily identified in the near-field fluorescence images. The distinctive protein banding structure of the myofibril was also seen clearly in the shear-force images without any labelling requirement. With the microscope in the transmission mode, resolution of the fluorescence images was degraded significantly by excessive specimen thickness (>1 μm), whereas the shear-force images were less affected by specimen thickness and more affected by poor adherence to the substrate. Although the exact mechanism generating contrast in the shear-force images is still unknown, shear-force imaging appears to be a promising new imaging modality.     

Shear force near-field optical microscope based on Q-controlled bimorph sensor for biological imaging in liquid

Shear force near‐field microscopy on biological samples in their physiological environment loses considerable sensitivity and resolution as a result of liquid viscous damping. Using a bimorph‐based cantilever sensor incorporating force feedback, as recently developed by us, gives an alternative force detection scheme for biological imaging in liquid. The dynamics and sensitivity of this sensor were theoretically and experimentally discussed. Driving the bimorph cantilever close to its resonance frequency with appropriate force feedback allows us to obtain a quality factor (Q‐factor) of up to 10³ in water, without changing its intrinsic resonance frequency and spring constant. Thus, the force detection sensitivity is improved. Shear force imaging on mouse brain sections and human skin tissues in liquid with an enhanced Q‐factor of 410 have shown a high sensitivity and stability. A resolution of about 50 nm has been obtained. The experimental results suggest that the system is reliable and particularly suitable for biological cell imaging in a liquid environment.     

Nanoscopic observation of a gold nanoparticle-conjugated protein using near-field scanning optical microscopy

This study describes a single gold nanoparticle (AuNP)-based observation of biomolecular interaction using a near-field scanning microscope (NSOM) in transmission mode. To observe streptavidin molecules, a glass surface was first patterned with a micro-scale line of (3-aminopropyl)trimethoxysilane (APTMS) by micro-contact printing (μCP) with a subsequent reaction of N-hydroxysuccinimide (NHS)-biotin. The AuNP-conjugated streptavidin was then applied to the biotin-modified glass surface and NSOM was employed to detect the resulting specific interaction between streptavidin and biotin on the glass surface. Using the optical and topological images generated from the NSOM analysis, the interaction could be observed at the nanoscopic scale. This study demonstrates that the NSOM is a powerful tool for the detection of protein interactions at the nanoscopic level when the protein is conjugated with AuNPs.     

Magnetic domain imaging with a scanning near-field optical microscope using a modified Sagnac interferometer

We report on the combination of a scanning near-field optical microscope and a modified Sagnac interferometer for magnetic-domain imaging in the reflection mode. The Sagnac interferometer is used for detection of the magneto-optical Kerr effect. Since the interferometer is inherently insensitive to polarization changes caused by topography effects, magnetic-domain imaging is not limited to samples with flat surfaces. In this way, it is possible to image magnetic bits written on the tracks of a magneto-optical disc that has a rather pronounced surface profile.     

散射型近场扫描光学显微镜的探针振动参数优化研究

基于原子力显微镜平台设计了可见-近红外波段的散射型近场扫描光学显微镜,通过理论模型计算和实验测量,分析了散射探针振动的调制振幅和扫描反馈幅值对近场信号的影响。研究表明:与探针针尖尺寸相近的调制振幅有利于抑制背景散射噪声及优化近场信号的信噪比;当探针扫描反馈幅值与自由空间调制振幅之比大于90%时,可基本消除探针扫描过程中非简谐振动对近场成像测量的影响。     

A scanning near-field optical microscope having scanning electron tunnelling microscope capability using a single metallic probe tip

A new microscope system that has the combined capabilities of a scanning near-field optical microscope (SNOM) and a scanning tunnelling microscope (STM) is described. This is achieved with the use of a single metallic probe tip. The distance between the probe tip and the sample surface is regulated by keeping the tunnelling current constant. In this mode of operation, information about the optical properties of the sample, such as its refractive index distribution and absorption characteristics, can be disassociated from the information describing its surface structure. Details of the surface structure can be studied at resolutions smaller than the illumination wavelength. The performance of the microscope is evaluated by analysing a grating sample that was made by coating a glass substrate with gold. The results are then compared with the corresponding SNOM and STM images of the grating.     

