Kenzo Miyazaki

Origin of periodicity in nanostructuring on thin film surfaces ablated with femtosecond laser pulses

We report physical processes responsible for the periodic nanostructure formation in femtosecond-laser ablation of thin film surfaces. It has been found that an initial random distribution of nanoscale ablation traces is periodically structured with an increase in superimposed laser pulses or fluence on diamond-like carbon film used as the target. The results show that the formation of periodicity can be attributed to the excitation of surface plasmon polaritons to induce the periodic enhancement of local fields in the surface layer. The estimated field period is in good agreement with the observed size of nanostructures.

Efficient deep-ultraviolet generation by frequency doubling in β-BaB 2 O 4 crystals

Efficient and tunable coherent ultraviolet (205-310 nm) generation in beta-BaB(2)O(4) crystals has been studied using Type I phase matching at room temperature. A second-harmonic (SH) -generation efficiency eta of more than 10% was obtained at the SH wavelength lambda(SH) greater, similar 206 nm. The maximum eta was 36% for a fundamental dye-laser power of 150 kW, and eta at lambda(SH) = 310 nm was found to be 4-6 times larger than that in ammonium dihydrogen phosphate.

Mechanism of femtosecond-laser-induced periodic nanostructure formation on crystalline silicon surface immersed in water

Focused on silicon surface in water, superimposed multiple shots of linearly polarized 800-nm, 100-fs, 10-Hz laser pulses at lower fluence than the single-pulse ablation threshold are shown to produce two kinds of periodic nanostructures with almost constant periods of 150 nm and 400 nm. Surface plasmon polaritons excited in the surface layer illustrates well the formation of nanostructures and its dynamic properties observed. Pump and probe measurements of the ultrafast change in surface reflectivity during the interaction have demonstrated that the multiple low-fluence fs pulses are crucial to the nanostructuring through the accumulation of non-thermal bonding structure change and the subsequent nanoscale ablation.

Ultrafast dynamics of periodic nanostructure formation on diamondlike carbon films irradiated with femtosecond laser pulses

Using a pump-probe technique the authors have measured reflectivity of diamondlike carbon (DLC) film irradiated with femtosecond laser pulses to understand dynamic processes responsible for periodic nanostructure formation. The results have shown that characteristic reflectivity change observed as a function of superimposed laser shots is closely associated with the nanostructure formation and the bonding structure change to induce surface swelling, leading to a conclusion that the nanostructure formation on the DLC surface is certainly preceded by the bonding structure change. The nanoscale ablation to produce the nanostructure is discussed based on the local field generation on the surface.

The White Light Supercontinuum Is Indeed an Ultrafast White Light Laser

We identify the white light supercontinuum generated by femtosecond Ti-sapphire laser pulses as an ultrafast white light laser because the relative coherence lengths of all the wavelength components of the supercontinuum are essentially the same as that of the laser pulse when compared to an incoherent white light source.

Nanoscale ablation on patterned diamondlike carbon film with femtosecond laser pulses

The authors have studied the origin of nanostructure formation on diamondlike carbon film in femtosecond laser ablation at low fluence. Using the thin film target patterned with submicrometer-size stripes, they have observed that the nanostructure starts to be formed on the crest of stripes along the direction perpendicular to the laser polarization. The experimental results have shown that nanoscale ablation for the nanostructuring would preferentially be initiated by the enhancement of localized electric field on the stripe surface with high curvature.

High‐power discharge‐pumped F2 molecular laser

Output energy of a discharge‐pumped F2 molecular laser has been greatly increased by an extremely intense excitation in a sufficiently high‐pressure laser gas mixture. The maximum vacuum ultraviolet output energy was 112 mJ for an 8 atm laser gas mixture, which is almost an order of magnitude larger than that obtained so far with discharge devices. The peak power deposition into the discharge plasma was estimated to be 23 MW/cm3 for the maximum output energy. Use of a short electrode spacing was found to be effective to increase the output energy.

Observation of revival structure in femtosecond-laser-induced alignment of N2 with high-order harmonic generation

We report a new sensitive method using high-order harmonic generation to observe revival structure in fs-laser induced alignment of a rotational wave packet of molecules. Pump and probe fs-laser pulses with a time delay were focused collinearly into a pulsed N2 gas jet, so that the pump pulse induces alignment of a ground-state rotational wave packet, and the delayed probe pulse produces harmonic radiation from the aligning molecules. The harmonic signal observed as a function of time delay has clearly demonstrated a typical time-dependent revival structure in the field-free alignment of molecules.

A Comment on Lightning Control Using a Femtosecond Laser

The current technique of lightning control using nanosecond CO2 laser pulses and its limitations are analysed. The advantages of replacing the CO2 laser pulses by intense femtosecond laser pulses without changing the basic design of the lightning control tower are analysed and discussed.

High-order harmonic generation in rare gases with intense subpicosecond dye laser pulses

A systematic study of high-order harmonic generation in rare gases has been made for the first time with intense (2*1014 W cm-2, 616 nm) dye laser pulses, where the harmonic radiation up to the 41st order in He, the 37th in Ne, the 19th in Ar, the 17th in Kr and the 13th in Xe is observed. The individual harmonic distributions represent a plateau that is preceded by an intensity minimum and formed from an energy region near the ionization limit. The importance of dynamic resonances and shifting ionization limit is discussed to illustrate the plateau formation process observed.