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.     

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).     

Optical characterization of probes for photon scanning tunnelling microscopy

The photon scanning tunnelling microscope is a well-established member of the family of scanning near-field optical microscopes used for optical imaging at the sub-wavelength scale. The quality of the probes, typically pointed uncoated optical fibres, used is however, difficult to evaluate in a direct manner and has most often been inferred from the apparent quality of recorded optical images. Complicated near-field optical imaging characteristics, together with the possibility of topographically induced artefacts, however, has increased demands for a more reliable probe characterization technique. Here we present experimental results obtained for optical characterization of two different probes by imaging of a well-specified near-field intensity distribution at various spatial frequencies. In particular, we observe that a sharply pointed dielectric probe can be highly suitable for imaging when using p -polarized light for the illumination. We conclude that the proposed scheme can be used directly for probe characterization and, subsequently, for determination of an optical transfer function, which would allow one to deduce from an experimentally obtained image of a weakly scattering sample the field distribution existing near the sample surface in the absence of the probe.     

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.     

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.     

Coaxial probes for scanning near-field microscopy

This paper deals with the development of coaxial aperture tips integrated in a cantilever probe for combined scanning near-field infrared microscopy and scanning force microscopy. A fabrication process is introduced that allows the batch fabrication of hollow metal aperture tips integrated on a silicon cantilever. To achieve the coaxial tip arrangement a metal rod is deposited inside the hollow tip using the focused ion beam technique. Theoretical calculations with a finite integration code were performed to study the transmission characteristics of coaxial tips in comparison with conventional aperture probes. In addition, the influence of the geometrical design parameters of the coaxial probe on its optical behaviour is investigated.     

Characterization of reflection scanning near-field optical microscopy and scanning tunnelling optical microscopy/photon scanning tunnelling microscopy working in preliminary approach constant height scanning mode

The resolution in near-field images is currently determined by the visual inspection of recorded images. One of the major questions in near-field optical microscopy is 'what resolution can be reached, the tip-to-sample distance being known?' This knowledge is critical when choosing the scanning step and the distance between the tip and the sample, in a preliminary scan. This preliminary scan is often the only way to detect the interesting parts of the sample, with limited risk of tip crash and topographical artefacts. The method proposed here needs two scans of the same area, of the same sample, in constant height mode, recorded at two tip-to-sample distances. The pseudo-transfer function is the ratio of the Fourier transform of these two data maps. This function enables the evaluation of the limit of resolution. Theoretical considerations are introduced to assess the method.     

Imaging of optical disc using reflection-mode scattering-type scanning near-field optical microscopy

A phase-change optical disc was observed using a reflection-mode scattering-type scanning near-field optical microscope (RS-SNOM). In an a.c.-mode SNOM image, the 1.2 μm × 0.6 μm recording marks were successfully observed although the data were recorded on the groove. In contrast, no recording marks could be resolved in a d.c.-mode SNOM image. These results are in good agreement with those from a numerical simulation using the finite difference time domain method. The resolution was better than 100 nm with a.c.-mode SNOM operation and the results indicate that recording marks in phase-change optical media can be directly observed with the RS-SNOM.     

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.     

Plasmon-coupled tip-enhanced near-field optical microscopy

Near the cut‐off radius of a guided waveguide mode of a metal‐coated glass fibre tip it is possible to couple radiation to surface plasmons propagating on the outside surface of the metal coating. These surface plasmons converge toward the apex of the tip and interfere constructively for particular polarization states of the initial waveguide mode. Calculations show that a radially polarized waveguide mode can create a strong field enhancement localized at the apex of the tip. The highly localized enhanced field forms a nanoscale optical near‐field source.     

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.     

散射式太赫兹扫描近场光学显微技术研究

基于散射式近场探测原理,设计并搭建了散射式太赫兹扫描近场光学显微系统(THz s-SNOM),实现了纳米量级空间分辨率的太赫兹近场显微成像测量。该系统以输出频率范围为0.1~0.3THz的太赫兹倍频模块为发射源,通过纳米探针的针尖产生纳米光源与样品相互作用,并将样品表面的倏逝波转化为可在远场测量的辐射波。通过探针逐点扫描样品表面,同时获得了样品表面的形貌图和太赫兹近场显微图。该系统的显微分辨率取决于探针针尖的曲率半径,而与太赫兹波的波长无关。使用该系统测量了金薄膜/硅衬底样品和石墨烯样品的近场显微图,结果表明,近场显微的空间分辨率优于60nm,波长与空间分辨率之比高达λ/26000。     

Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force

We develop a novel optical microcantilever for scanning near-field optical microscopy controlled by atomic force mode (SNOM/AFM). The optical microcantilever has the bent channel waveguide, the corner of which acts as aperture with a large tip angle. The resonance frequency of the optical microcantilever is 9 kHz, and the spring constant is estimated to be 0.59 N/m. The optical microcantilever can be operated in contact mode of SNOM/AFM and we obtain the optical resolution of about 200 nm, which is as same size as the diameter of aperture. We confirm that the throughput of optical microcantilever with an aperture of 170 nm diameter would be improved to be more than 10⁻⁵.     

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.     

Investigation of dyed human hair fibres using apertureless near-field scanning optical microscopy

We present the first studies of dyed human hair fibres performed with an apertureless scanning near‐field optical microscope. Samples consisted of 5‐µm‐thick cross‐sections, the hair fibres being bleached and then dyed before being cut. Hair dyed with two molecular probes diffusing deep inside the fibre or mainly spreading at its periphery were investigated at a wavelength of 655 nm. An optical resolution of about 50 nm was achieved, well below the diffraction limit; the images exhibited different optical contrasts in the cuticle region, depending on the nature of the dye. Our results suggest that the dye that remains confined at the hair periphery is mainly located at its surface and in the endocuticle.     

Imaging an optical disc by the combined use of scanning tunnelling microscopy and scanning electron microscopy

We present the data obtained by scanning tunnelling microscopy combined with scanning electron microscopy of the digitally encoded structure on a stamper used to fabricate optical discs. The combination allows us to focus the STM tip on a preselected spot with a precision of ?0·3 μm. The data show the superiority of STM for a more detailed characterization of shape, width, length, height and fine structure appearing on the sample. We also show the influence of tip shape on STM resolution. Simultaneous use of both microscopes is possible but high electron doses produce an insulating layer of contaminants thick enough to make STM operation impossible.     

Near-field scanning optical microscopy using polymethylmethacrylate optical fiber probes

We report the first use of polymethylmethacrylate (PMMA) optical fiber-made probes for scanning near-field optical microscopy (SNOM). The sharp tips were prepared by chemical etching of the fibers in ethyl acetate, and the probes were prepared by proper gluing of sharpened fibers onto the tuning fork in the conditions of the double resonance (working frequency of a tuning fork coincides with the resonance frequency of dithering of the free-standing part of the fiber) reported earlier for the case of glass fibers. Quality factors of the probes in the range 2000–6000 were obtained, which enables the realization of an excellent topographical resolution including state-of-art imaging of single DNA molecules. Near-field optical performance of the microscope is illustrated by the Photon Scanning Tunneling Microscope images of fluorescent beads with a diameter of 100 nm. The preparation of these plastic fiber probes proved to be easy, needs no hazardous material and/or procedures, and typical lifetime of a probe essentially exceeds that characteristic for the glass fiber probe.