We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials.

We discovered that chiral nanoneedles fabricated by vortex laser ablation can be used to visualize the helicity of an optical vortex. The orbital angular momentum of light determines the chirality of the nanoneedles, since it is transferred from the optical vortex to the metal. Only the spin angular momentum of the optical vortex can reinforce the helical structure of the created chiral nanoneedles. We also found that optical vortices with the same total angular momentum (defined as the sum of the orbital and spin angular momenta) are degenerate, and they generate nanoneedles with the same chirality and spiral frequency. DOI: 10.1103/PhysRevLett.110.143603 PACS numbers: 42.50.Tx, 42.50.Wk, 79.20.Ds, 79.20.Eb Light that has a helical wave front due to an azimuthal phase shift, expðiL Þ (where L is an integer known as the topological charge), carries orbital angular momentum, L@. Such light is referred as an optical vortex [1][2][3][4]. Optical vortices have been widely investigated for applications such as optical trapping and guiding [5][6][7], as well as superresolution microscopy [8,9]. Circularly polarized light has a helical electric field and a spin angular momentum, S@, associated with its circular polarization. Optical vortices with circular polarization exhibit both wave front and polarization helicities, and a total angular momentum, J@ [10-12], which is defined as the sum of the orbital and spin angular momenta. This angular momentum is evidenced by the orbital and spinning motions of trapped particles in optical tweezers.To date, several researchers have intensely studied the interaction of structured light, such as radially polarized beams, with plasmonic or metallic structures [13][14][15]. However, these previous studies mostly focused on optical properties, such as mode selection, plasmon focusing, etc., of plasmonic or metallic structures, including photonic crystals as well as plasmonic waveguides, prepared by conventional integrated photonic circuit techniques based on lithography and chemical etching. There are few reports on the use of structured light itself to form chiral structures on the nanoscale. Recently, we discovered that the helicity of a circularly polarized optical vortex can be directly transferred to an irradiated metal sample, resulting in the formation of chiral nanoneedles [16][17][18]. This is the first demonstration, to the best of our knowledge, of nanostructures created by structured light with angular momenta, and it clearly represents a new scientific phenomenon.We have also investigated control of the chirality of formed nanoneedles by changing the sign of the optical vortex helicity. Chiral nanostructures have the potential to form many new material structures [19], including planar chiral metamaterials [20,21] and plasmonic nanostructures [22,23]. They can also be used to selectively identify the chirality of chemical composites in nanoscale imaging systems, such as atomic force and scanning tunnel microscopes [24][25][26][27].However, it is currently unclear whe...

and various applications particularly at the microscopic scale. This includes optical trapping and manipulation, [4][5][6] optical telecommunications, [7,8] quantum physics, [9] and "super-resolution" microscopy with a spatial resolution beyond the diffraction limit. [10][11][12] New applications for OAM fields have also been proposed in environmental optics and free-space telecommunication. As an example OAM states can potentially propagate though air turbulence with lower degradation than a conventional Gaussian beam. [13,14] New physical effects include studies such as the rotational Doppler effect originating from interactions of such fields and rotating objects that may provide new technologies for remote sensing of rotating bodies in astrophysics. [15,16] In this area of original physical demonstrations, recent studies have shown that irradiation by such an OAM field can twist materials, such as metal, silicon, azo-polymer, and even a liquid-phase resin with the help of spin angular momentum (SAM) of circularly polarized light with the rotating electric field. This can thus shape helical nano/microstructures. [17][18][19][20][21][22][23][24] Going beyond fundamental aspects, twisted materials created by such OAM fields may provide a new understanding of interactions between optical fields and matter on the subwavelength scale that may reveal new physics, e.g., spin-orbit coupling effects or unconventional optomechanical effects for moving beyond traditional forms of optical manipulation. Furthermore, twisted materials created by such OAM fields, will provide a new understanding of interactions between optical fields and matter on a subwavelength scale. For example, the twisted materials manifest the role of both SAM and OAM of light fields and the coupling of SAM and OAM (so-called spin-orbit coupling) effects. This coupling is key not only for understanding fundamental physics but also for controlling optical material structures.Conventional optical tweezers rely on the field gradients near the diffraction limited focus of a laser beam to hold mesoscopic particles solely with focused light beams. However, the technique often fails to efficiently trap and manipulate particles at the nanoscale because the gradient force scales with the volume of the particle (for a dielectric object) and is proportional to the particle's polarizability. In this domain, advanced materials science and nanofabrication technologies have made remarkable progress in structured devices, e.g., photonic crystals, metamaterials, and metasurfaces. These devices offer novel trapping geometries to enhance the interaction between optical fields and materials at the nanoscale. Furthermore, these devices allow the control of 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...

We demonstrate the generation of high-quality tunable terahertz (THz) vortices in an eigenmode by employing soft-aperture difference frequency generation of a vortex output and a Gaussian mode. The generated THz vortex output exhibits a high-quality orbital angular momentum (OAM) mode with a topological charge of ℓTHz = ±1 in a frequency range of 2-6 THz. The maximum average power of the THz vortex output obtained was ~3.3 µW at 4 THz.

The current trends in stimulated Brillouin scattering and optical phase conjugation are overviewed. This report is formed by the selected papers presented in the “Fifth International Workshop on stimulated Brillouin scattering and phase conjugation 2010” in Japan. The nonlinear properties of phase conjugation based on stimulated Brillouin scattering and photo-refraction can compensate phase distortions in the high power laser systems, and they will also open up potentially novel laser technologies, e.g., phase stabilization, beam combination, pulse compression, ultrafast pulse shaping, and arbitrary waveform generation.