Yasushi Oshikane

Observation of nanostructure by scanning near-field optical microscope with small sphere probe

Abstract Step and terrace structure has been observed in an area of 1 μm × 1 μm on the cleaved surface of KCl–KBr solid-solution single crystal by scanning near-field optical microscope (SNOM) with a small sphere probe of 500 nm diameter. Lateral spatial resolution of the SNOM system was estimated to be 20 nm from the observation of step width and the scanning-step interval. Vertical spatial resolution was estimated to be 5–2 nm from the observation of step height and noise level of photomultiplier tube (PMT). With applying a dielectric dipole radiation model to the probe surface, the reason why such a high spatial resolution was obtained in spite of the 500 nm sphere probe, was understood as the effect of the near-field term appeared in the radiation field equations.

Atomically resolved scanning tunneling microscopy of hydrogen-terminated Si(001) surfaces after HF cleaning

Atomic structures of hydrogen-terminated Si(001) surfaces after HF cleaning are investigated by scanning tunneling microscopy. It is revealed that the surface is macroscopically rough but is composed of terraces and steps. Inside a terrace, 1×1 structures are formed. This corresponds to the ideal 1×1 dihydride structure. The step edges run along the 〈110〉 direction. On the other hand, the 1×1 dihydride structure disappears when the surface is subsequently rinsed with ultrapure water, because every other dihydride row of the ideal 1×1 structure is preferentially etched in ultrapure water.

Atomic structures of hydrogen-terminated Si(001) surfaces after wet cleaning by scanning tunneling microscopy

Scanning tunneling microscopy observations are performed on a H-terminated Si(001) surface treated with HF solutions and ultrapure water with very low dissolved oxygen and total organic carbon contents. Over a large area, row structures are observed in [110] and [110] directions. Pyramidal-shaped etch pits are also observed, which are caused by anisotropic etching by OH ions. Detailed images clearly show 2×1 periodic structures. It is suggested that every other row of the ideally dihydride 1×1 surface is etched preferentially by OH ions. This explains the mechanism by which the smallest etch pits are formed.

Development of a scanning near-field optical microscope with a probe consisting of a small spherical protrusion

Abstract The probe of the scanning near-field optical microscope (SNOM) is a dielectric sphere 500 nm in diameter on a transparent substrate. The probe sphere is illuminated by evanescent waves which are formed by the incidence of a HeNe laser with the wavelength of 632.8 nm under the condition of total internal reflection. The light from the probe is collected by a conventional microscope through the substrate. The detected light intensity varies markedly when a sample is brought into the near-field around the probe. The variation of the detected light intensity in the near-field depends on the complex index of refraction of samples; the smaller the real part of the refractive index, the more marked the increase of the detected light intensity. This result is explained through the use of an electric dipole model for the electromagnetic interaction between the probe and sample. The vertical and lateral resolutions of about 1 nm and 10 nm, respectively, are obtained for a standard sample which is prepared by vacuum evaporation of metal.

Scanning tunneling microscopy study of hydrogen-terminated Si(001) surfaces after wet cleaning

Abstract A scanning tunneling microscopy (STM) study of the atomic structure of hydrogen-terminated Si(001) surfaces after wet cleaning is presented. Surface morphologies strongly depend on wet cleaning procedures. After being dipped into dilute HF solution, the surface is constructed by piling round, small terraces along the 〈110〉 direction. On the other hand, atomic rows along the 〈110〉 direction are clearly observed when the surface is subsequently rinsed by ultrapure water, and the corrugation pattern is identified as a 2×1 structure. With STM and Fourier-transform infrared attenuated total reflection (FTIR–ATR) observations, it is proposed that etching with ultrapure water proceeds in two steps. First, atomic steps perpendicular to dihydride rows of the upper layer are etched quickly to form step edges parallel to the dihydride rows and to produce large terraces with fewer steps. Second, etching occurs inside a terrace. Every other row of an ideally 1×1 dihydride terrace is removed preferentially in ultrapure water.

3D-FEM analysis of SPP excitation through nanoholes in asymmetric metal-insulator-metal structure at tip of circular truncated conical fiber

3D-electromagnetic (EM) analysis of surface plasmon polaritons (SPPs) excited by a single-mode (SM) propagation of visible lightwave in an optical fiber has been studied with a 3D-FEM package based on a finite element method. End of the fiber is formed to be a circular cone by wet etching process, and is FIBed to make a circular truncated conical shape with a flat circular surface a few micrometers in diameter. The flat end is covered with three layers of asymmetric metalinsulator- metal structure, thin metallic layer (M1), thick insulator layer (I), and thick metallic layer (M2), respectively. The outermost M2 layer has FIBed nanoholes to convert light waves at the extremity of the fiber into SPPs efficiently, and a bright tiny point light source will be generated on the surface of the M2 layer. In this study, the 3D-FEM models consists of both the MIM structure and the shrinking optical fiber tip coated with a metallic thin film has been designed and analyzed numerically. By applying perfect electric conductor and perfect magnetic conductor to planes containing the axis of rotation, the FEM model has a quarter of the circular truncated conical shape. The FEM analysis is formed in two steps. At the first step, a FEM mode analysis is performed to obtain a solution corresponding to the SM propagation in the fiber. The second level of action is the FEM analysis of EM field in the whole of model to find a stationary solution with the solution of mode analysis. Characteristic of wavelength-dependent excitation, propagation, and focusing of the SPPs will be presented with several experimental results of trial products of the fiber tip.

