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 microm 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 approximately 63 times larger than that of the large-diameter probe, but the power is about 1/25 of that of the large probe.     

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.     

Near-field fluorescence imaging of single molecules with a resolution in the range of 10 nm

We demonstrate fluorescence imaging of single molecules, by near-field scanning optical microscopy (NSOM), using the illumination-collection mode of operation, with an aperture probe. Fluorescence images of single dye molecules were obtained with a spatial resolution of 15 nm, which is smaller than the diameter of the aperture (20 nm) of the probe employed. Such super-resolution may be attributable to non-radiative energy transfer from the molecules to the coated metal of the probe since the resolution obtained in the case of conventional NSOM is limited to 30–50 nm due to penetration of light into the metal.     

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO₂ tip on a Si cantilever was successfully fabricated by selective etching of the SiO₂ tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10–100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.     

Microfabricated silicon dioxide cantilever with subwavelength aperture

We have developed a microfabricated SiO₂ cantilever with subwavelength aperture for scanning near-field optical microscopy (SNOM), to overcome the disadvantages of conventional optical fibre probes such as low reproducibility and low optical throughput. The microcantilever, which has a SiO₂ cantilever and an aperture tip near the end of the cantilever, is fabricated in a reproducible batch process. The circular aperture with a diameter of 100–150 nm is formed by a focused ion-beam technique. Incident light is directly focused on the aperture from the rear side of the cantilever using a focusing objective, and high optical throughput (10⁻² to 10⁻³) is obtained. The microcantilever can be operated as a SNOM probe in contact mode or in dynamic mode.     

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.     

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.     

Optical switching due to whispering gallery modes in dielectric microspheres coated by a Kerr material

We numerically evaluated the optical responses of a nonlinear microsphere put on prisms, i.e. in prism‐coupling geometry where the incident light excites the WGM through near‐field coupling. As numerical calculations, we employed the finite‐difference time‐domain method taking into account Kerr nonlinearity. The sphere was coated by a Kerr material with 160 nm thick. The third‐order nonlinear susceptibility of the Kerr material χ⁽³⁾ was assumed to be 7 × 10^?14[m² V^?2]. The diameter of the sphere was assumed to be 1 µm. The numerical results have shown that the control and/or the signal lights can induce the optical switching‐like variation in the reflectance. Such a nonlinear response of the sphere has been interpreted by the variation in the dielectric constant of the sphere due to a Kerr nonlinearity.     

Photoluminescence imaging of phosphor particles using near-field optical microscope with UV light excitation

We have developed fibre probes suitable for 325 nm UV light excitation and a photoluminescence near-field scanning optical microscope (NSOM) and demonstrated the photoluminescence imaging of phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) particles. The probe was fabricated by a two-step-etching method that we developed. The probe had a large taper angle at the top of the probe and a small taper angle at the root. The NSOM image was different from the topographical structure but roughly reflected the corresponding features of the particles. The inhomogeneity of the photoluminescence intensity between BAM particles was observed in the NSOM image. The photoluminescence intensity with various bandpass filters showed differences between the individual particles, which means that they have different spectra.     

Visualizing filamentous actin on lipid bilayers by atomic force microscopy in solution

The surface structure of actin filaments (F-actin) was visualized at high resolution, by atomic force microscopy (AFM) in aqueous solution, in large paracrystals prepared on positively charged lipid monolayers. The increased stability of these closely packed specimens allowed us to show that both the long pitch (38 nm) and the monomer (5.8 nm) can be directly resolved by AFM in the contact mode. The right-handed helical surface, distinguishable in high resolution images, was compared with reconstructed models based on electron microscopy. The height of the rafts, a measure of the actin filament diameter, was 10 ± 1 nm, whereas the smaller inter-filament distance, 8 ± 1 nm, was consistent with interdigitation of the filaments. The 10 ± 1 nm F-actin diameter is in good agreement with the results of fibre X-ray diffraction. As such specimens are relatively easy to prepare without specialized equipment, this method may allow the study of the thin filaments in which F-actin-associated proteins are also present.     

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.     

Single molecule mapping of the optical field distribution of probes for near-field microscopy

The most difficult task in near-field scanning optical microscopy (NSOM) is to make a high quality subwavelength aperture probe. Recently, we have developed high definition NSOM probes by focused ion beam (FIB) milling. These probes have a higher brightness, better polarization characteristics, better aperture definition and a flatter end face than conventional NSOM probes. We have determined the quality of these probes in four independent ways: by FIB imaging and by shear-force microscopy (both providing geometrical information), by far-field optical measurements (yielding throughput and polarization characteristics), and ultimately by single molecule imaging in the near-field. In this paper, we report on a new method using shear-force microscopy to study the size of the aperture and the end face of the probe (with a roughness smaller than 1.5 nm). More importantly, we demonstrate the use of single molecules to measure the full three-dimensional optical near-field distribution of the probe with molecular spatial resolution. The single molecule images exhibit various intensity patterns, varying from circular and elliptical to double arc and ring structures, which depend on the orientation of the molecules with respect to the probe. The optical resolution in the measurements is not determined by the size of the aperture, but by the high optical field gradients at the rims of the aperture. With a 70 nm aperture probe, we obtain fluorescence field patterns with 45 nm FWHM. Clearly, this unprecedented near-field optical resolution constitutes an order of magnitude improvement over far-field methods like confocal microscopy.     

