Takashige Omatsu

Optical Vortices Illumination Enables the Creation of Chiral Nanostructures

We discovered that optical vortices with an annular spatial form and an orbital angular momentum owing to a helical wave front enable us to twist materials, such as metal, silicon and azo-polymer, to form various structured matters including microneedles, chiral nanostructures and chiral surface reliefs. Such structured matters will potentially open the door to advanced devices, for instance, silicon photonic device, biomedical microelectro-mechanical systems, ultrasensitive detector for chiral chemical composites and plasmonic metasurfaces for chiral chemical reactions.

Helical microfibers created by photopolymerization with light fields possessing orbital angular momentum (Conference Presentation)

Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Such microstructures have been optimized for optical manipulation and are finding application elsewhere, including micro-optics, mechanical microstructures and polymer crystallography. Furthermore, due to self-focusing, photopolymerization can form a waveguide, which develops into an optical fibre as long as submillimeters. Importantly, to date virtually all photopolymerization studies have been performed with incident light fields possessing planar wavefronts and simply exploit the beam intensity profile. Here we investigate photopolymerization of ultraviolet curing resins with a light field possessing orbital angular momentum (OAM). We show that the annular vortex beam breaks up via modulation instability into the m-microfibers, depending on the azimuthal index m of an incident optical vortex. These microfibers exhibit helical structures with chirality determined by the sign of m and mirror the helical nature of the incident vortex beam wavefront. We have developed a numerical model based on the Beam Propagation Method that captures the key experimental observations for a variety of optical vortices characterized by their azimuthal index m. This research opens up a range of new vistas and has broad consequences for the fields of structured light, new approaches to writing novel mesoscopic structures and applications such as in detecting or sorting the OAM mode (e.g. photonic lanterns) in areas including optical communications and manipulation.

Optical vortex induced chiral mass-transport of azo-polymer through two photon absorption

We discover that 1.06 μm picosecond vortex pulses induce chiral mass-transport to form a single-armed chiral surface relief in azo-polymer through two photon absorption process. The surface relief exhibits a diameter of a 2.5μm, i.e. 0.7 times of diffraction limit.

Sub-millimeter helical fiber created by Bessel vortex beam illumination

We demonstrate a self-written sub-millimeter (>300 μm) helical fiber in a photo-cure resin by irradiation of non-diffractive 1st-order Bessel beam with an orbital angular momentum. The twisted direction of the helical fiber can be controlled by only reversing the sign of the topological charge of Bessel beam.

Shrinking optical vortex to the nanoscale

Optical beams with Orbital Angular Momentum (OAM) can potentially be used to probe forbidden transitions. However, the size of the vortex beam has to be comparable to that of an atom, molecule or an artificial atom. We propose and demonstrate a de-magnifying hyperlens allowing reducing the size of the vortex beam to the nanometer scale.

Bottle beam generation from a frequency-doubled Nd:YVO4 laser with a tightly end-pumping geometry

We first demonstrated the generation of an optical bottle beam with a zero-intensity region surrounded by threedimensional bright regions from an intra-cavity frequency-doubled Nd:YVO4 laser with a nearly hemispherical cavity configuration. We also numerically analyzed the components of the generated bottle beam which consists of a series of Laguerre–Gaussian modes. The experimental results can be excellently reconstructed with the theoretical model.

Fabrication of hollow microneedles by optical vortex illumination

Molding technologies are mostly employed to fabricate microneedles (MNs) for medical and cosmetic applications, such as drug delivery and skin care systems. However, such technologies are ill suited for bioabsorbable materials with high viscosity, thus, there is no report concerning hollow bioabsorbable MNs, so far. We propose a novel laser processing technology, which enables us to fabricate a hollow bioabsorbable MN by simply irradiating both optical vortex for fabricating MNs and Gaussian beam for drilling holes on a hyaluronic acid sheet.

Nano-needle fabrication based on optical vortex laser ablation

We found that the optical vortex could fabricate a nano-scaled metal needle (nano-needle). Optical vortices, having a helical (twisted) wavefront owing to a phase singularity, carry an angular momentum. The angular momentum of the optical vortices is transferred to the melted (or vaporized) metal during ablation process, resulting in the formation of the nano-needle. The tip curvature of the nano-needle with a height of 8.4u2005µm was measured to be <50nm, corresponding to less than one-twentieth of the laser wavelength (1064nm). We also demonstrated the fabrication of a two-dimensional 5 x 5 nanoneedle array. The direction of the nano-needle could be also controlled by changing an incident angle of the optical vortex onto the metal target.We found that the optical vortex could fabricate a nano-scaled metal needle (nano-needle). Optical vortices, having a helical (twisted) wavefront owing to a phase singularity, carry an angular momentum. The angular momentum of the optical vortices is transferred to the melted (or vaporized) metal during ablation process, resulting in the formation of the nano-needle. The tip curvature of the nano-needle with a height of 8.4u2005µm was measured to be <50nm, corresponding to less than one-twentieth of the laser wavelength (1064nm). We also demonstrated the fabrication of a two-dimensional 5 x 5 nanoneedle array. The direction of the nano-needle could be also controlled by changing an incident angle of the optical vortex onto the metal target.