宮本 克彦

We have found that large chiral symmetry breaking in chiral crystallization can be achieved by irradiating a several milliwatts focused laser to a plasmonic nanolattice immersed in a stagnant NaClO3 saturated aqueous solution. Several hundreds of chiral crystals with the same handedness showed up in the solution after the laser irradiation in contrast to spontaneous crystallization. In situ microscopic observation for the early stage of the crystallization in the vicinity of the focal spot revealed that microbubble generation followed by large supersaturation increase, in which supersaturation reaches 360%, promotes several numbers of crystal nucleation in the vicinity of the bubble as "mother" crystal. The generation of the microbubble induced Marangoni convection, the velocity of which reaches several hundreds of micrometers per second, crushing the first appearing chiral crystal into pieces by microfluidic shear. Namely, secondary nucleation caused by microfluidic shear amplified the number of "daughter" crystals with the same handedness. This spatiotemporally controllable micromixing experiment realized by laser irradiation gives us not only a novel route bridging a light and chiral symmetry breaking but also the novel method to observe the early stage dynamics of the secondary nucleation, which was hard to observe by conventional observation technique, in real time.

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 present the new structured materials, i.e. string-shaped Au nano-structures, formed by employing the optical vortex ablation processing on an Au thin film. We also address the filament formation of Au particles by deposition of optical vortex pulse. Such structured Au particles have the potential to pave the pathway towards advanced chemical reaction.

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.

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

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

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

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.

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

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 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.

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.

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.

Optical vortex illumination enables the creation of silicon needles with a height of ~14 μm and a thickness of ~2 μm. Silicon needle formation requires an optical vortex pulse with a pulse duration of 10~20 ps, so as to create thermally-melted silicon with fewer heat diffusion effects.

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.

We demonstrated a widely tunable 1-mu m optical vortex laser formed from a 0.532-mu m optical vortex pumped optical parametric oscillator with a singly-resonant cavity configuration employing cascaded non-critical phase-matching LiB3O5 crystals. With this system, the topological charge of the pump beam can be selectively transferred to the signal or idler output, and a vortex output in the wavelength range of 850-990nmor 1130-1300nmcould be obtained. A maximum signal vortex output energy of 0.9 mJ was achieved, corresponding to an optical efficiency of 10%. (C) 2015 Optical Society of America

We present terahertz (THz) wave generation using type II phase matching polarization combination via difference frequency generation (DFG) with lithium niobate (LiNbO3) crystal. A collinear phase matching has not been realized so far because LiNbO3 crystal has a large absorption and high refractive index dispersion. In this paper we proposed that using type II phase matching is able to avoid these defects and generate the THz wave. We experimentally demonstrated the forward and backward collinear type II phase matching method, and confirmed that THz wave is generated by using simple equipment. This method will be applied for the applications for a small THz wave sources. (C) 2015 The Japan Society of Applied Physics

We demonstrated high average power, diffraction-limited (M-2 similar to 1.1) picosecond output from a sapphire face-cooled Nd:YVO4 slab amplifier at 1064 nm in a multi-pass geometry. Average output power of 44.5 W was achieved at an optical efficiency of 56%. A second harmonic power of 24.1 W was also obtained at an input fundamental power of 36 W, corresponding to a conversion efficiency of 67%. (C) 2015 Optical Society of America

A novel sensing technique is developed that makes use of evanescent terahertz (THz) waves without directly detecting the THz wave. The technique is demonstrated for measuring changes in the electric field of near-infrared light, transcribed from changes in the electric field of THz waves. The method will enable sensing of various materials in the THz-frequency region, to provide knowledge about chemical reactions that require real-time tracking.

© Springer Japan 2015. This chapter describes a research on chiral metal structures on the nanoscale (chiral metal nanostructures) formed by the irradiation of an optical vortex with orbital angular momentum. The purpose of the present research has been to clarify the relationship between the orbital, spin, and total angular momenta of light and nanostructures through laser ablation processes. As a result, the orbital angular momentum of the optical vortex is transferred to the metal so as to create a chiral nanostructure response by the optical vortex helicity. The chirality of the nanostructures can also be selectively controlled merely by changing the sign of the total angular momentum. The total angular momentum of light further determines the spiral spatial frequency of the chiral nanostructures. By adjusting the numerical aperture of a focusing objective lens and the incident laser power, a chiral nanostructure with a tip curvature of less than 40 nm, corresponding to 1/25th of the laser wavelength (1,064 nm), was successfully fabricated. The chemical composition of the nanostructure was almost identical to that of the substrate. A two-dimensional 5×5 nanostructure array was also fabricated. We also address a chiral surface relief (termed ‘conch’-shaped relief) formation in an azo-polymer through photo-isomerization process.

