Temperature of matter increases under intense photoirradiation owing to photothermal conversion. The photothermal effect is sometimes a significant issue in optical manipulation usually requiring intense optical fields. Quantitative evaluation of local temperature under photoirradiation can, therefore, provide indispensable information for optical manipulation. In a previous work, we have applied fluorescence correlation spectroscopy (FCS) to monitor the temperature under the optical trapping condition in water, ethanol, and ethylene glycol. We pointed out that analyses of diffusion time of fluorescent dyes could provide information about temperature on the basis of temperature-dependent viscosities of the solvents. In the present work, the FCS thermometry was applied to seven solvents including primary aliphatic alcohols, to examine universal applicability of the method. To verify the experimental results, numerical simulations were performed on the basis of two-dimensional heat conduction at a stationary state. The numerical results on the temperature field satisfactorily reproduced the experimental data, proving that the FCS thermometry is applicable to ordinary solvents. In addition, we also performed numerical simulations on velocity fields in the solvent, to evaluate contribution of natural convection under typical optical trapping condition at light intensity of ∼MW cm − 2. It was revealed that the contribution of the natural convection is not negligible for mass transfer in the solvents.
Fluorescence correlation spectroscopy was applied to the evaluation of the local heating at the focal spot of nearinfrared laser for optical trapping. Based on the translational diffusion coefficient of probe dyes at the focal spot in solution, the relation between temperature rise and incident laser power, ΔT/ΔP, were determined for water, ethylene glycol, 1-pentanol, 1-hexanol, 1-octanol, 1-nonanol, and 1-decanol. The value of ΔT/ΔP linearly increased with a/l (a and l is the absorption coefficient and thermal conductivity of solvent, respectively) as predicted by a simple theoretical model.
We fabricated semiconductor cadmium selenide (CdSe) quantum dots via the pulsed laser ablation in the superfluid helium. The fabricated quantum dots showed blue-shifted fluorescence due to the strong quantum confinement effect. The fluorescence blinking phenomena were also observed indicating the single photon emission process. Our proposed scheme is a simple, robust, and reliable method to fabricate quantum dots and to introduce the highly fluorescence nanoparticles into superfluid helium appropriate for resonant optical manipulation and nano-tracers for liquid helium visualization.
Gold nanoparticles (Au NPs) exhibit strong light absorption due to localized surface plasmon resonance (LSPR), and efficiently convert light energy into heat under illumination. Heat transfer from Au NPs to surrounding matrices induces an increase in temperature, resulting in nanobubbles generation owing to explosive evaporation of the medium. In particular, stationary bubbles can be produced by illuminating CW laser for single Au NPs. These stationary bubbles in microscopic region drive fluid convection of medium and suggest the potential application to the manipulation of colloidal particles and molecules. In the present work, we have investigated the thermo-physical properties of the stationary bubbles and fluid convection of surrounding water by integrating experimental results with those by the theoretical calculation.
Remote acceleration of a molecular recognition will open an avenue for the control of various biological functions.
Here, we have developed a new principle for the rapid macroscopic assembly based on the light-induced molecular
recognition via nanoparticles. Remarkably, as an application of this principle, we have demonstrated the submillimetre
network formation triggered by light-induced hybridization of zmol-level DNA within a few minutes. This finding will be
used for the rapid and highly sensitive genetic screening without fluorescent labeling.
Fluorescence correlation spectroscopy (FCS) was applied to investigate molecular translational diffusion in the solution of water, ethylene glycol, and heavy water under gradient light field of a near infrared (NIR) laser beam. The diffusion times of Rhodamine-6G in ethylene glycol and Rhodamine-123 in water became faster with an increase in the NIR laser power owing to absorption of the NIR light by the solvents. We also applied the radiation pressure of the NIR laser light to cadmium telluride (CdTe) nanoparticles dispersed in heavy water, resulting in increase in the average number of the CdTe particles in the confocal volume with increasing the NIR laser power.
Laser manipulation technique was applied to the patterning of single nano/microparticles in solution at room temperature. Individual gold nanoparticles were optically manipulated to the surface of a glass substrate in ethylene glycol. An ultraviolet laser beam was focused to the nanoparticle, which led to the transient temperature elevation of the particle, resulting in its photothermal fixation. A set of gold nanoparticles was aligned in the anisotropic optical potential well of a tightly focused laser beam with linear polarization and was adhered onto the substrate through the same photothermal method keeping their alignment. Combination of a microstereolithography with the laser trapping method enabled us to fabricate three-dimensional microstructures of resin and fix microparticles to them.
Laser manipulation technique was applied to patterning of single nanoparticles onto a substrate one by one in solution at room temperature. Individual polymer nanoparticles were optically manipulated to the surface of glass substrate in ethylene glycol solution of acrylamide, N,N'-methylenebis(acrylamide), and commercial radical photoinitiator. An ultra violet (UV) laser beam was focused to the nanoparticle, which led to generation of sub-μm sized acrylamide gel around the particle. The polymer nanoparticles were incorporated into the polymerized gel and fixed onto the substrate. A single gold nanoparticle was optically trapped and moved to the surface of the glass substrate in ethylene glycol. Additional irradiation of the UV laser light induced transient melting of the particle, resulting in its adhesion to the substrate. By the use of the present methods, arrangement of individual polymer and gold nanoparticles on any pattern was achieved.