M. Obara

Gold nanoparticles as nanoheaters and nanolenses in the processing of different substrate surfaces

We present results of our recent study on the heating process and near field localization arising when gold nanoparticles are irradiated by ultrashort laser pulses at wavelength of 800 nm. The system under consideration consists of Au nanoparticles with diameter of 40, 80, or 200 nm in vacuum or deposited on different substrates. Substrate materials with different dielectric properties are used in order to sense and visualize the nanoparticle heating and near field electromagnetic distribution. The theoretical analysis is based on the optical properties obtained by the Mie scattering theory. The absorption coefficients calculations are implemented in a two-temperature heat model for estimation of the nanoparticle temperature. The near field distribution in the vicinity of the particles is calculated by the finite difference time domain (FDTD) method. It is found that at even moderate laser fluences the temperature of the particle can reach a value sufficient for bubble formation in biological tissues. The analysis of the near field distribution shows that when the particle is deposited on a substrate surface, the dielectric properties of the substrate define the spatial distribution and the enhancement of the near field intensity. The observed localization and field enhancement may result in a precise modification of the substrate with a resolution defined only by the nanoparticle size. Such modifications are experimentally observed in different substrates.

Epitaxial growth of Nd:YAG thin films by pulsed laser deposition

Abstract Epitaxial growth of Nd:Y3Al5O12 (Nd:YAG) films on SGGG (Zr- and Sc-doped Gd3Ga5O12) and YAG substrates by pulsed laser deposition using a KrF excimer laser at substrate temperatures of 800°C to 910°C was investigated. The composition of the films was investigated by Rutherford backscattering spectroscopy and the crystallinity by X-ray diffraction analysis. The optical properties of Nd:YAG thin films were investigated by photoluminescence (PL) measurements. The PL spectrum of the Nd:YAG film on the YAG substrate exhibited 4 F 3/2 → 4 I 9/2 and 4 F 3/2 → 4 I 11/2 transitions of Nd3+ ions in YAG which were almost the same as those of the Nd:YAG bulk crystal.

Noble metallic nanostructures: preparation, properties, applications

The process of formation and the characteristics are studied of noble metal nanostructures created by pulsed laser ablation in vacuum. Femtosecond (fs) and nanosecond (ns) laser systems lasing at different wavelengths are used. Several different modifications of the pulsed lased deposition (PLD) technique, as off-axis deposition and glancing angle deposition configurations are used to create nanostructures. Laser annealing of single or bimetal thin films is used to fabricate alloyed nanostructures. The possibility is demonstrated of tuning the optical properties of gold nanostructures on flexible substrates. Different experimental techniques, as fast photography, optical emission spectroscopy, FE-SEM, AFM, TEM, and Raman spectroscopy are applied to characterize the noble metallic nanostructures produced. The optical spectra of the Au and Ag nanostructures are also studied experimentally and theoretically. The theoretical simulation methods used are: molecular dynamic (MD), finite difference time domain (FDTD) and a method based on the generalized multi-particle Mie (GMM) theory. Applications of noble metal nanostructures to surface enhanced Raman spectroscopy (SERS) and biophotonics are briefly considered.

Biomedical material ablation by flexible-hollow-fiber-delivered double pulses

In this study, we demonstrate a high quality beam delivery using a flexible small-core hollow fiber and the ablation processing of hydroxyapatite (HAp) and collagen with fs double pulses through a bending hollow fiber. HAp is a main component of bones and teeth. We adopted two new methods: One is the use of a hollow fiber with a 320-mum core diameter in order to improve the output beam intensity profile by eliminating the high order modes. The other one is an fs double pulse laser transmission scheme with a tailored ps-order delay time, which is earlier than the ablation onset. Since the optical damage threshold of the hollow fiber depends on the peak intensity of the incident laser pulse, therefore, the double pulse transmission enabled us to deliver higher pulse energies than the single pulse even by keeping the fs laser ablation characteristics. By adopting these two new methods of flexible hollow fiber, double pulse transmission and ablation processing, we delivered high energy enough to ablate HAp and collagen. We demonstrated the delivery of a near Gaussian output beam profile, and high ablation rates

Waveguide-based Bragg filters inside bulk glasses integrated by femtosecond laser processing

In this paper, for the first time to our best knowledge, we report on an integration technique of waveguides and Bragg gratings for fabrication of waveguide-based Bragg filters.

Formation and initial evolution of nanoparticles at ultrashort laser ablation of gold: molecular dynamics simulation

The formation of nanoparticles at ultrashort laser ablation of gold in vacuum is investigated theoretically. The analyses of the nanoparticle formation mechanisms and their initial evolution are performed on the basis of molecular dynamics (MD) simulation. The study is carried out for Au target irradiated by laser pulses of 100 fs duration at laser wavelengths of 800 nm. The evolution of the ablation process is monitored for time interval of few hundreds of picoseconds. The size distribution of the nanoparticles and their velocity distribution are obtained as a function of the laser fluence. The results indicate that the nanoparticles are formed in the stage of superheated material decomposition and phase explosion and fragmentation are the main mechanisms leading to their formation. The results for velocity and size distributions are compared to the available experimental ones and good agreement is observed.

