The crystallization of glycine in unsaturated solution is made possible by laser trapping of its molecular clusters due to photon pressure of a focused continuous wave near-infrared laser beam. Always one single crystal is spatiotemporally formed at a focal spot, and then it undergoes dissolution, eventually leading to repetitive crystallization and dissolution. The polymorph characterization of the crystal formed in unsaturated solution confirmed the γ-form, which is not obtainable by conventional crystallization methods. The preparation probability of the γ-form compared to the α-form is much higher than that in the supersaturated solution.

The polymorph control of glycine in D 2 O solution is successfully demonstrated by a laser trapping technique using a linearly or circularly polarized CW near-infrared laser beam. Focusing each laser beam into an air/solution interface of the solution always generates the stable crystal polymorph of either αor γ-form at the focal spot. The formation probability of each polymorph strongly depends on various experimental conditions of laser polarization, power, and solution concentration. For the supersaturated and saturated solutions, circularly polarized laser irradiation enhances γ-crystal formation, while for the unsaturated solution the laser polarization effect becomes prominent and linearly polarized laser light at a specific power provides the maximum γ-crystal probability of 90%. The present polymorphism is achieved by laser-induced effects such as concentration increase, temperature elevation, and molecular rearrangement, whose mechanism is discussed in view of laser polarization dependence of these effects.

Crystal polymorph of glycine is controlled by tuning the power of a linearly polarized continuous wave 1064-nm laser beam. Upon focusing the beam to the air/solution interface of a supersaturated glycine/D 2 O solution, its single crystal is spatiotemporally formed at a focal spot within a few seconds to a few tens of seconds. Fourier transform infrared measurement and single-crystal X-ray crystallographic analysis of the fabricated single crystal reveal that two polymorphs of Rand γ-forms are prepared depending on the laser power. The probability of γ-form preparation, which is not available under ambient conditions, arises up to 50% at 1.3 W laser power after an objective lens. The mechanism of the polymorph control is discussed in view of both photon pressure and local temperature elevation due to laser irradiation at the focal spot.

The formation, dissolution, and transfer of a millimeter-scale dense liquid droplet are demonstrated by focusing a CW near-infrared laser beam into a thin film of glycine solution in heavy water. The entire process is investigated by directly monitoring the temporal change in the two-dimensional surface profile using a laser displacement meter. Upon laser irradiation, the surface depression is initially induced by laser heating, followed by the formation of the shallow convex-shaped droplet around the focal spot, in which the droplet is always in contact with the surrounding solution through the ultrathin solution layer. After the laser is switched off, the dissolution occurs through the recovery from the ultrathin layer toward the original solution film. When the laser is set to the outside of the droplet, the solution depression is similarly induced, and subsequently the droplet starts moving toward the focal spot. These processes are summarized and discussed in view of laser-induced effects of concentration increase and temperature elevation.

Laser trapping of molecules and proteins in solution at room temperature is made possible by irradiating 1064-nm continuous-wave (CW) laser with power around 1 W. Although conventional small molecules are not trapped at the focal point, molecules that can form clusters upon assembling and proteins whose size is close to 10 nm are gathered, giving unique assembly structure. Glycine in H2O shows crystallization, urea in D2O gives a millimeter-sized giant droplet, and cobalt oxide-filled ferritin protein confirms assembly followed by precipitation. Solute concentration, solvent, and laser power are important factors for determining trapping and assembling phenomena, and the laser focal position is very critical. These unique behaviors are realized by setting the irradiation at the air/solution surface, inside the solution, and at the glass/solution interface. Laser trapping-induced crystallization, liquid/liquid phase separation, and precipitation are compared with the previous results and considered. After summarizing the results, we describe our future perspective and plans.

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A single dense liquid droplet of urea is formed by irradiating a focused continuous wave near-infrared laser beam to a glass/solution interface of a thin film of the unsaturated D 2 O solution though its dynamic deformation. Conversely, in the supersaturated solution, neither droplet formation nor large solution deformation is observed. This can be explained on the basis of its high viscosity. In addition, crystal growth and dissolution are demonstrated by focusing the laser beam close to the crystal generated in the solution. All results are here discussed in view of local temperature elevation, mass transfer due to convection, and laser trapping of the clusters due to photon pressure, by comparing with experimental results for glycine.

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