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.     

Writing subwavelength-sized structures into aluminium films by thermo-chemical aperture-less near-field optical microscopy

An optical near-field at the tip of an atomic force microscope probe is utilised to pattern aluminium thin films on glass substrates by photo-thermally induced corrosion in water. Aluminium forms a thin passivating oxide layer when immersed into neutral water at room temperature. Owing to the high energy density of the near-field, the metal below the probe tip can be heated to 100°C due to absorption of the light, which then provokes breakdown of the passivation and metal corrosion. The localised near-field is generated by tip-induced enhancement of an evanescent field originating from a laser beam, that is totally internally reflected at the glass–aluminium–water interface. The process is governed by surface plasmons excited in the aluminium film by the evanescent waves and the field enhancement of the probe tip. Holes of 40 nm diameter and lines below 100 nm width have been written into a 20-nm-thick aluminium film. Applications of the scanning probe lithography process may include the one-step fabrication of point contacts or contact masks for near-field optical lithography and reactive ion etching.     

Moulded photoplastic probes for near-field optical applications

The inexpensive fabrication of high-quality probes for near-field optical applications is still unsolved although several methods for integrated fabrication have been proposed in the past. A further drawback is the intensity loss of the transmitted light in the 'cut-off' region near the aperture in tapered optical fibres typically used as near-field probes. As a remedy for these limitations we suggest here a new wafer-scale semibatch microfabrication process for transparent photoplastic probes. The process starts with the fabrication of a pyramidal mould in silicon by using the anisotropic etchant potassium hydroxide. This results in an inverted pyramid limited by < 111 > silicon crystal planes having an angle of ∼ 54°. The surface including the mould is covered by a ∼ 1.5 nm thick organic monolayer of dodecyltrichlorosilane (DTS) and a 100-nm thick evaporated aluminium film. Two layers of photoplastic material are then spin-coated (thereby conformal filling the mould) and structured by lithography to form a cup for the optical fibre microassembly. The photoplastic probes are finally lifted off mechanically from the mould with the aluminium coating. Focused ion beam milling has been used to subsequently form apertures with diameters in the order of 80 nm. The advantage of our method is that the light to the aperture area can be directly coupled into the probe by using existing fibre-based NSOM set-ups, without the need for far-field alignment, which is typically necessary for cantilevered probes. We have evidence that the aluminium layer is considerably smoother compared to the 'grainy' layers typically evaporated on free-standing probes. The optical throughput efficiency was measured to be about 10⁻⁴. This new NSOM probe was directly bonded to a tuning fork sensor for the shear force control and the topography of a polymer sample was successfully obtained.     

Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with ≈150 nm lateral resolution, ≈250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump–probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump–probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within ≈1 μm of the implanted stripes, an effect which could not have been resolved with a far-field system.     

Shear force imaging of DNA in a near-field scanning optical microscope (NSOM)

A near-field scanning optical module has been constructed as an accessory for a Nanoscope IIIa commercial scanning probe microscope. Distance feedback and topographic registration are accomplished with an uncoated optical fibre scanning tip by implementation of the shear force technique. The tip is driven by a piezoelectric actuator at a resonance frequency of 8–80 kHz. A laser diode beam is scattered by the tip and detected by a split photodiode, with lock-in detection of the difference signal. The amplitude ( r ) and phase (τ) responses were characterized as a function of the calibrated tip–sample separation. Using an r cos τ feedback signal, imaging of pUC18 relaxed circular plasmid DNA spread on mica precoated with cetylpyridinium chloride was achieved. The apparent width (28 ± 5 nm) was approximately four times that achieved by scanning force measurements with the same instrument; the apparent height of the DNA (0.6 ± 0.3 nm) was similar with the two techniques. These results demonstrate the applicability of the shear force signal for imaging biological macromolecules according to topography and in conjunction with the optical signals of a near-field scanning optical microscope (NSOM).     

Fabrication of near-field optical apertures in aluminium by a highly selective corrosion process in the evanescent field

A simple, one‐step process to fabricate high‐quality apertures for scanning near‐field optical microscope probes based on aluminium‐coated silicon nitride cantilevers is presented. A thin evanescent optical field at a glass–water interface was used to heat the aluminium at the tip apex due to light absorption. The heat induced a breakdown of the passivating oxide layer and local corrosion of the metal, which selectively exposed the front‐most part of the probe tip from the aluminium. Apertures with a protruding silicon nitride tip up to 72 nm in height were fabricated. The height of the protrusion was controlled by the extent of the evanescent field, whereas the diameter depended on the geometry of the probe substrate. The corrosion process proved to be self‐terminating, yielding highly reproducible tip heights. Near‐field optical resolution in a transmission mode of 85 nm was demonstrated.     

Inducing superconductivity at a nanoscale: photodoping with a near-field scanning optical microscope

The local modification of an insulating GdBa₂Cu₃O_6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub-wavelength light source of wavelength λ_exc = 480–650 nm, locally generating photocarriers in an otherwise insulating GdBa₂–Cu₃O_6.5 thin film. Of the photogenerated electron–hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at λ = 1.55 μm at room temperature show that photocarriers can be induced and constrained to move on a ≈200 nm scale for all investigated λ_exc. Photogenerated wires present a superconducting critical temperature T _c = 12 K with a critical current density J _c = 10⁴ A cm⁻². Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (≈ 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.     

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.