Takashi Tokizaki

In situ detection of faradaic current in probe oxidation using a dynamic force microscope

A faradaic current on the order of a sub-pico-ampere was detected while fabricating two-dimensional oxide nanostructures on H-passivated Si(001) surfaces. The detected faradaic current has been shown to faithfully reflect the degree of probe oxidation with a clear dependence on the variation of voltage and the tip speed. The faradaic current in dynamic mode can serve as a sensitive monitor of the nano-oxidation reaction for implementing precise closed-loop control of the oxide growth.

Nanometer-sized optical waveguides fabricated by anodic oxidation using a scanning near-field optical microscope

We have fabricated an optical waveguide with a subwavelength cross section that propagates light. A metal–oxide core is partially embedded into a metal clad by anodic oxidation using the probe tip of a scanning near-field optical microscope (SNOM). Then, using the SNOM in transmission mode we have evidence of light propagating more than 5 μm in the waveguide whose core width and thickness are 300 and 70 nm, respectively.

Laser scanning microscope for low temperature single molecule and microscale spectroscopy based on gradient index optics

A scanning optical microscope for low temperature imaging and spectroscopy with a gradient index rod-shaped microlens as an objective lens is presented. The solid immersion effect enhances the resolution to 310 nm of the full-width at half-maximum at the wavelength of 545 nm. Laser scanning mechanism located outside an optical cryostat enables to achieve large scanning ranges independent of temperature. The performance is demonstrated at 1.6 K on single molecules of terrylene in a dodecane crystal and on molecular J aggregates in thin polymer films.

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.

Nano-oxidation and in situ faradaic current detection using dynamic carbon nanotube probes

Carbon nanotube-attached atomic force microscope probes were successfully used without nanotube bending to make simultaneous precision nano-oxidation and faradaic current measurements in the dynamic mode. Probe oxidation on H-passivated Si(001) surfaces was carried out by two methods involving vector-scan and raster-scan with a much higher resolution and precision compared to the nanofabrication by standard cantilevers. Faradaic current of the order of a sub-picoampere was detected during nano-oxidation using a carbon nanotube probe, accurately reflecting the subtle difference in the oxidation reaction. The minute current detection through the AFM tip is sensitive enough for the detection of very thin oxides and small-sized features. The dimension of the meniscus during nano-oxidation, which is indispensable for establishing the mechanism model, was evaluated, based on the in situ faradaic current detection and edge broadening.

Large Enhancement of Third-Order Optical Susceptibility in CuCl Quantum Dots Embedded in Glass

We report third-order optical nonlinearity of CuCl microcrystallites with radii of 1.5-8.0 nm in glass. We measured absolute values of third-order susceptibility χ(3) using degenerate four-wave mixing and imaginary χ(3) by absorption saturation measurements. The highest value of χ(3) was obtained to be 2×10-6 esu for microcrystallites with the mean radius of 4 nm which were embedded in the glass with the volume fraction of about 0.5%. We found that the figure of merit, χ(3)/ατ, where α and τ are the absorption coefficient and the response time, respectively, is also increased upon an increase of radii in the range of 1.5-4.5 nm.

Why nano-oxidation with carbon nanotube probes is so stable: I. Linkage between hydrophobicity and stability

Carbon nanotube (CNT) probes enhance the stability of the nano-oxidation process under dynamic-mode operation. In this paper we investigate how the hydrophobic nature of the CNT allows oxide nanostructures to be fabricated with constant aspect ratio over a wide range of relative humidity values. In particular, we characterize oxide growth by measuring both the integrated ionic current and volume expansion. Behaviour of different CNT probes was compared to assess individual stability and performance under identical voltage and humidity conditions. While much remains to be established about the relationship between exposure conditions and dynamic-mode parameters on nanoscale oxide fabrication, hydrophobicity is a key factor in the improved reliability of CNT probes over conventional ones.

Fabrication and evaluation of nanometer-sized metal oxide structures on composite metal thin films using scanning near-field optical microscope

Nanometer-scale metal–oxide structures with optical transparency have been fabricated on metal films by anodic oxidation using a probe tip of a scanning near-field optical microscope (SNOM). We discuss the thickness and the optical transparency of the oxides depending on the bias voltage applied to samples for oxidation. For titanium (Ti) films, the thickness of the oxide increases up to 100 nm linearly with the bias voltage. For composite films consisting of titanium–aluminum (Ti–Al) and titanium–silver (Ti–Ag) layers, the bias-voltage dependence of the oxide thickness is altered at some bias voltages, which concur with those required for the oxidation of the upper Ti layers. Moreover, the oxides have been observed optically by the same SNOM as a transmission mode. The transmittance at the oxide increases with the growth of the oxides, because of the decrease of the thickness of the metal films due to the oxidation. The result also suggests that Ti–, Al– and Ag–oxides fabricated by this method are optically transparent.

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.

Self-synchronized voltage pulse application for dynamic force microscope nano-oxidation

A self-synchronized voltage pulse application method was demonstrated as an effective control of the nano-oxidation using a dynamic force microscope. By creating a cyclic water meniscus using the large oscillation amplitude, self-synchronized voltage pulses can be realized during application of a continuous voltage to the sample. The modification of the probe-sample distance was equal to the change in the applied voltage pulse duration on the order of submicroseconds. This method realized the ultrastable control of the water meniscus formation.