Polarization contrast in reflection near-field optical microscopy with uncoated fibre tips

Using cross-hatched, patterned semiconductor surfaces and round 20-nm-thick gold pads on semiconductor wafers, we investigate the imaging characteristics of a reflection near-field optical microscope with an uncoated fibre tip for different polarization configurations and light wavelengths. It is shown that cross-polarized detection allows one to effectively suppress far-field components in the detected signal and to realize imaging of optical contrast on the sub-wavelength scale. The sensitivity window of our microscope, i.e. the scale on which near-field optical images represent mainly optical contrast, is found to be ≈100 nm for light wavelengths in the visible region. We demonstrate imaging of near-field components of a dipole field and purely dielectric contrast (related to well-width fluctuations in a semiconductor quantum well) with a spatial resolution of ≈100 nm. The results obtained show that such a near-field technique can be used for polarization-sensitive imaging with reasonably high spatial resolution and suggest a number of applications for this technique.     

Labelling quality and chromosome morphology after low temperature FISH analysed by scanning far-field and near-field optical microscopy

A non‐enzymatic, low temperature fluorescence in situ hybridization (LTFISH) procedure was applied to metaphase spreads and interphase cell nuclei. In this context ‘low temperature’ means that the denaturation procedure of the chromosomal target DNA usually applied by heat treatment and chaotropic agents such as formamide was completely omitted so that the complete hybridization reaction took place at 37 °C. For LTFISH, the DNA probe had to be single‐stranded, which was achieved by means of separate thermal denaturation of the DNA probe only. The DNA probe pUC1.77 was used for all LTFISH experiments. The labelling quality (number of binding sites, relative background intensity, relative intensity of major and minor binding sites) was analysed by confocal laser scanning microscopy (CLSM). An optimum in specificity and signal quality was obtained for 15 h hybridization time. For this hybridization condition of LTFISH, the chromosomal morphology was analysed by scanning near‐field optical microscopy (SNOM). The results were compared with the morphology of chromosomes after (a) labelling of all centromeres using the same chemical treatment in the FISH procedure but with the application of target denaturation, and (b) labelling of all centromeres using a standard FISH protocol including thermal denaturation of the DNA probe and the chromosomal target. Depending on the FISH‐procedure applied, SNOM images show substantial differences in the chromosome morphology. After LTFISH the chromosome morphology appeared to be much better preserved than after standard FISH. In contrast, the application of the LTFISH chemical treatment accompanied by heat denaturation had a very destructive influence on chromosomal morphology. The results indicate that, at least for certain DNA probes, specific chromosome labelling can be obtained without the usually applied heat and chemical denaturation of the DNA target, resulting in an apparently well preserved chromatin morphology as visualized by SNOM. LTFISH may be therefore a useful labelling technique whenever the chromosomal morphology had to be preserved after specific labelling of DNA regions. Binding mechanisms of single‐stranded DNA probes to double‐stranded DNA targets are discussed.     

Dynamic etching method for fabricating a variety of tip shapes in the optical fibre probe of a scanning near-field optical microscope

A novel etching method for an optical fibre probe of a scanning near-field optical microscope (SNOM) was developed to fabricate a variety of tip shapes through dynamic movement during etching. By moving the fibre in two-phase fluids of HF solution and organic solvent, the taper length and angle can be varied according to the movement of the position of the meniscus on the optical fibre. This method produces both long (sharp angle) and short (wide angle) tapered tips compared to tips made with stationary etching processes. A bent-type probe for a SNOM/AFM was fabricated by applying this technique and its throughput efficiency was examined. A wide-angle probe with a 50° angle at the tip showed a throughput efficiency of 3.3 × 10⁻⁴ at a resolution of 100 nm.