This review deals with mechanistic aspects of pulsed laser-induced fabrication of noble metal nanoparticles. This fabrication method is developed over the past two decades and has attracted much attention of researchers in various fields including laser chemistry, materials chemistry and nanobiotechnology. Highlights of this review are advantages and disadvantages inherent to the fabrication method involving complex processes such as vapor bubble generation under high-intensity laser irradiation of microscopic metal surfaces, leading to nanoparticle formation.

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Fabricating functional submicrometer particles is difficult by conventional particle fabrication techniques that use top-down or bottom-up approaches. Pulsed-laser melting in liquid is a versatile technique for crystalline submicrometer spherical particles of various materials. Space-selective pulsed heating of particles that are suspended in liquid by laser irradiation plays an important role in raising the particle temperature over the melting point and quenching melt droplets for spherical particle formation. The balance between high heat capacity at the larger particle side and low optical absorption at the smaller particle side leads to the selective formation of submicrometer spherical particles. We demonstrate some critical application examples of submicrometer spherical particles with both size-derived and bulk functions by reducing the size to the submicrometer scale. Key Words: Submicrometer, Particles, Melting, Liquid, Spheres

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It has been shown that cavitation bubbles play essential roles in syntheses of nanoparticles by laser ablation in liquids. The growth of nanoparticles inside the cavitation bubble was observed by a laser light scattering experiment. The structure of synthesized nanoparticles was affected by the dynamics of the cavitation bubble, indicating that nanoparticles were exposed in the high-pressure, high-temperature reaction field of the collapsed cavitation bubble. In addition, an experiment carried out rather recently suggests that the cavitation bubble stores significant amount of electrical charges.

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We review a one-step method to fabricate microstructure on a glass plate using laser-induced backside wet etching (LIBWE). The LIBWE method is based on liquid laser ablation with a conventional pulsed laser at UV or visible wavelength. We fabricated a well-defined deep microarray without cracks or chipping formations on the glass plate by the LIBWE method. The system allows us to use the rapid prototyping of the high precision surface microfabrication of glass and other transparent materials as laser direct-write processing in a conventional atmospheric environment.

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We briefly review femtosecond laser induced chemical reactions and nanomaterial productions in a liquid phase. Focused intense femtosecond laser pulses create a high density of reactive species such as solvated electrons and cation radicals of the solvent. The solvated electrons efficiently reduce the metal ions followed by their nanoparticle formations. Multimetallic nanoparticles, which are expected to be utilized as electromagnetic materials and catalysts, are produced. In the cases of neat organic solvents, polyynes and carbon nanoparticles are formed. We also presented the production of heteroatom-doped carbon nanoparticles, which are candidates of oxygen reduction catalysts in fuel cells. By using aqueous organic solutions and/or water/organic bilayer solutions, hydrophilic carbon nanoparticles are formed by the successive oxidation of organic molecules by hydroxyl radicals. Nanoparticle synthesis by reactive species in the liquid phase might be an alternative means of widely used laser ablation in liquid.

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We present laser trapping-controlled crystal growth of protein, hen egg-white lysozyme (HEWL). A continuous-wave near-infrared laser beam of 1064 nm is focused at a point 10 μm away from the generated HEWL crystal. Laser trapping of HEWL liquid-like clusters in solution firstly forms a millimeter-sized highly concentrated domain, inside which the crystal growth of the specific target crystal is realized and controlled. At the beginning of the irradiation, the crystal growth rate is extremely suppressed compared to that in spontaneous crystal growth, while it is enhanced by the continuous irradiation. The suppress and the enhancement of crystal growth rate can be controlled by changing laser power and polarization direction. The dynamics and mechanism of this unique crystal growth is discussed from the viewpoint of large, stable domain formation consisting of the HEWL liquid-like clusters and by considering the rigidity and anisotropy of the molecules/clusters in the domain.

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When an intense infrared femtosecond laser is focused inside an aqueous solution through an objective lens, shock and stress waves are generated at the laser focal point after effective multi-photon and excited-state absorptions. These waves propagate from the laser focal point and act in its vicinity with biological cells as an impulsive force, which we applied to manipulate animal and plant cells. In this review, we explained a photo-excitation mechanism at the laser focal point and indicated an advantage of the femtosecond laser-induced impulsive force in comparison with other pulse lasers. We observed the behaviors of animal and plant cells with impulsive force action by high-speed imaging and interpreted their differences from the mechanism to protect the cell from external force.

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