尾松 孝茂

© 2020 IOP Publishing Ltd. Although semiconductor to metal phase transformation of MoTe2 by high-density laser irradiation of more than 0.3 MW cm-2 has been reported, we reveal that the laser-induced-metal (LIM) phase is not the 1T′ structure derived by a polymorphic-structural phase transition but consists instead of semi-metallic Te induced by photo-thermal decomposition of MoTe2. The technique is used to fabricate a field effect transistor with a Pd/2H-MoTe2/LIM structure having an asymmetric metallic contact, and its contact properties are studied via scanning gate microscopy. We confirm that a Schottky barrier (a diffusion potential) is always formed at the Pd/2H-MoTe2 boundary and obstacles a carrier transport while an Ohmic contact is realized at the 2H-MoTe2/LIM phase junction for both n- and p-type carriers.

We report on the first demonstration of picosecond optical vortex-induced chiral surface relief in an azo-polymer film due to two-photon absorption isomerization. The chiral surface relief exhibits an extremely narrow defocusing tolerance without undesired outer rings due to the Airy pattern of highly focused light. Such chiral surface relief reflects a z-polarized electric field with an azimuthal helical phase caused by spin-orbital angular momentum coupling.

We report on the first demonstration of picosecond optical vortex-induced chiral surface relief in an azo-polymer film due to two-photon absorption isomerization. The chiral surface relief exhibits an extremely narrow defocusing tolerance without undesired outer rings due to the Airy pattern of highly focused light. Such chiral surface relief reflects a z-polarized electric field with an azimuthal helical phase caused by spin-orbital angular momentum coupling. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.

Recent work has shown that irradiation with light possessing orbital angular momentum (OAM) and an associated phase singularity, that is an optical vortex, twists a variety of materials. These include silicon, azo-polymer, and even liquid-phase resins to form various helically structured materials. This article provides a review of the unique helical-structured materials created and the novel fundamental phenomena enabled by this interaction between both the spin angular momentum and the OAM of light with matter. Such light-induced helical-structured materials will potentially lead to advanced photonic devices, for instance, metamaterials for ultrasensitive detection and reactions for chiral chemical composites.

We have developed a widely tunable terahertz (THz) vortex source with an average power of 3 μW and a frequency tuning range of 2-6 THz. The handedness of the THz vortex output in this system is also controlled.

We demonstrate the generation of an ultra-widely tunable mid-infrared (6–18 μm) optical vortex output with a moderate pulse energy from a AgGaSe2 difference frequency generator pumped by an optical vortex parametric oscillator. The handedness of the vortex output can be controlled/selected by swapping the lasing frequencies of the signal and idler outputs and rotating the AgGaSe2 crystal by 90 deg.

We reversibly controlled phase conversion between a microdroplet of a NaClO3 unsaturated aqueous solution and a metastable single crystal, which is usually a short-lived phase in spontaneous crystallization, simply by irradiating a tightly focused visible continuous-wave (CW) laser to the microdroplet. The laser irradiation allowed the metastable crystal to generate and stably grow without a polymorphic transformation. This successful metastable phase control is attributed to the combination of the advantage of optical trapping-induced nucleation that nucleation takes place from unsaturated mother solution and the advantage of microdroplet method, which suppresses additional nucleation leading to the transformation. In situ observation shows the crystal dissolves when the laser irradiation is stopped, whereas the laser irradiation stabilizes the crystal even if the size of the crystal becomes larger than that of focal spot. These observations indicate that a change in the relative magnitudes of chemical potentials between solution/crystalline phases. This change is possibly promoted via crystal growth by trapping of crystalline clusters in optical potential well formed on a crystal surfaces originating from "light propagation" through the crystal. Our results shed a light not only on polymorph control but also on a method to prepare a longer-lived achiral precursor for analysis on achiral-chiral transition by "freezing" a kinetic pathway of chiral crystallization.

