Go Obara

Plasmonic and Mie scattering control of far-field interference for regular ripple formation on various material substrates

We present experimental and theoretical results on plasmonic control of far-field interference for regular ripple formation on semiconductor and metal. Experimental observation of interference ripple pattern on Si substrate originating from the gold nanosphere irradiated by femtosecond laser is presented. Gold nanosphere is found to be an origin for ripple formation. Arbitrary intensity ripple patterns are theoretically controllable by depositing desired plasmonic and Mie scattering far-field pattern generators. The plasmonic far-field generation is demonstrated not only by metallic nanostructures but also by the controlled surface structures such as ridge and trench structures on various material substrates.

Growth of high spatial frequency periodic ripple structures on SiC crystal surfaces irradiated with successive femtosecond laser pulses

We present experimentally and theoretically the evolution of high spatial frequency periodic ripples (HSFL) fabricated on SiC crystal surfaces by irradiation with femtosecond laser pulses in a vacuum chamber. At early stages the seed defects are mainly induced by laser pulse irradiation, leading to the reduction in the ablation threshold fluence. By observing the evolution of these surface structures under illumination with successive laser pulses, the nanocraters are made by nanoablation at defects in the SiC surface. The Mie scattering by the nanoablated craters grows the periodic ripples. The number of HSFL is enhanced with increasing pulse number. At the edge of the laser spot the Mie scattering process is still dominant, causing the fabrication of HSFL. On the periphery of the spot SiC substrate remains a semiconductor state because the electron density in the SiC induced by laser irradiation is kept low. The HSFL observed is very deep in the SiC surface by irradiating with many laser pulses. These experimental results are well explained by 3D FDTD (three-dimensional finite-difference time-domain) simulation.

Femtosecond laser near-field nanoablation patterning using Mie resonance high dielectric constant particle with small size parameter

We demonstrate near-field nanohole patterning using a Mie resonance, small size parameter particle for nanofabrication technology regardless of substrate’s refractive index. Maximal enhancement factor and nearly smallest spot diameter among the same size dielectric particles are simultaneously obtainable on both low-refractive-index SiO2 and high-refractive-index Si substrates with a 200 nm particle of magnetic quadrupole resonance scattering mode (n∼2.7) at 400 nm excitation wavelength. Circular nanoholes with approximately 100 nm in diameter were fabricated on both substrates using a 200 nm amorphous TiO2 particle (n=2.66+0.024i) even with lower laser fluences than a half ablation threshold of the bare substrates.

Direct observation of surface plasmon far field for regular surface ripple formation by femtosecond laser pulse irradiation of gold nanostructures on silicon substrates

We have directly observed the interference ripple pattern between surface plasmon far field by gold nanosphere and the incident laser on silicon substrate. We explained the ripple formation using three-dimensional finite-difference time-domain simulation method. Nanosphere is an origin for regular ripple formation due to Mie scattering. We present a new method to control the plasmonic far-field pattern using an arbitrary gold nanostructure on the silicon substrate. Previously, the formed ripples were not regular but wavy because they were formed incoherently through the self organization process originating from the random surface roughness. The ripple structure was well controlled coherently.

Near-field interaction of two-dimensional high-permittivity spherical particle arrays on substrate in the Mie resonance scattering domain

We describe theoretical and experimental results on near-field interaction of two-dimensionally (2D) arrayed, high-permittivity spherical particles on a substrate in the Mie resonance scattering domain for surface nano-patterning processing. When a touching particle pair of Mie resonance particles on the substrate is considered, an electromagnetic mode different from the single particle mode is excited inside the particles, resulting in an intensity enhancement in a gap between two hotspots at particle-substrate contact points. As for 2D hexagonal close-packed particle arrays on the substrate, the refractive index of particle exhibiting a maximal enhancement factor for the 2D particle arrays is found to be shifted from the Mie resonance conditions for the single particle system.

