We studied the unconventional laser trapping of nanoparticles using strong optical nonlinearity. If the laser frequency is resonant with the electronic transition of particles, the optical force is enhanced, and optical nonlinearity easily appears. This article discusses the influence of strong optical nonlinearity on the exerted optical force. Further, as a possible effect arising from nonlinearity, super-resolution trapping by optical vortex is theoretically demonstrated.

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The optical vortex beam has recently attracted significant attention because of carrying the orbital angular momentum. We clarified that multipolar plasmons of metal nanodisks can be selectively excited by circularly-polarized optical vortex beams. The orbital and spin angular momenta are transferred from vortex photons to localized plasmons. Unfortunately, the mode volume of this plasmonic nanodisk resonator is sub-micrometer dimension that is restricted by the diffraction limit of the surface plasmon wave. In order to realize single-nanometer-sized cavities, we designed the tailored plasmonic structure consisting of metal multimer surrounding a nanogap. This structure makes it possible to localize the vortex field into the gap space with conserving the orbital and spin angular momenta. The transfer of angular momenta from nano-vortex photons to molecules or nanoparticles causes rotational radiation pressure, i. e., optical torque, which induces nano-vortex flow of molecules/particles and may lead to chiral structuring of molecule/particle assemblies. We experimentally demonstrated rotational manipulation of a polymer nano-bead using a gold nano-prism trimer structure.

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Orbital angular momentum （OAM） of light is receiving much attention from the viewpoints not only of fundamental physics but also of applications. Photonic structures are becoming important tools for controlling/generating OAM in an efficient and stable manner. Here we overview some of schemes for generating light with OAM using photonic structures such as metasurfaces and microring resonators. We also discuss a possibility for generating light possessing OAM from a single quantum dot embedded in a microring resonator or in a photonic crystal nanocavity.

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This review describes the interaction between circularly polarized light and metallic nanostructures, the characteristics of chiral plasmons, and their near-field interaction with matters in the vicinity. In the near-field region of the nanostructure, especially in the near-field region of plasmonic materials, strongly twisted electromagnetic fields are generated, and strong optical activity locally arises. The selection rule for optical activity (optically active materials are chiral) is broken for local optical activity. Circularly polarized electromagnetic fields can be generated in the vicinity of any materials under resonance conditions, regardless of the chirality of the material or the optical field. Due to the strongly twisted optical near-fields, the molecules in the vicinities of metal nanostructures show optically active behavior. We expect these properties utilized as a basis to manipulate chiral electromagnetic fields and to provide novel chiral properties of materials.

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Levitated optomechanics is an emerging area of study enabled by optically trapped mesoscopic particles in vacuum. This opens the path to a range of new opportunities and insights at the classical-quantum interface. We review our recent work on the dynamics of an optically levitated microparticle in vacuum placed in an optical vortex beam. The dynamics are dictated by the orbital angular momentum of the field. The microparticle is confined within a Laguerre-Gaussian beam and orbits the annular beam profile with increasing angular velocity as the air drag force is reduced, as a result of reducing the background pressure. Furthermore, we extend this to explore the particle dynamics in a complex threedimensional optical potential with orbital angular momentum in vacuum. The potential is formed by the generation of a “perfect vortex” beam, which upon propagation, evolves to a Bessel beam. We show that the optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle, which produces a rich variety of orbital motions with respect to the beam propagation axis.

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Ultrathin optical fibers have emerged as efficient and versatile platforms for studying light-matter interactions. Owing to their geometry, they are characterized by intense evanescent fields extending beyond the fiber surface. These fields can carry both spin and orbital angular momentum of guided light. Complex spatial intensity, phase, and polarization profiles can be generated at the fiber waist by propagating higher order fiber modes. In this paper, we review applications of ultrathin optical fibers, with an emphasis on optical manipulation at the micro- and nanoscale. We also discuss mode content and angular momentum of light guided by ultrathin fibers.

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Optical vortices, i.e. helical light beams, carry a donut spatial intensity profile and an orbital angular momentum, arising from their helical wavefronts. In recent years, they have been intensely studied in a variety of fields, such as optical tweezer and manipulation, optical telecommunication, quantum information, and fluorescence microscope. In particular, we and our co-workers discovered that optical vortices twist melted or vaporized materials to establish chiral structured materials on a nano/micro scale. Such chiral structures allow potentially the development of chirality sensors, chiral chemical reactors, and optical meta-materials. In this article, we review various chiral structures formation based on orbital angular momentum transfer effects. We also address the spin-orbital angular momentum coupling effects in chiral surface relief formation in an azo-polymer film.

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Among typical active matter such as self-propelled micro-objects, the characteristic collective motion originating from the hydrodynamic interaction between constituents has been observed in both biological and artificial systems. In this study, we report various kinds of autonomous collective motion of micrometer-sized particles in a one-dimensional driven system by an angular momentum of light. The regular stationary and dynamic arrangements resembling “crystals” or “clusters” is induced by longranged hydrodynamic interaction between particles in low Reynolds numbered system. A transition in the collective motion has been observed by varying the size ratio of the particles in binary-sized systems. We have also studied the influence of spatial confinement on the collective motion and obtained a complex motion phase diagram as a function of angular momentum and spatial confinement.

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Vortex beams, which include phase or polarization singularities, have attracted attention and provided new approaches to spectroscopic studies. We demonstrate in this paper the application of vortex pulses to four-wave-mixing (FWM) spectroscopy which is a major technique to investigate the coherent dynamics of excitons in semiconductors. The measurements provide some unique properties of GaN excitons: FWM using optical vortex demonstrates the coherent orbital angular momentum dynamics significantly exceeds the exciton dephasing time and its decay reflects inhomogeneity of rotational phase gradient; FWM using polarization vortex realizes a snap-shot measurement of optical anisotropies in GaN films, allowing us to evaluate the uniaxial strain energies and the spin-exchange interaction constant with high accuracy and sensitivity.

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We have proposed an the electronic CRDS which a light source and a receiver are opposed to each other instead of two high reflectance mirrors and an optical signal is converted into an electric signal by a receiver, fed back to the light source. In this paper, it was verified that by measuring CO2 concentration, it can be measured with higher sensitivity than general single pass absorption spectroscopy. As a result, although it was not as good as conventional CRDS, it was able to measure with 26 times higher sensitivity than general single pass absorption spectroscopy.

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For optical sensing, an annular beam is widely used because of its ability to self-transform into a nondiffractive beam when it propagates in air. This ability, however, is difficult when it propagates in random media because of scattering. To analyze the characteristics of annular beam propagation in random media, we proposed a new wavefront analysis method based on geometric optics approximation. In this algorithm, we adopted a simplified geometric optics approximation and iterative calculation based on forward scattering and simulated the attenuation and the scattering waveform of the annular beam in random media. By this simulation, we got a non-diffractive beam waveform at the optical axis with a certain propagation distance and media concentrations. The numerical analysis results were compared with the experimental results.

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