The unique properties of optical vortex beams, in particular their spiral wavefront, have resulted in the emergence of a wide range of unique applications for this type of laser output. These applications include optical tweezing, free space optical communications, microfabrication, environmental optics, and astrophysics. However, much like the laser in its infancy, the adaptation of this type of laser output requires a diversity of wavelengths. We report on recent progress on development of optical vortex laser sources and in particular, focus on their wavelength extension, where nonlinear optical processes have been used to generate vortex laser beams with wavelengths which span the ultraviolet to infrared. We show that nonlinear optical conversion can be used to not only diversify the output wavelength of these sources, but can be used to uniquely engineer the wavefront and spatial properties of the laser output.

We demonstrate an ultra-broadband (>2-octave band) tunable optical vortex laser comprising an optical-vortex-pumped optical parametric oscillator by employing a nanosecond pulse (similar to 10 ns) green laser and cascaded non-critical phase-matching LiB3O5 crystals (45mm long each). With this system, an optical vortex output was produced over an extremely wide wavelength range of 0.67-2.57 mu m. (C) 2017 The Japan Society of Applied Physics

Picosecond pulsed frequency-doubled optical vortices were generated using a pair of beta-BaB2O4 crystals with their c axes inverted. This arrangement produced high-quality ultraviolet vortex output with low spatial separation of the phase singularity at a conversion efficiency of similar to 40%. We also discuss the theoretical spatial form and beam propagation of the ultraviolet vortex output. (C) 2017 Optical Society of America

An end-pumped Nd: YVO4 laser under selective pumping is used to excite lasing modes with transverse patterns performed to exhibit the characteristics of multiple spots arranged on elliptical features near degenerate cavities. The spatial distribution of elliptical lasing modes is clearly revealed to be localized on the nonplanar ray orbits, so-called nonplanar elliptical modes, which possess large fractional orbital angular momentum. Moreover, temporal dynamics for the output emission of nonplanar elliptical modes are verified to obtain self-mode-locked operation. We further numerically manifest not only the influence of radial-asymmetry distributions on the vortex structures of nonplanar elliptical modes, but also the vector field of transverse lasing modes altered with twisting phase structures in the propagation direction. (C) 2017 Optical Society of America

Structured light beams, such as optical vortices, vector beams, and non-diffracting beams, have been recently studied in a variety of fields, such as optical manipulations, optical telecommunications, nonlinear interactions, quantum physics, and 'super resolution' microscopy.. Their unique physical properties, such as annular intensity profile, helical wavefront and orbital angular momentum, give rise to a plethora of new, fundamental light-matter interactions and device applications. Recent progress in nanostructured materials, including metamaterials and metasurfaces, opened new opportunities for structured light generation on the microscale that exceed the capabilities of bulk-optics approaches such as computer generated holography and diffractive optics. Furthermore, structured optical fields may interact with matters on the subwavelength scale to yield new physical effects, such as spin-orbital momentum coupling. This special issue of Optics Express focuses on the state-of-the-art fundamental research and emerging technologies and applications enabled by the interplay of "structured light" and "structured materials". (C) 2017 Optical Society of America

A helical surface relief can be created in an azo-polymer film simply by illuminating circularly polarized light with spin angular momentum and without any orbital angular momentum. The helicity of the surface relief is determined by the sign of the spin angular momentum. The illumination of circularly polarized light induces orbital motion of the azo-polymer to shape the helical surface relief as an intermediate form; a subsequent transformation to a non-helical bump-shaped relief with a central peak creates a final form with additional exposure time. The mechanism for the formation of such a helical surface relief was also theoretically analyzed using the formula for the optical radiation force in a homogeneous and isotropic material. (C) 2017 Optical Society of America

