Yasutaka Hanada

Characterization and mechanism of glass microwelding by double-pulse ultrafast laser irradiation

We investigated the physical mechanism of high-efficiency glass microwelding by double-pulse ultrafast laser irradiation by measuring the dependences of the size of the heat-affected zone and the bonding strength on the delay time between the two pulses for delay time up to 80 ns. The size of the heat-affected zone increases rapidly when the delay time is increased from 0 to 12.5 ps. It then decreases dramatically when the delay time is further increased to 30 ps. It has a small peak around 100 ps. For delay time up to 40 ns, the size of the heat-affected zone exceeds that for a delay time of 0 ps, whereas for delay time over 60 ps, it becomes smaller than that for a delay time of 0 ps. The bonding strength exhibits the same tendency. The underlying physical mechanism is discussed in terms of initial electron excitation by the first pulse and subsequent excitation by the second pulse: specifically, the first pulse induces multiphoton ionization or tunneling ionization, while the second pulse induces electron heating or avalanche ionization or the second pulse is absorbed by the localized state. Transient absorption of glass induced by the ultrafast laser pulse was analyzed by an ultrafast pump-probe technique. We found that the optimum pulse energy ratio is unity. These results provide new insights into high-efficiency ultrafast laser microwelding of glass and suggest new possibilities for further development of other ultrafast laser processing techniques.

UV waveguides light fabricated in fluoropolymer CYTOP by femtosecond laser direct writing

We have fabricated optical waveguides inside the UV-transparent polymer, CYTOP, by femtosecond laser direct writing for propagating UV light in biochip applications. Femtosecond laser irradiation is estimated to increase the refractive index of CYTOP by 1.7 x 10(-3) due to partial bond breaking in CYTOP. The waveguide in CYTOP has propagation losses of 0.49, 0.77, and 0.91 dB/cm at wavelengths of 632.8, 355, and 266 nm, respectively.

Colour marking of transparent materials by laser-induced plasma-assisted ablation (LIPAA)

We demonstrate colour marking of a transparent material using laser-induced plasma-assisted ablation (LIPAA) system. After the LIPAA process, metal thin film is deposited on the surface of the ablated groove. This feature is applied to RGB (red, green and blue) colour marking by using specific metal targets. The metal targets, for instance, are Pb3O4 for red, Cr2O3 for green and [Cu(C32H15ClN8)] for blue colour marking. Additionally, adhesion of the metal thin film deposited on the processed groove by various experimental conditions is investigated.

Laser-induced plasma-assisted ablation (LIPAA): fundamental and industrial applications

The laser-induced plasma-assisted ablation (LIPAA) process developed by our group, in which a single conventional pulsed laser is only used, makes it possible to perform high-quality and high-speed glass microfabrication. Up to the present, this process has been widely applied for micromachining of various transparent hard and soft materials. In this process, the laser beam first passes through the glass substrate since the laser beam has no absorption by the substrate. Then, the transmitted beam is absorbed by a solid target (typically a metal), located behind the substrate so that the target is ablated, resulting in plasma generation. Due to the interaction of the laser beam and the laser-induced plasma, significant ablation takes place at the rear surface of the substrate. Recently, we have developed the proto-type LIPAA system using a second harmonic of diode pumped Q-switched Nd:YAG laser for the practical use. In this paper, we demonstrate micromachining, crack-free marking and color marking of glass materials. Additionally, selective metallization of glass and polyimide by the LIPAA process followed by metal chemical-plating is investigated. A possible mechanism of LIPAA is also discussed based on the results from double pulse irradiation using near-IR fs laser, transient absorption measurement and plasma-conductivity measurement.

3D MICROSTRUCTURING OF GLASS BY FEMTOSECOND LASER DIRECT WRITING AND APPLICATION TO BIOPHOTONIC MICROCHIPS

Three-dimensional (3D) microfabrication of photostruc- turable glass by femtosecond (fs) laser direct writing is demonstrated for manufacture of biophotonic microchips. The fs laser direct writ- ing followed by annealing and successive wet etching can fabricate the hollow microstructures, achieving a variety of microfluidic components and microoptical components in a glass chip. One of the interesting and important applications of the 3D microfluidic structures fabricated by the present technique is inspection of living microorganisms. The microchips used for this application are referred to as nanoaquarium. Furthermore, the optical waveguide is written inside the glass by the fs laser direct writing without the annealing and the successive etching. It is revealed that integration of the microfluidic and microoptical com- ponents with the optical waveguides in a single glass chip is of great use for biochemical analysis and medical inspection based on optical sensing.