Nanofabrication of hexagonally arrayed holes on an aluminum surface using femtosecond laser pulses

Nano-processing using a near-field generated by irradiating a laser to the nano-particle is becoming a new emerging nanotechnology, because it can fabricate nano-size structures of sub-laser-wavelength. In this paper, we would like to report on the nanohole fabrication on an aluminum (Al) surface by femtosecond lasers due to near-field enhanced effects. The particle size used experimentally is 450 nm and 820 nm (PS, n=1.59) in diameter. The used laser fluence ranges from the bellow-ablation-threshold fluence to the above-threshold fluence of Al films. The ablation threshold of Al films is 400 mJ/cm2 approximately. The Al films were deposited on the silicon substrate by conventional vacuum evaporation method. Particles were successfully arrayed in an hexagonal closed-packed structure (hep) so that the near-field generates uniformly on each particle

Self-fabrication of avoid array in fused silica by femtosecond laser

In this paper, we report on the self-fabrication of sub-micrometer-sized void array in fused silica using Ti:sapphire femtosecond lasers. We have investigated the effect of the focusing condition on the shape of the fabricated void. It is found that the shape of the voids was varied with the depth of a focusing point from the sample surface, because the self-focusing process has a significant effect on the generation of the voids

Waveguide Bragg gratings written inside bulk glasses by femtosecond laser

Fiber Bragg gratings are one of the key optical devices for optical communications and optical sensing. Usually fiber Bragg gratings are made holographically or with phase masks. However both techniques have inherent drawbacks as follows: in a holographic technique, the stability of the interference fringe patterns is a critical technical issue while in a phase mask technique, expensive masks are needed. Recently femtosecond lasers have demonstrated to make Bragg gratings inside a commercially available optical fiber. The first demonstration was to fabricate long period fiber Bragg gratings in single mode optical fibers b focused femtosecond lasers irradiation through a 0.46 NA objective lens. Another approach to make Bragg gratings in optical fibers was demonstrated using a phase mask technique. Additionally Bragg gratings have been fabricated inside bulk glasses using femtosecond lasers. However there have been no reports on waveguide Bragg grating fabrication inside bulk glasses using only femtosecond lasers. The fabrication of waveguide Bragg gratings inside bulk glasses using femtosecond lasers is very attractive for integrated photonic device fabrication. In this paper, we will present a novel integration technique of waveguides and Bragg gratings to fabricate waveguide Bragg gratings in glasses using femtosecond lasers. Femtosecond (fs) lasers can induce permanent refractive index changes inside various transparent materials via nonlinear absorptions. If fs lasers are focused on optical waveguides written previously by fs lasers to make waveguide Bragg gratings, it was hard to get enough refractive index difference between the waveguide and the Bragg grating because an amount of fs laser induced refractive index change is limited. Then the diffraction efficiency of the laser propagating through the waveguide by the grating is quite low. Moreover the light confinement in the fs laser written waveguides is weak due to the small refractive index difference ∆n, and consequently the Bragg grating written in the waveguide reflects an only small amount of the propagating laser power in the waveguides. In this paper, we present that one can induce long cylindrical refractive index changes as long as 200 μm that are produced with a 0.65 NA objective lens. Next, we integrate the waveguide and the Bragg grating by observing the emitted light from the laser induced plasma with a CCD camera in order to fabricate the waveguide Bragg gratings. Among the waveguide Bragg filters, waveguides are embedded in a volume type Bragg grating. To fabricate the Bragg gratings, we examined refractive index changes induced by fs laser pulses with a high NA objective lens. We used a Ti:sapphire fs chirped pulses amplification system that operates at 150 fs, a center wavelength of 800 nm and a repetition rate of 1 kHz in our experiment. The laser pulses are focused into borosilicate glass using a 0.65 NA microscope objective lens. If the fs laser pulses are focused using the 0.65 NA objective lens with a proper input energy to induce a refractive index change (around 1μJ in this case), the refractive index change is found as a cylindrical shape that is about 10 μm long along the laser axis. However if the effective NA is reduced down to 0.1, it is possible to increase the length of refractive index change as shown in Fig. 1. These refractive index changes are produced under the conditions that the effective NA is 0.1, puslewidth is 150 fs and irradiation time is 60 s without any translation of the target glass.

Sum frequency generation of CW deep-UV lights using an external cavity with dual wavelength enhancement

We have developed an all solid-state coherent light source, which can generate 252.41 nm light. The developed laser system consists of two-stage external cavities. In this study, we report the output performance of the 2nd stage external cavity in this laser system. The output beam (373 nm) from the 1st stage and a single-mode CW Ti:sapphire ring laser (780 nm) are delivered into the BBO-installed external cavity to resonate both light to the cavity simultaneously, which make the power of each light in the cavity increased, so that we can obtain 252.41 nm light efficiently by sum-frequency mixing with a BBO crystal.