Toshiaki Kondo

Femtosecond laser interference technique with diffractive beam splitter for fabrication of three-dimensional photonic crystals

A simple optical interference method to fabricate microperiodic structures was demonstrated. Femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Temporal overlap of the split femtosecond pulses, which requires 10 μm order accuracy in optical path lengths, was automatically achieved by this optical setup. One-, two-, and three-dimensional periodic microstructures with micrometer-order periods were fabricated using this method.

Multiphoton fabrication of periodic structures by multibeam interference of femtosecond pulses

Femtosecond laser pulses are useful for laser microfabrication through multiphoton absorption. However, it is difficult to create interference of femtosecond pulses for the fabrication of periodic structures. In this letter, we report the fabrication of two-dimensional periodic structures by means of multibeam interference of femtosecond pulses. Scanning electron microscopy revealed a rod structure arranged into a square lattice. The possibility of controlling the period of the lattice, rod thickness, and rod shape were demonstrated.

Holographic lithography of periodic two-and three-dimensional microstructures in photoresist SU-8

Micro-fabrication of periodic structures was performed by holographic lithography technique in SU-8 photoresist using a simple and versatile experimental arrangement based on a diffractive beam-splitter. High-fidelity two- and three-dimensional microstructures fabricated with sub-micrometric resolution in large areas of approximately 1 mm diameter. The structures are potentially usable as elements of micro-fluidic systems (e.g., Brownian ratchets), and templates for photonic crystal devices (e.g., mirrors, collimators, superprisms).

Fabrication of three-dimensional periodic microstructures in photoresist SU-8 by phase-controlled holographic lithography

Three-dimensional (3D) periodic dielectric structures were recorded in a commercial photoresist SU-8 by exposure to intensity patterns created by five interfering coherent femtosecond Ti?:?sapphire laser pulses. Periodic symmetry of these patterns was controlled by the choice of mutual convergence angles of the beams as well as their phases. The fabricated structures are mechanically and chemically stable, well-connected frameworks of SU-8 regions in air; the smallest dimensions of SU-8 features are in the sub-micrometre range. The structures exhibit a high degree of long-range order across areas as large as 1?mm in cross-section.

Optimization of antireflection structures of polymer based on nanoimprinting using anodic porous alumina

The optimization of antireflection (AR) structures of polymer composed of an ordered array of tapered pillars was studied. An anodic porous alumina mold with precisely controlled tapered holes was prepared and used for photoimprinting of the polymer. The reflectance of the obtained AR polymer structures with conical pillars was evaluated through the measurement of transmittance. Among the AR structures with pillars with various slopes, those with a gradually changing slope exhibited the lowest reflectance. The fabrication process will be effective for the formation of an AR surface with minimized reflectance over a large sample area.

Surface-Enhanced Raman Scattering in Multilayered Au Nanoparticles in Anodic Porous Alumina Matrix

Three-dimensional arrays of Au nanoparticles in an anodic porous alumina matrix were applied to a substrate for the measurement of surface-enhanced Raman scattering (SERS). The dependences of the SERS intensity of pyridine on the number of Au nanoparticle layers, and the gap size between nanoparticles were examined. The SERS intensity was in a good linear relationship with the number of layers of nanoparticles. The intensity also strongly depended on the gap between nanoparticles. This dependence is thought to originate from the enhancement of electric field of incident light with the decrease in the gap between nanoparticles.

Photo-structuring of As 2 S 3 glass by femtosecond irradiation

The nonlinear absorption coefficient of As(2)S(3) glass has been measured to be 2.0 cm/GW for femtosecond pulses at 800 nm. Femtosecond laser structuring via two photon absorption in bulk As(2)S(3) glass by erasable and permanent photo-darkening is demonstrated using both holographic and direct multi-beam laser writing.

Fabrication of photonic crystal structures by femtosecond laser-induced photopolymerization of organic-inorganic film

We report on a fabrication of photonic crystal structures in organic-inorganic hybrid films by a laser interference technique. Films containing the methacrylic group, which is photopolymerable by an adequate laser light, are prepared using a sol-gel technique from a mixture of organosilicate alkoxide and zirconium alkoxide modified by methacrylic acid. For the photopolymerization, the coated films on glass substrate are exposed to the interference light which is arranged with a square lattice in about 1 μm spacing. From microscopic images of microstructures produced by the photopolymerization, the influence of changes in conditions such as pre-bake temperature, photoinitiator and irradiation energy (laser power and duration time) on periodic structure is investigated. Adjusting the conditions, 2D and 3D photonic crystal structures with the micrometer-order period are formed in organo-silicate-zirconate hybrid materials.

Cross-Striped Ordered Arrays of Au Nanoparticles in Anodic Porous Alumina Matrix

Cross-striped composite structures of Au nanoparticles in anodic porous alumina matrix were fabricated by alternately carrying out anodization and electrodeposition. The length of air gap between Au nanoparticles could be precisely controlled based on the conditions of the cycles of anodization of Al and electrodeposition of Au. The process is simple, has high throughput, and will be useful for the preparation of several types of functional optical devices which require ordered three-dimensional nanostructures.

Fabrication of three-dimensional photonic crystals by femtosecond laser interference

We describe a novel microfabrication method based on interference of several coherent laser pulses in photosensitive media. The method allows to transform the periodic multidimensional interference patterns into periodic modulation of dielectric properties of the material, and is therefore potentially suitable for the photonic crystal fabrication in materials like photoresists, photosensitive glasses, and others. We have fabricated one, two, an three-dimensional photonic crystals with different lattices and sub-micrometer periods. The fabricated structures have high structural quality, as evidenced by confocal and scanning electron microscopes. Furthermore, using numerical simulations we explore the possibilities to obtain body-centered-cubic and diamond photonic crystal lattices by varying optical phases of the interfering beams. Numerical simulations are also used to reveal photonic bandgap properties of some 2D photonic crystals, fabricated using this technique.