Focused irradiation of femtosecond laser pulses can induce nucleation of various proteins at low supersaturation, where high quality protein crystals can be obtained. Recently we studied the nucleation mechanism by fast-imaging methods and found that the rapid shrinkage (microsecond-scale) of a laser-induced cavitation bubble creates a local high concentration of protein. Actually, the surface of cavitation bubbles is known to be a preferred region for protein adsorption. Hence, protein molecules adsorbed at the surface during the expansion will be then gathered to the center region by the rapid shrinking of cavitation bubbles. These results indicate the femtosecond laser technique is based on a fundamental crystallization parameter, concentration, and can work as a nucleation trigger of various materials. In this paper, we introduce our recent studies of the nucleation mechanism and the development of the technique for protein nucleation at lower supersaturation.

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There describe research results that control of crystal growth is achieved by applying photon pressure of a focused CW laser beam close to glycine crystals generated spontaneously in solution, and that the Ostwald ripening are successfully demonstrated. Note that photon pressure generated by the focused laser can make it possible to cause a growth of the crystal located far from the focal point with about 1μm. We confirm that a large dense area generated around the focal spot plays an important role for the crystal growth. Two novel phenomena of the formation of mm-sized dense liquid droplet and crystallization in unsaturated solution related to the formation of the large dense area are also presented. These provide a principal insight into the elucidation of mechanism and process of this long-distance crystal growth.

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Photochemical reaction of protein triggers crystallization. Residual Trp or Tyr radical intermediate proteinsare produced and react with other proteins to form dimers. Some of the dimers grow to crystal. The dimers whose configurations are similar to adjacent molecules in crystal are expected to grow. We call these dimers as a template molecule. In the case of photochemical reaction of lysozyme, seven kinds of dimers combined at Trp-Trp or Tyr-Tyr residuals are produced. Template molecule is a dimer combined at Tyr^<53>-Tyr^<53> residuals.

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Anisotropic Ag nanoparticles grow epitaxially on TiO_2 single crystals by photoelectrochemical reduction of Ag^+. Under different conditions and on different substrates, biaxially and uniaxially orientated crystalline nanorods or pyramids, uniaxially oriented triangular nanoplates, or trapezoidal pyramids are grown preferentially, without need for a masking layer. The oriented anisotropic Ag nanoparticles would be applied to plasmon-based photofunctional materials and devices that are responsive to polarized visible and/or near infrared light, including near infrared photochromic materials.

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Formation of nanocrystals from spherical nanoparticles induced by photoirradiation is introduced. The shape conversionhas been observed in various experimental conditions such as weak photoirradiation using fluorescence light and strong photoirradiation using laser light. Although the mechanisms of these processes are different from each other, some characteristic properties of silver nanoparticles, such as the efficient absorption of visible light and the decrease in the stability with the decrease in the size, play important roles in both processes.

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We described efficient formation of water aerosol/droplet upon irradiating wet ambient air by a UV lamp (Hg lamp, 185nm) or UV laser (ArF laser, 193nm) in the laboratory. Particle formation was observed by light scattering and mass spectroscopy. With an observation of reaction-intermediates, namely ozone and HO_2 radical, and computer simulations of the gas phase reactions, we proposed the mechanism of photochemical hydrogen-peroxide formation, which serves as the nucleus for water condensation. It is shown that in the present systems a small supersaturation can make particle growth by thermodynamic considerations of the system.

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