Shingo Kanehira

Ion exchange in glass using femtosecond laser irradiation

We explain the occurrence of ion exchange and an index profile around the focal point inside a commercial crown glass formed by femtosecond laser irradiation. The index profile in the photoinduced area has a ring-shaped pattern, which indicates that local densification occurred in the glass. An irregular surface reflecting the density distribution is formed around the focal point by dry etching process using a focused ion beam. By the irradiation of femtosecond laser pulses, the effect of ion exchange between the focal point and the surrounding area is also observed in the area in which local densification occurred.

Formation mechanism of element distribution in glass under femtosecond laser irradiation

We report on the formation mechanism of element distribution in glass under high-repetition-rate femtosecond laser irradiation. We simultaneously focused two beams of femtosecond laser pulses inside a glass and confirmed the formation of characteristically shaped element distributions. The results of the numerical simulation in which we considered concentration- and temperature-gradient-driven diffusions were in excellent qualitative agreement with the experimental results, indicating that the main driving force is the sharp temperature gradient. Since the composition of a glass affects its refractive index, absorption, and luminescence property, the results in this study provide a framework to fabricate a functional optical device such as optical circuits with a high-repetition-rate femtosecond laser.

Observation of laser-induced stress waves and mechanism of structural changes inside rock-salt crystals.

The structural changes inside rock-salt crystals after femtosecond (fs) laser irradiation are investigated using a microscopic pump-probe technique and an elastic simulation. The pump-probe imaging shows that a squircle-shaped stress wave is generated after the fs laser irradiation as a result of the relaxation of thermal stress in the photoexcited region. Pump-probe crossed-Nicols imaging and elastic simulation elucidate that shear stresses and tensile stresses are concentrated in specific regions during the propagation of the stress wave. The shear stresses and tensile stresses observed in this study can explain the characteristic laser-induced structural changes inside rock-salt crystals.

Photoinduced microchannels inside silicon by femtosecond pulses

We reported on the fabrication of microchannels in the interior of silicon wafers using a femtosecond laser of 800nm wavelength, which was in the absorption region of silicon. The scanning electron micrographs showed that microchannels were induced inside the silicon wafer when the femtosecond laser beam was focused inside the wafer. The aspect ratio of the microchannel cross section decreased with the increase in scan velocity of the laser. The formation of the photoinduced microchannels probably resulted from the microexplosions due to both the linear absorption and avalanche ionization.

Optically produced cross patterning based on local dislocations inside MgO single crystals

Here, the authors demonstrate a unique technique to form dense dislocations locally inside a single crystal with a rocksalt-type structure using femtosecond laser irradiation for the first time, to our knowledge. Cross-shaped patterns of micrometer size, originating from densely introduced dislocations, are formed around the focal point. The authors controlled three-dimensional propagation of dislocations by adjusting the pulse energy of femtosecond laser and numerical aperture of objective lens.

Fabrication and characterization of silicon antireflection structures for infrared rays using a femtosecond laser

We focus on IR sensors with lower reflection for the wavelength around 10 μm, strongly awaited for detecting human bodies. A concave structure was designed as a more suitable reflection-free structure for IR light, and an optical system with a femtosecond laser was employed for verification of the effectiveness of the structure. The microstructures prepared through this process were fabricated and optically measured using SEM, FT-IR, and Raman spectroscopy. The measurement revealed that good reflection-free structures were realized for IR sensors with lower reflection for the wavelength of around 10 μm.

Controllable hydrogen release via aluminum powder corrosion in calcium hydroxide solutions

Abstract Mechanism of aluminum powder corrosion in calcium hydroxide solutions was investigated via various analyses of residues after its hydrolysis reaction and pH change of the solutions to control the release of hydrogen gas. It was revealed that the reaction with a release of hydrogen gas consisted of katoite formation and hydration of aluminum. The katoite formation, which is progressed by the reaction among calcium ion, aluminum hydroxide anion (Al(OH)4−), and water molecules at the first step, is important for the promotion of aluminum corrosion and moderate release of hydrogen gas.

Formation of Elemental Distribution in Glass Using Thermal Accumulation with Femtosecond Laser Irradiation

Elemental migration inside a glass was induced space-selectively and microscopically by high-repetition femtosecond(fs) laser irradiation. The tendency of the elemental migration depended on the strength of the bond between cations and oxygen ions:strongly bonded ions like Si or Al migrated to the center of the irradiated spot, whereas weekly bonded ions such as Ca migrated to the outside. Judged from analyzed temperature distribution, this phenomenon may be due to the thermomigration(Soret effect). The refractive index distribution was modified locally by controlling elemental distribution and optical waveguide was formed in phosphate and borate glasses.

Electromagnetic properties of photonic crystals with diamond structure containing defects

Three-dimensional photonic crystals with a diamond structure, which are composed of the TiO 2 -based ceramic particles dispersed in an epoxy lattice, were fabricated by stereolithography. The diamond structure showed a photonic band gap in the 14.3-17.0 GHz range along the Γ-K direction, which is close to the band calculation using the plain wave expansion method. Two types of lattice defects-air cavity and dielectric cavity-were introduced into the diamond structure by removing a unit cell of diamond structure or inserting a block of the lattice medium into the air cavity. The transmission of millimeter waves affected by multiple reflections at the defects was measured in the photonic band gap. Resonant frequencies in the defects were calculated and compared with the measurement results.

Three-dimensional deposition of silicon from silicate glass with dispersed metallic aluminum by a femtosecond laser

We have fabricated silicon structure in silicate glass prepared with metallic aluminum in the starting material, using femtosecond laser irradiation and subsequent annealing. Small Si-rich structures such as oxygen-deficiency (O-deficiency) defects or Si clusters transform into nano-sized Si particles by the focusing irradiation of the laser. Then the Si-rich structures grow into micro-size particles due to the thermite reaction promoted by heat treatment. We determine the effect of focused laser pulse on the Si deposition process by using a time-resolved transient lens method with a sub-picosecond laser pulse. Localized high-temperature, high-pressure, and the generation of shock waves appear to be very important in forming the Si-rich structures that ultimately grow into Si particles. The diffusion of oxygen by shock waves and the existence of Al-rich structures help form Si-rich structures as Si-O bonds continuously break under high temperature. The focusing irradiation of femtosecond lasers is very useful for fabricating Si structures inside glass.