Extracting and focusing of surface plasmon polaritons inside finite asymmetric metal/insulator/metal structure atapex of optical fiber by subwavelength holes

We have been studied a finite asymmetric metal-insulator-metal (MIM) structure on glass plate for near-future visible light communication (VLC) system with white LED illuminations in the living space (DOI: 10.1117/12.929201). The metal layers are vacuum-evaporated thin silver (Ag) films (around 50 nm and 200 nm, respectively), and the insulator layer (around 150 nm) is composed of magnesium fluoride (MgF2). A characteristic narrow band filtering of the MIM structure at visible region might cause a confinement of intense surface plasmon polaritons (SPPs) at specific monochromatic frequency inside a subwavelength insulator layer of the MIM structure. Central wavelength and depth of such absorption dip in flat spectral reflectance curve is controlled by changing thicknesses of both insulator and thinner metal layers. On the other hand, we have proposed a twin-hole pass-through wave guide for SPPs in thick Ag film (DOI: 10.1117/12.863587). At that time, the twin-hole converted a incoming plane light wave into a pair of channel plasmon polaritons (CPPs), and united them at rear surface of the Ag film. This research is having an eye to extract, guide, and focus the SPPs through a thicker metal layer of the MIM with FIBed subwavelength pass-through holes. The expected outcome is a creation of noble, monochromatic, and tunable fiber probe for scanning near-field optical microscopes (SNOMs) with intense white light sources. Basic experimental and FEM simulation results will be presented.

Design and fabrication of active spectral filter with metal-insulator-metal structure for visible light communication

Visible light communication with LED is an important ICT for the ubiquitous network society. However visible light communication has the speed limit in the conventional blinking LED method. Therefore an active spectral filter would be useful in order to input information signals onto the LED spectrum. Plasmonic spectral filter based on a metalinsulator- metal (MIM) structure is one of the candidates of such active filter. We will explain our progress of fabrication of the MIM structures with the vacuum deposition technique and compare their absorption properties with the theoretical prediction.

Plasmonic active spectral filter in VIS-NIR region usingmetal-insulator-metal (MIM) structure on glass plate

Interaction between surface plasmons at two interfaces inside a meta-insulator-metal (MIM) structure is one of the interesting physical phenomena in nanophotonics. We have started to create a plasmonic active spectral filter based on the MIM structure for a developing white light-emitting diode (LED) visible-light communication. An optical active filter at visible region assisted by surface plasmon resonance (SPR) in MIM structure of vacuum-deposited thin films on glass substrate has been studied both experimentally and theoretically. Interface between the first thin silver layer (M1, around 50 nm-thick) and bulk glass slide is appropriate for excitation of SPR at particular wavelength and incident angle of illumination light. And spatial extension of the SPR wave may cause an effective propagating mode confined in the insulator layer (I, around 150 nm-thick) by both M1 and the second thick silver layer (M2, around 200 nm-thick). Such an energy conversion from the illuminating light to the propagating SPR modes corresponds to an evident absorption dip on spectral reflectance curve of the MIM structure, and the shape of dip may vary widely in response to material and configuration of the MIM. The spectral and angular reflectance of the prototypical MIM structure has been measured by spectrophotometer for P- and S-polarized light because the plasmonic effect inside the MIM structure depends strongly on the polarization of light. Such the characteristic reflection feature has also been studied by using both the usual transfer matrix method and 2D electromagnetic simulation based on the finite element method. In this talk, several striking and preliminary MIM prototypes will be introduced and discussed.

3D-FDTD and experimental analysis of a resonant microcavity probe for high-resolution SNOM

A simple novel probe for Scanning Near-field Optical Microscope (SNOM) is proposed. The probe consists of a small protrusion on a micro resonator. The resonator and protrusion is a spherical micro sphere with diameter of 50-70 μm, 1.5 μm, respectively. The resonator is a polystyrene latex sphere and the protrusion is a polymethyl methacrylate sphere. The s-polarized laser (Ti:Sapphire laser) beam, which illuminates the resonator through an evanescent wave, can be tuned to the resonant frequencies. The resonance occurred in the sphere is a traveling wave resonance, which is called MDRs (Morphology Dependent Resonances) or WGMs (Whispering Gallery Modes). The internal resonant wave could generate an intensive evanescent field on the surface of the resonator. The small protrusion on the resonator combines with the evanescent field and could acts as a high sensitive probe for SNOM. A clear image of the protrusion illuminated by the evanescent field on the resonator was observed with the cooled CCD detector. The brightness of the image of the protrusion depends on the laser wavelength. The optical characteristics of the resonant probe is also studied by a Finite-Difference Time- Domain method.