Power dependence of laser trapping crystallization

CW 1064 and 800 nm lasers were used. It should be necessary to check the efficiency of laser trapping crystallization from low to high power range. We define the crystallization probability as follow; experimental observation time is restricted to 30 min. If crystallization is observed within 30 min, then it is counted as successful crystallization. If crystallization is not observed within 30 min, it is counted as failed case.

Heat generation by focused trapping laser cannot be ignored since long time laser irradiation will make the solvent evaporated. Although we tried to reduce the evaporation of

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solvent by using a closed chamber as a container, solution height still slowly decreased. In this condition, the edge of the solution will possibly be dried and generate the crystal. A 1~2 hour laser irradiation time is not a suitable for spatiotemporally controlled laser trapping crystallization experiment. To avoid such complication, we decide a maximum laser irradiation time to be 30 min.

We examined 1064 nm CW laser trapping crystallization with 200, 400, 600 and 800 mW trapping laser power and focusing to the air/solution interface of saturated solution (S.S. = 1.0) for 30 min. However we can observe crystal generation only with laser power above 600 mW as shown in Fig. 3.1a. CW laser trapping crystallization of 800 nm CW laser with 0.9 SS solution showed similar crystallization power dependency. We need more than 600 Fig. 3.1 Power dependence of laser trapping crystallization with a focused (a) CW 1064 nm and (b) CW 800 nm laser. Laser was focused to the air/solution interface of the glycine solution. Power of 200, 400, 600, and 800 mW CW laser was examined for 1064 nm with 1.0 SS solution; Power of 400 and 600 mW CW laser was examined for 800 nm with 0.9 SS solution. The examined crystallization time is 30 minutes.

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mW under examined condition as we can see in Fig. 3.1b. As we mentioned in previous section, crystallization can be initiated from high concentration region such as dense liquid droplet. Those results imply that the formation of local high concentration region generation by photon pressure is necessary for laser trapping crystallization, and 200 - 400 mW was not sufficient to induce nucleation of the crystal. Stronger power of laser will induce molecular assembly more efficiently [21]. It implies the local high concentration of solute induced by focused laser is depending on laser power. The growth rate of amino acid aggregates decreased with decreasing the laser power. We also can suggest that 30 min focusing of low power trapping laser is not enough strong to make surrounding of focal spot higher than the critical concentration for nucleation.

Fig. 3.2 Images of glycine crystal generation and growth by CW laser trapping.

Saturation degree is 0.9 SS. Laser power is 600 mW.

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Figure 3.2 shows snapshots of glycine crystal generation and growth at the air/solution interface. After few minutes of CW laser irradiation, the seed crystal was generated at the focal spot and trapped by the focused laser. The resulting crystal from low power laser showed the same crystallization process with the crystal from high power laser.

In spite of lower absorption coefficients of H₂O and D₂O at 1064 nm, heat effect by laser irradiation cannot be neglected when the intense laser beam is focused. The temperature

elevation by laser irradiation has estimated by applying fluorescence correlation spectroscopy recently. Temperature elevation which is defined as T/P (K/W in unit) at the

focal spot is about 22-24 K/W in H₂O and 2 K/W in D₂O, respectively, with high NA objective lens (N.A. = 1.35) [64]. It suggests that if we use H₂O as a solvent, drastic temperature elevation increases the solubility of glycine and makes the crystallization more difficult. Therefore we have been using D₂O as a solvent of laser trapping crystallization in order to decrease the thermal effect [51]. We followed these information and used D₂O as a solvent of glycine solution to suppress temperature elevation during laser trapping.

When we use liquid thin film of glycine solution which was formed by dropping a small amount of solution onto the cover glass (solution layer thickness: <100 mm), it showed local surface deformation and height change during laser trapping crystallization process as shown in Fig. 3.3. Height was first lowered and liquid film become thin. After kept very thin condition of this liquid film for a while, surface height was started elevating and eventually

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crystal formation was observed. The solution height elevation is attributed to the dense liquid droplet formation as reported [55]. In contract to thin solution layer formed on cover glass, surface height/shape of the sample solution in cut glass vial showed no local surface deformation and negligibly slowly height decreasing. Figure 3.4 shows the solution height change during trapping laser focusing to the solution surface. A sample which was not irradiated showed typically less than a few m decrease. Degree of height decreasing showed negligible power dependency in the range of 200~600 mW. All sample showed about 20 m decrease for 30 min irradiation of trapping laser. Laser-induced surface deformation is almost negligible, and crystallization process may not be strongly affected.

Fig. 3.3 Solution height change during 1064 nm CW laser irradiation in cover glass case, laser was focused to the air/solution interface of the thin glycine solution layer formed on the glass substrate. Above curve was obtained with 1.32 SS solution. Laser power is 1.0 W.

Asterisk indicates the timing when crystallization occurred.

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