Hironaga Uchida

Tip-sample interactions in the scanning tunneling microscope for atomic-scale structure fabrication

In a scanning tunneling microscope (STM) operated in ultra-high vacuum, if we place a well-prepared W tip above the Si(111)-7×7 surface at a separation of ~1 nm and apply an appropriate voltage pulse to it, we can extract a single Si atom from a predetermined position routinely at room temperature. The extracted Si atoms are redeposited onto the surface with a certain probability, their positions always being at a fixed crystallographic site. The redeposited Si atoms can be displaced intentionally to other crystallographically equivalent sites. In case of the Si(001)-2×1 surface, usually two Si atoms forming a dimer are extracted together. For both surfaces, Si atoms at crystallographically different sites including step edges are extracted with different probabilities. The microscopic mechanisms of these processes are discussed.

Three-dimensional magnetophotonic crystals based on artificial opals

We fabricated and experimentally investigated three-dimensional magnetophotonic crystals (3D MPCs) based on artificial opals. Opal samples with three-dimensional dielectric lattices were impregnated with different types of magnetic material. Magnetic and structural properties of 3D MPCs were studied by field emission scanning electron microscopy, x-ray diffraction analysis, and vibrating sample magnetometer. We have shown that magnetic materials synthesized in voids of opal lattices and the composites obtained have typical magnetic properties.

Single-atom manipulation on the Si(111)7 × 7 surface by the scanning tunneling microscope (STM)

Abstract Methods to manipulate single Si atoms on the Si(111)7 × 7 surface at room temperature, using a scanning tunneling microscope (STM) operated in ultrahigh vacuum, are reported. The extraction and redeposition of single Si atoms and the displacement of the redeposited Si atoms can be done with a W tip, if the tip is carefully cleaned, by applying a voltage pulse between tip and sample with its polarity, magnitude and duration being selected appropriately.

Magnetophotonic crystals—a novel magneto-optic material with artificial periodic structures

When the constitutive materials of photonic crystals (PCs) are magnetic, or even only a defect in PCs is magnetic, the resultant PCs exhibit very unique optical and magneto-optical (MO) properties. They are called magnetophotonic crystals (MPCs) providing the spin-dependent nature in PCs. The strong photon confinement in the vicinity of magnetic defects results in large enhancement in linear and nonlinear magneto-optical responses of the media. Novel functions, such as band Faraday effect, magnetic super-prism effect and non-reciprocal or magnetically controllable photonic band structure, are predicted to occur theoretically. In this paper, preparations of 1D, 2D, and 3D MPCs and their fundamental properties are described based upon our recent investigations.

Fabrication of Atomic-Scale Structures on Si(111)-7×7 Using a Scanning Tunneling Microscope (STM)

A single Si atom or a group of a few Si atoms can be extracted from a predetermined position of the Si(111)-7×7 surface through field evaporation with a scanning tunneling microscope (STM). A method of tip preparation which makes Si atom extraction reliable is presented. The results of many individual Si atom extraction experiments are discussed on the basis of the statistics of extraction with the Si(111)-7×7 unit cell.

Nonlinear magneto-optical Kerr effect in garnet magnetophotonic crystals

Magnetization-induced second-harmonic (SH) generation is studied in magnetophotonic crystals formed from a magnetic garnet spacer located between two dielectric Bragg reflectors, at the resonance of the fundamental radiation with the cavity mode. Longitudinal nonlinear magneto-optical Kerr effect manifests itself in the rotation of the polarization plane of a reflected SH wave which reaches a value up to 250°/μm. A magnetization-induced variation of the SH intensity is observed in transversal configuration with magnetic contrast up to 0.3.

Deposition and subsequent removal of single Si atoms on the Si(111)‐7×7 surface by a scanning tunneling microscope

Single Si atoms can be deposited onto a Si(111)‐7×7 sample surface from the tip of a scanning tunneling microscope, the Si atoms to be deposited being previously picked up by the tip from another place on the sample surface. The crystallographic position of these deposited Si atoms changes as their density increases. The deposited Si atoms can be re‐removed by picking them up again with the tip, the substrate atomic arrangement remaining unperturbed. It is also possible to fill Si atom vacancies on the sample surface with a Si atom deposited from the tip.

Magneto-optical properties of three-dimensional magnetophotonic crystals

Properties of three-dimensional magneto-photonic crystals based on artificial opals were studied by optical spectroscopy. We have investigated an influence of magnetic materials embedded into opal lattice on optical and magnetic properties of synthesized composites. It was shown that increase of volume fraction of magnetic material in opal lattice leads to decrease of transmissivity of synthesized composites. We measured Faraday rotation of opal-magnetite composites and found considerable changes in the Faraday rotation inside the (111) photonic bandgap.

Magnetization-induced second- and third- harmonic generation in magnetophotonic crystals

The results of our recent studies of magnetization-induced nonlinear-optical second- and third-order effects in magnetophotonic crystals and magnetophotonic microcavities are surveyed. Magnetophotonic crystals (MPCs) are fabricated from a stack of four repetitions of lambda/4-thick layers of Bi-substituted yttrium iron garnet (Bi:YIG) and lambda/4-thick SiO2 layers. Magnetophotonic microcavities (MMCs) are formed from two dielectric (nonmagnetic) Bragg reflectors and ferromagnetic cavity spacers that are lambda/2-thick Bi:YIG layers. The nonlinear magneto-optical Kerr effect (NOMOKE), both in magnetization-induced second-harmonic generation (MSHG) and magnetization-induced third-harmonic generation (MTHG), is observed in MMCs at wavelengths of the resonant microcavity modes. Magnetization-induced variations of MSHG and MTHG intensities, as well as magnetization-induced shift of phase and rotation of polarization of second and third-harmonic waves, are observed in proper - transversal, longitudinal, or polar - NOMOKE configurations. Manyfold enhancement of the absolute values of both the MSHG and MTHG intensities are attributed to the localization of the resonant fundamental radiation in Bi:YIG microcavity spacers. The NOMOKE in MSHG intensity is observed in MPCs in the spectral range of photonic bandgap (PBG) edges. The MSHG intensity reveals enhancement by a factor of more than 10^2 if the fundamental wavelength is tuned in the vicinity of the PBG edge. This enhancement is attributed to the fulfillment of the phase-matching conditions for MSHG effect in layered structures with periodic modulation of both optical (magneto-optical) and nonlinear optical parameters.

Magnetization-induced third harmonic generation in magnetophotonic microcavities

Magnetization-induced third harmonic generation was experimentally observed in thin films of bismuth-dopedyttrium-iron garnet. The magnitude of magnetization-induced nonlinear optical response was enhanced through building-in a garnet film into a photonic-crystal microcavity. It was shown that the observed magnetization-induced variations in the third-harmonic intensity were caused by the internal homodyne of interfering weak magnetic and strong nonmagnetic polarization components of the cubic nonlinearity of the garnet.