We provide a novel laser-induced crystallization mechanism which explains crystallization induced by visible laser trapping of silver nanoparticles (AgNPs) at the air/unsaturated mother solution interface from the focal spot [Niinomi et al.CrystEngComm 2016, 18, 7441-7448]. Simultaneous in situ microscopic observation of Raman scattering and polarized-light image revealed that the optical trapping of nanoparticles that exhibit surface-enhanced Raman scattering (SERS) triggers the crystallization, showing the excitation of localized surface plasmon resonance (LSPR) significantly promotes the crystallization. Numerical analysis of temperature distribution based on the combination of finite-difference time-domain electromagnetic and finite-difference heat transfer calculations shows that temperature reaches 390 degrees C at the focal spot because of plasmonic heating, the energy dissipation of the plasmon-enhanced electromagnetic field as heat. A conceivable mechanism of the crystallization is local increment of supersaturation caused by local solvent evaporation via the Plasmonic heating. This plasmonic heating assisted laser-induced nucleation process has the possibility to provide not only a novel approach for spatiotemporal control of crystallization but also a novel nucleation field based on nonlinear light matter interaction originating from the plasmon-enhanced electromagnetic near field through heterogeneous nucleation on the surface of plasmonic particles.

Optical vortex, possessing an annular intensity profile and an orbital angular momentum (characterized by an integer termed a topological charge) associated with a helical wavefront, has attracted great attention for diverse applications due to its unique properties. In particular for terahertz (THz) frequency range, several approaches for THz vortex generation, including molded phase plates consisting of metal slit antennas, achromatic polarization elements and binary-diffractive optical elements, have been recently proposed, however, they are typically designed for a specific frequency. Here, we demonstrate highly intense broadband monocycle vortex generation near 0.6 THz by utilizing a polymeric Tsurupica spiral phase plate in combination with tilted-pulse-front optical rectification in a prism-cut LiNbO3 crystal. A maximum peak power of 2.3 MW was obtained for THz vortex output with an expected topological charge of 1.15. Furthermore, we applied the highly intense THz vortex beam for studying unique nonlinear behaviors in bilayer graphene towards the development of nonlinear super-resolution THz microscopy and imaging system.

Understanding the origin of the phonon modes of highly efficient electro-optic crystals is very important for designing materials and for optimizing their photonic applications. Here we investigate the origin of phonon modes in the 0.1-15 THz range of the benchmark electrooptic OH1 (2-(3-(4-hydroxystyry1)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal, which is interesting due to its large electro-optic coefficient and high THz-wave generation efficiency. The phonon modes (and vibrational absorption properties) of OH1 crystals are evaluated theoretically by periodic density functional theory and also experimentally by THz absorption spectroscopy. The the oretical calculations are well-matched with experimental results. The THz absorption properties are highly anisotropic; the amplitude of the vibrational absorption is the largest along the polar c-axis compared to the other two crystallographic axes. For comparison, the vibrational absorption modes of the OH1 molecule in the gas phase are also calculated. The calculated vibrational absorption spectrum of OH1 crystalline powder appears similar to that of the OH1 molecule in the gas phase. However, the molecular vibrational motions in the crystalline state are coupled motions of vibrational motions in the gas phase. Interestingly, the vibrational mode of the torsion of the O-H bond with the largest absorption strength in the gas phase is in the crystal inhibited due to the crystal field effect. The origin of the intense phonon modes of OH1 crystals is mainly related to relatively strong distortions of the push pull pi-conjugated system including electron donor and acceptor groups.

Frequency doubling of optical vortices is demonstrated with an optical-optical efficiency exceeding 70%, using a spiral phase plate at a fundamental vortex energy of 10.6 mJ. Beam propagation of the doubled vortex output is also investigated both experimentally and theoretically. Spatial transforms in the output during propagation are observed. (C) 2016 Optical Society of America

We developed an octave-band tunable optical vortex laser based on a 532 nm optical vortex pumped optical parametric oscillator with a simple linear-cavity configuration by employing cascaded non-critical phase-matching LiB3O5 crystals. The optical vortex output was tunable from 735 to 1903 nm. For a pump energy of 9 mJ, an optical vortex pulse energy of 0.24-2.36 mJ was obtained, corresponding to an optical-optical efficiency of 0.3-26%. (C) 2016 Optical Society of America