Contribution of defect on early stage of LIPSS formation

We investigated an early stage of laser-induced periodic surface structure (LIPSS) formation to elucidate the contribution of defects on the formation. 4H-SiC crystals were irradiated by multiple pulses of femtosecond laser with different laser spot sizes. We observed the decrease in formation thresholds of high-spatial-frequency LIPSS (HSFL) and low-spatial-frequency LIPSS (LSFL) with the increased irradiated laser spot size. For smaller laser spot size, HSFL was only formed at the periphery of LSFL formation area, whereas for larger spot size, HSFL was randomly distributed within the laser spot. Our results are coincident with the hypothesis that the existence of defects in crystal contributes to the early stage on the formation of LIPSS, in which the electron excitation via one or two photon absorption in a defect site cause local nanoablation at a laser fluence under the intrinsic ablation threshold, followed by the formation of a nanovoid, which act as a scatterer, and interference of scattered wave and laser pulses lead to HSFL formation.

Evolution of Femtosecond Laser-Induced Surface Ripples on Lithium Niobate Crystal Surfaces

We fabricated periodic ripple structures on the surface of a lithium niobate crystal by irradiation with femtosecond laser pulses and observed the evolution of these structures under irradiation with successive laser pulses. After just a few laser pulses we observed nanorod-shaped craters, aligned with each other but randomly distributed over the surface. The nanocraters are caused by nanoablation at defects in the crystal surface. With increasing pulse number, side-lobed nanocraters appear and light scattered from the initial nanorod-shaped craters at the crystal surface interferes with the incident light, causing the formation of periodic structures.

Calculational studies on controllability of nanosphere propulsion by using femtosecond laser-excited enhanced near field

In this study, we propose a thrust/propulsion of nanospheres by using near field excited by femtosecond laser to control the sphere velocity and propelled angle. We investigated the dependence of parameters which contribute to the sphere velocity and propelled angle on the basis of the optical intensity by the three-dimensional (3D) finite-difference time-domain (FDTD) method.

Efficient femtosecond laser surface patterning using high dielectric constant particles with small size parameter

We present results on near-field ablation using Mie resonance high dielectric constant particles with small size parameter for establishing a new downsizing technique for nanopatterning. In this article, we first describe a comparative study of near-field properties on substrates using metallic and dielectric nanoparticle. The results indicate that combination of particle and substrate for efficient localized near-field nano-processing is important for selecting either metallic or dielectric particle. We then demonstrate nanoablation using a Mie resonance high dielectric constant small particle. Theoretical calculations clarified that the maximal enhancement factor and spot diameter close to the smallest size are obtainable on both low-refractive-index (SiO2) and high-refractive-index (Si) substrates using a 200 nm Mie resonance dielectric particle (n~2.7) at magnetic quadrupole mode with 400 nm excitation wavelength. Experimental results with 200 nm amorphous TiO2 particles (n=2.66+0.024i) by 400 nm femtosecond laser irradiation verified that clear circular nanoholes with about 100 nm in diameter were fabricated on both substrates even with laser fluence lower than a half ablation threshold of the bare substrates. As for nanopatterning with two-dimensionally arrayed 200 nm amorphous TiO2particles, cohesion of nanoholes was observed in high laser fluence regime due to inter-particle near-field interaction.

Nano-surface patterning by femtosecond laser for plasmonic surface optical applications

For plasmonic surface optical applications, localized optical field distribution properties in the vicinity of gold particles on a silicon substrate by backward and forward irradiation are presented. It is technically difficult to fabricate nanostructure on the surface by conventional forward laser incidence to the substrate because gold nanoparticles easily aggregate to form double-layered particle arrays. We calculated enhanced optical field properties in order to pattern the substrate surface only with a template of the bottom-layered particle arrays in case that the backward irradiation of femtosecond laser is used in the system of aggregated double-layered gold nanoparticle arrays. With the backward irradiation, the optical field intensity in the substrate for the double-layered hexagonal arrays is found to be only 30% lower than the mono-layered system. Moreover, near-field cannot be generated with the forward irradiation. As a result, only the backward irradiation scheme is found to be effective for uniform surface nanopatterning at enhanced plasmonic near-field zones.