We demonstrated widely tunable mid-infrared (6.3-12 mu m) optical vortex pulse generation from a ZGP difference frequency generator pumped by a 2 mu m optical vortex laser with a cascaded KTP geometry. The mid-infrared vortex output carried the same topological charge as that of the 1 mu m pump output without any destruction. A pulse energy of >135 mu J was obtained in the wavelength region of 6.3-7.0 mu m. (C) 2014 Optical Society of America

The frequency-doubling of a picosecond vortex fiber laser, formed of a 1-mu m picosecond master laser and a large-mode-area fiber amplifier by using a nonlinear LiB3O5, crystal, was performed. A maximum second-harmonic power of 7.7 W was achieved, corresponding to a conversion efficiency of 31 %. The second harmonic had an annular spatial form owing to a phase singularity with a doubled topological charge, and its wavefront helicity was selectively controlled by tuning the stress applied to the fiber amplifier.

This paper describes the first demonstration of ultraviolet (266nm) vortex generation using the combination of a frequency-doubled nanosecond green laser, a spiral phase plate, and a periodically bonded beta-BaB2O4 device. For a laser pumping energy of 9.1 mJ, an ultraviolet vortex energy of 1.24 mJ was obtained, corresponding to a conversion efficiency of 13.7%. (C) 2014 Optical Society of America

A terahertz (THz) spiral phase plate with high transmission (>90% after Fresnel correction) and low dispersion has been developed based on the Tsurupica olefin polymer. Direct observations of the topological charge (both magnitude and sign) of a THz vortex beam are performed by using a THz camera with tilted lens focusing and radial defect introduction. The vortex outputs with a topological charge of +/- 1 (or +/- 2) are obtained at a frequency of 2 (or 4) THz. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

We have discovered that a novel chiral structured surface relief (termed 'conch'-shaped surface relief) with a height of over 1 mu m can be formed in an azo-polymer film merely by employing circularly polarized optical vortex irradiation with a total angular momentum of j = +/- 2. The temporal evolution of the conch-shaped surface relief in the azo-polymer film was also observed. The results provide physical insight into how the angular momentum of light is transferred to a material through mass transport by cis-trans photo-isomerization. Such conch-shaped surface reliefs with chirality, in which functional chemical composites can be doped, enable new applications, such as planar chiral metamaterials, plasmonic holograms, and identification of chiral chemical composites.

We report terahertz (THz) wave generation by satisfying Cherenkov phase-matching condition in both s and p polarizations. A dual-wavelength optical parametric oscillator is constructed from two potassium titanium oxide phosphate crystals pumped by a frequency-doubled Nd:YAG laser. By rotating the orientation of both a lithium niobate crystal (LiNbO3) and the polarization of the pump waves, the polarization of the THz wave changes. Due to the difference in the refractive index and absorption, the output power for p polarization is one tenth that for s polarization. A tuning range from 0.2 to 6.5 THz is obtained for s polarization, and from 0.2 to 4.2 and 5.4 to 6.9 THz for p polarization. The extraction efficiency is improved by changing the angle of prism for p polarization, and a large phase change occurs at total internal reflection. Consequently, p-polarized THz waves are optimal for spectroscopic applications. (C) 2014 Optical Society of America

We demonstrated broadband terahertz (THz) wave generation by satisfying the noncollinear phasematching condition with a reflected signal beam. We constructed a dual-wavelength optical parametric oscillator with two potassium titanium oxide phosphate crystals pumped by a frequency-doubled Nd:YAG laser. The collinear pump and signal waves were irradiated into a lithium niobate crystal. The pump and the signal waves were reflected at the crystal surface. Because the pump and the signal waves have a finite beam diameter, when the reflected signal wave and unreflected pump wave were irradiated at the correct angle, the noncollinear phase-matching condition was satisfied. By changing the incident angle to the crystal, broadband THz-wave generation with a range of over 0.2-7.2 THz was achieved. © 2013 Optical Society of America.

We present the first handedness control of an optical vortex output from a vortex-pumped optical parametric oscillator. The handedness of the optical vortex was identical to that of the pump vortex beam. Over 2 mJ, 2- μm optical vortex with a topological charge of ± 1 was achieved. We found that the handedness of a fractional vortex with a half integer topological charge can also be selectively controlled. © 2013 Optical Society of America.

We report a novel sensing technique that uses an evanescent terahertz (THz) wave, without detecting the THz wave directly. When a THz wave generated by Cherenkov phase matching via difference frequency generation undergoes total internal reflection, the evanescent THz wave is subject to a phase change and an amplitude decrease. The reflected THz wave, under the influence of the sample, interferes with the propagating THz wave and the changing electric field of the THz wave interacts with the electric field of the pump waves. We demonstrate a sensing technique for detecting changes in the electric field of near-infrared light, transcribed from changes in the electric field of a THz wave. © 2013 Optical Society of America.