We discovered that a nanosecond optical vortex pulse can create a chiral cone-shaped monocrystalline silicon (Si) nanostructure (chiral Si nanocone) by transferring its optical angular momentum to a monocrystalline Si substrate. The fabricated Si nanocone, with a length of 4.8 mu m and a tip curvature of approximate to 110nm, was fully monocrystalline, and it had a spiral structure with an 86nm line width on a conical surface. Furthermore, its chirality was also determined directly from the handedness of the optical vortex pulse. Such chiral Si nanocones may enable the development of novel silicon photonic devices as well as ultra-highly efficient photovoltaic devices.

Sum frequency generation of the fundamental and Stokes fields within an intracavity self-Raman vortex laser is demonstrated in a linear resonator configuration. In this system, the sum frequency field exhibits different spatial profiles in the near and far fields, and a time-varying topological charge which varies between values of +2 and 0. We present a theoretical model which supports these experimental observations.

It was discovered that optical vortices twist isotropic and homogenous materials, e.g., azo-polymer films to form spiral structures on a nano- or micro-scale. However, the formation mechanism has not yet been established theoretically. To understand the mechanism of the spiral surface relief formation in the azo-polymer film, we theoretically investigate the optical radiation force induced in an isotropic and homogeneous material under irradiation using a continuous-wave optical vortex with arbitrary topological charge and polarization. It is revealed that the spiral surface relief formation in azo-polymer films requires the irradiation of optical vortices with a positive (negative) spin angular momentum and a positive (negative) orbital angular momentum (constructive spin-orbital angular momentum coupling), i.e., the degeneracy among the optical vortices with the same total angular momentum is resolved. (C) 2016 AIP Publishing LLC.

The formation of a monocrystalline silicon needle by picosecond optical vortex pulse illumination was demonstrated for the first time in this study. The dynamics of this silicon needle formation was further revealed by employing an ultrahigh-speed camera. The melted silicon was collected through picosecond pulse deposition to the dark core of the optical vortex, forming the silicon needle on a submicrosecond time scale. The needle was composed of monocrystalline silicon with the same lattice index (100) as that of the silicon substrate, and had a height of approximately 14 mu m and a thickness of approximately 3 mu m. Overlaid vortex pulses allowed the needle to be shaped with a height of approximately 40 mu m without any changes to the crystalline properties. Such a monocrystalline silicon needle can be applied to devices in many fields, such as core-shell structures for silicon photonics and photovoltaic devices as well as nano- or microelectromechanical systems.

We present what is to our knowledge the first demonstration of handedness control of a midinfrared (6.0-12.5 mu m) vortex output from a 1 mu m vortex-pumped optical parametric oscillator by swapping the frequencies of the signal and idler outputs. Right-and left-handed vortex outputs of over 0.1 mJ were generated within a frequency range of 6.0 -11.0 mu m. Such a tunable midinfrared vortex laser with handedness selectivity will open the door to chiral organic materials processing. (C) 2015 Optical Society of America

Monolayer (MLG) and bilayer (BLG) graphene devices have been fabricated with integrated antennas and have been investigated for a wideband terahertz (THz) detection at room temperature (RT). The devices show opposite (metallic vs. semiconducting, respectively) temperature coefficients of their resistance, which enable us to achieve a reproducible THz response via bolometric heating. The bolometric nature of this response is inferred by determining the spectral density of the 1/f resistance noise exhibited by the devices, as a function of the incident THz power. With increasing power, the spectral density varies in the two devices in a manner that reflects the opposite signs of their resistance temperature coefficients. The bolometric response is furthermore confirmed for both devices by the variation of their Hooge parameter as a function of the THz power. Overall, these observations confirm the capacity of graphene devices for sensitive broadband THz detection near RT. (C) 2015 AIP Publishing LLC.