Transient electron excitation in laser-induced plasma-assisted ablation of transparent materials

We investigate the mechanism of laser-induced plasma-assisted ablation (LIPAA), by which high-quality and high-efficiency ablation of transparent materials, such as glass, can be performed with a single conventional pulsed laser. The laser-induced plasma induces transient absorption of the laser beam (532nm) by the glass substrate. The origin of the transient absorption is electron excitation by ions with kinetic energy more than approximately 10eV in the plasma, which is observed by measuring transient polarization change in the glass substrate applied with a high external pulsed electric field during the plasma-assisted electron excitation (plasma-conductivity measurement). A possible mechanism of LIPAA is proposed based on the results obtained.

Micromachining of transparent materials by laser-induced plasma-assisted ablation (LIPAA)

Laser-induced plasma-assisted ablation (LIPAA) process developed for glass materials has been applied for micromachining of a variety of transparent hard and soft materials. We have developed the proto-type LIPAA system using a second harmonic of diode pumped Q-switched Nd:YAG laser for the practical use. In this paper, micromachining and scribing of glass and sapphire is demonstrated using the developed system. Additionally, another application such as selective metallization of glass and polyimide with successive metal plating process is investigated. However, mechanism of this process is complex and still remains unknown. To have a better understanding of this process, double-pulse irradiation of a near-IR femtosecond (fs) laser with a delay time is also investigated. A possible mechanism is discussed based on the obtained results.

Micro pump fabrication by femtosecond laser direct writing for microorganism analysis in nano-aquarium

We demonstrate the fs laser fabrication of microchip with microchannel integrated with micro pump for the dynamic observation of microorganisms in water. Such microchips, referred to as nano-aquariums realize the efficient observation of microorganisms.

Nano-aquariums from ultrashort laser pulses

A large variety of organisms are presently living on the earth. Among these, microscopic-sized aquatic microorganisms have been surviving for more than 500 million years. Some of these microorganisms exhibit extremely rapid motion, which is unusual in the macro world in which we live, and can show unique 3D movements that appear to contradict gravity. Most are composed of a unit cell. Understanding the abilities and functions of these unit cells may offer insight into the cells that comprise more complex organisms, including human beings. The observation of microorganisms is currently a challenging subject for cell biologists. In the conventional microscopic observation system, a glass slide with a cover glass is generally used. However, the high numerical aperture of the objective lens limits both the field of the image and the depth of focus, thereby making it difficult to capture images of moving microorganisms. Indeed, it can take a very long time until a clear image is obtained. Amethod to reduce the amount of tracking required or the reliance on chance movements of the organisms is needed. To solve these problems, we applied our technique for fabricating 3D hollow microstructures inside photostructurable glass using femtosecond lasers to the manufacture of a special microchip, referred to as a nano-aquarium.1 The nano-aquarium can scale down the observation site by encapsulating the microorganism in a limited area while still providing enough space for motion. This makes it much easier to capture images of moving organisms in freshwater with a standard objective lens (see Figure 1). The nano-aquarium structures also have the advantage of keeping livingmicroorganisms fresh for a long time since such a structure prevents the evaporation of water as seen under the glass slide/cover glass system. To fabricate the nano-aquarium, we developed a technique that can be used to directly form 3D hollow microstructures with smooth internal surfaces inside photostructurable (Foturan) glass via femtosecond laser direct writing followed by anFigure 1. Illustration of microscopic observation of microorganisms using a nano-aquarium.

LIPAA technique and its possible impact on microelectronics (Invited Paper)

The laser-induced plasma-assisted ablation (LIPAA) process developed by our group, in which a single conventional pulsed laser is only used, makes it possible to perform high-quality and high-speed glass microfabrication. Up to the present, this process has been widely applied for micromachining of various transparent hard and soft materials. In this process, a laser beam is first directed to a glass substrate placed in vacuum or air. The laser beam passes through the substrate since the wavelength of the laser beam must have no absorption by the substrate for the LIPAA process. The transmitted laser beam is absorbed by a solid target (typically a metal), located behind the substrate. The target is then ablated, resulting in plasma generation. Due to the interaction of the laser beam and the laser-induced plasma, significant ablation takes place at the rear surface of the substrate. Recently, we have developed the proto-type LIPAA system using a second harmonic of diode pumped Q-switched Nd:YAG laser for the practical use. In this paper, we will demonstrate micromachining, crack-free marking, color marking and dicing of glass materials. Additionally, selective metallization of glass and polyimide by the LIPAA process followed by metal chemical-plating is investigated. The discussion includes mechanism and practical applications in micro-electronics industry of the LIPAA process.