Non-optical tip–sample distance control method for scanning near-field optical microscopy using a piezoresistive micro cantilever

A piezoresistive micro cantilever is applied to monitor the displacement of an optical fibre probe and to control tip–sample distance. The piezoresistive cantilever was originally made for a self-sensitive atomic force microscopy (AFM) probe and has dimensions of 400 µm length, 50 µm width and 5 µm thickness with a resistive strain sensor at the bottom of the cantilever. We attach the piezoresistive cantilever tip to the upper side of a vibrating bent optical fibre probe and monitor the resistance change amplitude of the strain sensor caused by the optical fibre displacement. By using this resistance change to control the tip–sample distance, the two-cantilever system successfully provides topographic and near-field optical images of standard samples in a scanning near-field optical microscopy (SNOM)/AFM system. A resonant characteristic of the two-cantilever system is also simulated using a mechanical model, and the results of simulation correspond to the experimental results of resonance characteristics.     

High-resolution constant-height imaging with apertured silicon cantilever probes

We present high-resolution aperture probes based on non-contact silicon atomic force microscopy (AFM) cantilevers for simultaneous AFM and near-infrared scanning near-field optical microscopy (SNOM). For use in near-field optical microscopy, conventional AFM cantilevers are modified by covering their tip side with an opaque aluminium layer. To fabricate an aperture, this metal layer is opened at the end of the polyhedral probe using focused ion beams (FIB). Here we show that apertures of less than 50 nm can be obtained using this technique, which actually yield a resolution of about 50 nm, corresponding to λ/20 at the wavelength used. To exclude artefacts induced by distance control, we work in constant-height mode. Our attention is particularly focused on the distance dependence of resolution and to the influence of slight cantilever bending on the optical images when scanning at such low scan heights, where first small attractive forces exerted on the cantilever become detectable.     

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.     

Excitation energy transfer of porphyrin in polymer thin films by time-resolved scanning near-field optical microspectroscopy

Thin films of water-soluble free-base porphyrin, 5,10,15,20-tetraphenyl-21H, 23H-porphinetetrasulphonic acid (TPPS) mixed with poly(diallyldimethyl ammonium chloride) (PDDA) have been prepared by a spin-coating method, in which the monomeric species were observed in the spin-coat film, whereas dimer was formed in the cast film prepared from TPPS/PDDA solution. Mesoscopic structures and dynamics of excitation energy migration and trapping of TPPS/PDDA spin-coat film have been analysed by time-resolved scanning near-field optical microspectroscopy (SNOM) and atomic force microscope. The observed film structure can be classified roughly into two parts: one is a large, flocculated polymer part, and the other is a smooth part widely spread around the flocculated polymers. In the smooth part, the observed spindle-like structure and circular hills and dips are essentially due to PDDA. The ellipsoidal small structures with ∼2µm length and <1 µm width in the flocculated polymer part show non-exponential fluorescence decays. The non-exponential dynamics originates from the excitation energy migration among TPPS monomers and energy trapping to dimers. From the analysis of fluorescence decay curves based on the equation developed by Klafter and Blumen, the spectral dimension has been estimated to be ∼1.46 for ellipsoidal structures. These results indicate that the distribution of the chromophore is inhomogeneous and a fractal-like structure exists even in the small domains determined by the resolution of the SNOM tip.     

