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.     

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 height regulation of a near-field scanning optical microscope probe tip

A nonoptical detection of the optical fibre tip has been developed. By detecting the output signal from a tiny piezoelectric detector attached to the vibrating fibre tip, the distance between the fibre tip and the sample has been successfully controlled. The frequency responses of the system composed of tip, the dither and the detector have been studied. The difference between the shear-force detection and the tapping-mode detection is discussed. It is found that the shear force exerted on the tip reduces the vibration amplitude with an unvaried resonance frequency. However, in the tapping mode, the resonance frequency varies with the tip–sample distance as the force is exerted on the fibre tip only within a half period. This requires better adjustments for the tapping-mode detection.     

A versatile multipurpose scanning probe microscope

A combined scanning probe microscope has been developed that allows simultaneous operation as a non‐contact/tapping mode atomic force microscope, a scattering near‐field optical microscope, and a scanning tunnelling microscope on conductive samples. The instrument is based on a commercial optical microscope. It operates with etched tungsten tips and exploits a tuning fork detection system for tip/sample distance control. The system has been tested on a p‐doped silicon substrate with aluminium depositions, being able to discriminate the two materials by the electrical and optical images with a lateral resolution of 130 nm.     

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.     

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.     

Photoconductive imaging in a photon scanning tunnelling microscope capable of point-contact current sensing using a conductive fibre probe

A photoconductive photon scanning tunnelling microscope was developed to investigate the point-contact photoconductive properties of condensed matter. In order to detect the current and the optical signal at a local point on a surface, we coated the edge of a bent type fibre probe with indium tin oxide. Thus it was possible to measure both photocurrent and optical property with subwavelength resolution. The performance of the novel microscope was evaluated by analysing an organic thin film of copper phthalocyanine (CuPc), which is known to be an efficient photoconductive material. Photocurrent and current–voltage characteristics were observed at the local point on the CuPc thin films. Furthermore, photoconductive images were obtained with topography and near-field optical imaging using this system. The photoconductive PSTM shows potential in various areas of future optics and electronics.     

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.     

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.     

Polarization properties of the near-field intensity reflected by metallic and dielectric one-dimensional structures

We investigate the state of polarization and near-field intensity distribution in the vicinity of rectangular groove objects ruled on metallic and dielectric materials. The sample is illuminated from the vacuum side by a linear combination of p- and s-polarised waves. Two rigorous methods of solution are used and compared in calculations of the total intensity at constant height when the light is incident normally onto the surface. Some calculations of the total intensity in the 'follow-the-profile mode' are also presented. It is shown that in the constant height mode, the contrast in the image can be reversed as the plane of observation moves away from the mean plane of the sample. We also found that the state of polarization depends strongly on the material and the distance to the plane of detection.     

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.     

Implementation of a short-tip tapping-mode tuning fork near-field scanning optical microscope

We present the implementation of a short‐tip tapping‐mode tuning fork near‐field scanning optical microscope. Tapping frequency dependences of the piezoelectric signal amplitudes for a bare tuning fork fixed on the ceramic plate, a short‐tip tapping‐mode tuning fork scheme and an ordinary tapping‐mode tuning fork configuration with an 80‐cm optical fibre attached are demonstrated and compared. Our experimental results show that this new short‐tip tapping‐mode tuning fork scheme provides a stable and high Q factor at the tapping frequency of the tuning fork and will be very helpful when long optical fibre probes have to be used in an experiment. Both collection and excitation modes of short‐tip tapping‐mode tuning fork near‐field scanning optical microscope are applied to study the near‐field optical properties of a single‐mode telecommunication optical fibre and a green InGaN/GaN multiquantum well light‐emitting diode.     

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.     

Image contrast of dielectric specimens in transmission mode near-field scanning optical microscopy: imaging properties and tip artefacts

Near-field scanning optical microscopy (NSOM) is a scanned probe technique utilizing a subwavelength-sized light source for high-resolution imaging of surfaces. Although NSOM has the potential to exploit and extend the experimental utility of the modern light microscope, the interpretation of image contrast is not straightforward. In near-field microscopy the illumination intensity of the source (probe) is not a constant value, rather it is a function of the probe–sample electronic environment. A number of dielectric specimens have been studied by NSOM to elucidate the contrast role of specimen type, topography and crystallinity; a summary of metallic specimen observations is presented for comparative purposes. Near-field image contrast is found to be a result of lateral changes in optical density and edge scattering for specimens with little sample topography. For surfaces with considerable topography the contributions of topographic (Z) axis contrast to lateral (X,Y) changes in optical density have been characterized. Selected near-field probes have also been shown to exhibit a variety of unusual contrast artefacts. Thorough study of polarization contrast, optical edge (scattering) contrast, as well as molecular orientation in crystalline specimens, can be used to distinguish lateral contrast from topographic components. In a few cases Fourier filtering can be successfully applied to separate the topographic and lateral contrast components.     

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.     

Evaluation of nano-optical probe from scanning near-field optical microscope images

We studied a nanometre-sized optical probe in a scanning near-field optical microscope. The probe profile is determined by using a knife-edge method and a modulated transfer function evaluation method which uses nanometre-sized line-and-space tungsten patterns (with spaces 1 μm to 50 nm apart) on SiO₂ substrates. The aluminium-covered, pipette-pulled fibre probe used here has two optical probes: one with a large diameter (350 nm) and the other with a small diameter (10 nm). The small-diameter probe has an optical intensity ≈63 times larger than that of the large-diameter probe, but the power is about 1/25 of that of the large probe.     

Probe design optimization for a high-resolution scattering-type scanning near-field optical microscope

The scattering-type scanning near-field optical microscope (SNOM) has a probe with a sharp tip for use in high resolution imaging. As sharp a tip as possible is generally considered ideal for the observations, but actually, a sharp tip does not always provide a high resolution SNOM image. We numerically examined the scattering property of the SNOM probe by the three dimensional finite difference time domain method. In this paper, we show the criterion for the ideal scattering probe which satisfies the simple relation between radius and taper angle of the tip.     

Optical-fibre scanning near-field optical microscope for cryogenic operation

We have developed a new type of scanning near-field optical microscope (SNOM) utilizing optical fibres. The probe tip is controlled by shear force feedback with a fibre interferometer and signal light is collected directly by a multimode fibre. These features make the SNOM head more compact and less sensitive to vibration. Further advantages of this new type of SNOM are that it obviates the need for optical windows in the cryostat and offers easy optical alignment.