Mesoscopic structures and dynamics of merocyanine J-aggregate studied by time-resolved fluorescence SNOM

Time-resolved fluorescence SNOM is used to probe the mesoscopic structure and dynamics of long-chain merocyanine (C₁₈MC) J-aggregates on glass plates prepared by spin coating, casting, and casting of water-soluble polymer films. A globular structure with an average diameter of ∼ 1 µm and a height of ∼ 50 nm was attributed to the J-aggregate of C₁₈MC in the spin-coating film. In polymer films, the bandwidth of the absorption of J-aggregate is much narrower in polyvinyl alcohol (PVA, ∼ 20 nm) than that in polyvinyl sulphate (PVS, ∼ 60 nm). We have demonstrated that the large bandwidth of the spectrum is due to the inhomogeneous distribution of the J-aggregate. The fluorescence image of the J-aggregate in PVA film was rather uniform, whereas non-uniform distribution of the fluorescence was observed in PVS film. The fluorescence of C₁₈MC J-aggregate in a small domain of PVA film was a single exponential decay with a lifetime as short as 19 ps, which was shorter than that in PVS film with a two-exponential decay (average lifetime of ∼ 25 ps). The fluorescence lifetime of the J-aggregate and its single exponential behaviour are considered to be indicators of the uniform distribution of the J-aggregate. The non-uniform distribution of the J-aggregate in PVS film was interpreted in terms of electrostatic interaction between PVS and merocyanine.     

Three-dimensional analysis of light propagation through uncoated near-field fibre probes

The near-field emission from uncoated tapered fibre probes is investigated for different probe geometries. The three-dimensional model calculations are based on Maxwell's curl equations and describe the propagation of a 10 fs optical pulse (λ = 805 nm) through tapers of different lengths and different diameters of the taper exit. The numerical evaluation is done with a finite difference time domain code. Two tapers with cone angles of 50°, with taper lengths of 1.5 µm and 1.0 µm and exit diameters of 100 nm and 520 nm, respectively, are considered. We find that without sample the short taper with large exit diameter optimizes both light transmission and spatial resolution. In the presence of a sample with a high dielectric constant, however, the spatial near-field distribution changes drastically for both taper geometries. We find a pronounced increase in spatial resolution, down to about 250 nm inside the medium. This collimation of the near-field distribution arises from interferences between emitted and reflected light from the sample surface and from a collimation effect that the field experiences in the high-index semiconductor material. The combination of high spatial resolution and transmission and collection efficiencies makes such probes interesting for spectroscopic investigations, as demonstrated by recent experiments.     

Propagation of surface plasmon polariton in nanometre-sized metal-clad optical waveguides

Using a local anodic‐oxidation method with a probe tip of a scanning near‐field optical microscope (SNOM) as the electrode, we have fabricated an oxide core with subwavelength dimensions on metal. The propagation of the surface plasmon polariton (SPP), which is excited at the interface between the oxide core and the metal clad, has been investigated using the same SNOM. Altering the wavelength of input light from 532 nm to 830 nm, the propagation length of the SPP extends from 2 µm to 6 µm. We carried out a simulation of the SPP propagation, and obtained a similar wavelength dependence.     

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.     

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.     

High-speed scanning by dual feedback control in SNOM/AFM

We have developed a high-speed scanning near-field optical microscope (SNOM)/atomic force microscope (AFM) system including dual feedback controllers. The system includes an additional piezoelectric actuator with fast response in the z direction and a correction circuit to eliminate unnecessary components from the feedback signal. From the measurement of a patterned chromium layer of 2 × 2 μm² checks on a quartz glass plate, we confirmed that our system had more effective feedback control and faster scanning than current SNOM/AFM systems that use only a piezo-tube. The scanning speed of the present system was estimated to be about five times faster than that of current SNOM/AFM systems.     

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.     

Successful application of spatial difference technique to electron energy-loss spectroscopy studies of Mo/SrTiO3 interfaces

The electron energy‐loss near‐edge structure (ELNES) of Mo/SrTiO₃ interfaces has been studied using high spatial resolution electron energy‐loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)‐orientated SrTiO₃ surfaces by molecular beam epitaxy at 600 °C. High‐resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)_Mo plane parallel to the substrate surface. Ti‐L_2,3 (～460 eV), O‐K (～530 eV), Sr‐L_2,3 (～1950 eV) and Mo‐L_2,3 (～2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface‐specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti‐L_2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO₃, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO₆ octahedra was disturbed. No interfacial Sr‐L_2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface‐specific O‐K edge was found, which differs from that of the bulk in both position (0.8 ± 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 ± 0.3 eV) occurred for the interfacial Mo‐L_2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO₃ was terminated with TiO₂. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.     

SNOM/STM using a tetrahedral tip and a sensitive current-to-voltage converter

Scanning near-field optical microscopes (SNOM) using the tetrahedral-tip (T-tip) with scanning tunnelling microscopy (STM) distance control have been realized in transmission and reflection mode. Both set-ups used ordinary STM current-to-voltage converters allowing measurement of metallic samples. In the transmission mode, a resolution of 10 nm to 1 nm with regard to material contrast can be achieved on binary metal samples. Because of the great near-field optical potential of the T-tip with respect to the optical resolution, it is a challenging task to find out whether these results can be transferred to non-metallic sample systems as well. This paper reports on a newly designed SNOM/STM transmission mode set-up using the tetrahedral-tip. It implements a sensitive current-to-voltage converter to widen the field of measurable sample systems. Beyond this, mechanical and optical measuring conditions are substantially improved compared to previous set-ups. The new set-up provides a basis for the routine investigation of metal nanostructures and adsorbed organic monolayers at resolutions in the 10 nm range.     

Far-field fluorescence microscopy with three-dimensional resolution in the 100-nm range

We report three-dimensional (3D) microscopy with nearly isotropic resolution in the λ/5 − λ/10 range. Our approach combines 4Pi-confocal two-photon fluorescence microscopy with image restoration. The 3D resolution is demonstrated with densely clustered beads as well as with F-actin fibers in mouse fibroblast cells. A comparison with unrestored two-photon confocal images reveals a total reduction of the uncertainty volume up to a factor of 15.     

Nano‐optical reversible switching of organic photochromes*

Photochromic fulgides are UHV‐deposited in ultrathin films exhibiting intrinsic nanostructures. The two isomeric states (C and E) are detected in absorption via scanning near‐field optical microscopy (SNOM). The optical image contrast measured in SNOM corresponds to the nanostructure topography observed in atomic force microscopy. Nano‐optical reversible switching of individual nanostructures is demonstrated and possible applications to rewritable optical recording on the nanoscale are discussed.     

Resolution enhancing using cantilevered tip-on-aperture silicon probe in scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) achieves a resolution beyond the diffraction limit of conventional optical microscopy systems by utilizing subwavelength aperture probe scanning. A problem associated with SNOM is that the light throughput decreases markedly as the aperture diameter decreases. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating the light field near the tip apex. However, a far-field illumination by a focused laser beam generates a large background scattering signal. Both disadvantages are overcome using the tip-on-aperture (TOA) approach, as presented in previous works. In this study, a finite difference time domain analysis of the degree of electromagnetic field enhancement is performed to verify the efficiency of TOA probes. For plasmon enhancement, silver is deposited on commercially available cantilevered SNOM tips with 20nm thicknesses. To form the aperture and TOA in the probes, electron beam-induced deposition and focused ion beam machining were applied at the end of the sharpened tip. The results show that cantilevered TOA probes were highly efficient for improvements of the resolution of optical and topological measurement of nanostructures.     

High resolution protein localization using soft X-ray microscopy

Soft X-ray microscopes can be used to examine whole, hydrated cells up to 10 µm thick and produce images approaching 30 nm resolution. Since cells are imaged in the X-ray transmissive 'water window', where organic material absorbs approximately an order of magnitude more strongly than water, chemical contrast enhancement agents are not required to view the distribution of cellular structures. Although living specimens cannot be examined, cells can be rapidly frozen at a precise moment in time and examined in a cryostage, revealing information that most closely approximates that in live cells. In this study, we used a transmission X-ray microscope at photon energies just below the oxygen edge (λ = 2.4 nm) to examine rapidly frozen mouse 3T3 cells and obtained excellent cellular morphology at better than 50 nm lateral resolution. These specimens are extremely stable, enabling multiple exposures with virtually no detectable damage to cell structures. We also show that silver-enhanced, immunogold labelling can be used to localize both cytoplasmic and nuclear proteins in whole, hydrated mammary epithelial cells at better than 50 nm resolution. The future use of X-ray tomography, along with improved zone plate lenses, will enable collection of better resolution (approaching 30 nm), three-dimensional information on the distribution of proteins in cells.     

Assessment of chloride secretion in human nasal epithelial cells by X-ray microanalysis

The genetic disease cystic fibrosis (CF) is due to defective epithelial chloride transport. Different treatments have been proposed that could restore chloride transport in CF patients. A new method is proposed for measuring the chloride secretion in easily accessible epithelial cells.
Fresh nasal epithelial cells were obtained by nasal brushing and made to attach to titanium grids for electron microscopy. Chloride efflux through the cystic fibrosis transmembrane regulator channel was stimulated by 20 µ m forskolin and 100 µ m isobutyl-methylxanthine (IBMX), in standard Ringer's solution (SR). Chloride efflux through the calcium-regulated channel was stimulated by 200 µ m adenosine triphosphate (ATP) in SR. The cells were rinsed after the exposure, in order to remove the experimental medium, frozen and freeze-dried. The elemental composition of the cells was determined by X-ray microanalysis.
Rinsing with distilled water or ammonium acetate appeared to cause damage to the cells, whereas rinsing with isotonic mannitol preserved the ionic composition. Stimulation of cells from healthy controls with forskolin and IBMX in a chloride-containing medium caused a significant (28 ± 6%) decrease in chloride concentration, which is indicative of net chloride efflux. In similar conditions, stimulation with ATP induced a 29 ± 5% decrease in the chloride concentration.
Stimulation of cells from CF patients with forskolin and IBMX in a chloride-containing medium caused no significant change in the intracellular chloride concentration, whereas ATP stimulation induced a response similar to that obtained in cells from healthy controls.
It is concluded that X-ray microanalysis of nasal epithelial cells may be used to determine chloride secretion in CF patients in an easily accessible cell type.     

Subsurface deformation of machined Al2O3 and Al2O3/5vol%SiC nanocomposite

Under machine grinding, material removal in monolithic Al₂O₃ is by intergranular fracture and grain pull-out. In comparison, under the same grinding conditions, an Al₂O₃/5%SiC nanocomposite undergoes significant surface grooving and intragranular fracture. The subsurface deformation mechanisms were investigated by cross-sectional transmission electron microscopy. For Al₂O₃, the residual deformation zone was localized very close to the surface in the first layer of grains, with dislocations occurring only within 1.5 µm of the top surface and a high density of basal twins penetrating to a depth of one single grain. Cracks were present along grain boundaries or basal twin interfaces. For Al₂O₃/SiC nanocomposites, the main residual plastic deformation is observed to be dislocations activated to a depth of about 10 µm (approx. 3–4 grains), with twinning rarely observed. Possible mechanisms by which the SiC particles influence the subsurface deformation and material removal modes are discussed.     

Fluorescence resonance energy transfer detected by scanning near-field optical microscopy

Fluorescence resonance energy transfer (FRET) between excited fluorescent donor and acceptor molecules occurs via the Förster mechanism over a range of 1–10 nm. Because of the strong (sixth power) distance dependence of the signal, FRET has been used to assess the proximity of molecules in biological systems. We used a scanning near-field optical microscope (SNOM) operated in the shared-aperture mode using uncoated glass fibre tips to detect FRET between dye molecules embedded in polyvinyl alcohol films and bound to cell surfaces. FRET was detected by selective photobleaching of donor and acceptor fluorophores. We also present preliminary results on pixel-by-pixel energy transfer efficiency measurements using SNOM.