The speed of optical transportation of microparticles was modified by changing dissipative force acting on the particles through the switching of the absorption spectrum of molecules contained in the particles. Linear and orbital motions of microparticles in water were achieved by using slightly focused laser beams. Polymer microparticles containing photo-chromic molecules under the photoirradiation configuration were transported by the optical force due to scattering. The transportation speed was increased by changing the color of the particles by the photoinduced isomerization from transparent (non-resonant) to resonant with the manipulation beams.
Microscopic orbital motion of small particles in water was achieved by using the optical dissipative force of a pair of non-coaxially counterpropagating laser beams in conjunction with the thermal fluctuation (Brownian motion) at room temperature.
In the present study, we have precisely tracked the three-dimensional (3D) motion of single nanoparticles under optical
trapping by focused femtosecond laser pulses for detecting radiation force due to simultaneous multiphoton absorption. A
300-nm polymer particle containing dye molecules was optically trapped with pulsed laser (800 nm, 200 fs at the focal
point). The 3D motion of the trapped polymer nanoparticle was observed with astigmatism fluorescence imaging method.
The trapping point shifted on the optical (Z-) axis towards the propagation direction of the femtosecond laser with
increasing laser power. The Z-displacement was dependent on incident laser power and, from which we concluded that
the positional shift along the Z-axis can be ascribed to the radiation force due to simultaneous two-photon absorption of
the laser pulses by the particle.
In the present study, we propose a combined (hybrid) approach of optical manipulation and photochemical reactions for achieving nano-photomechanical motions with small particles. Aa photochemical reactions, we have employed P-type and T-type photochromic reactions of diarylethene (DAE) and pyranoquinazoline (PQ) derivatives, respectively. Single polymer particle containing each of the photochromic compounds was optically trapped in water with a CW visible laser. At this stage, the particle experienced mainly gradient force and was trapped at the focal point of the CW laser. The absorption force was negligible because most of the photochromic molecules in the particle were in the colorless form. UV exposure induced the photoisomerization of the photochromic molecules, resulting in the increase in the number of colored forms. As a result, the absorption force acting on the particle increased and, the position of the particle shifted towards the light propagation direction depending on experimental condition. After turning the UV light off, the particle went back to the original position. The trapped particle thus underwent reciprocal motion synchronizing with the change of photo-response due to the photochromic reactions.
In the present study, we have detected multiphoton absorption force acting on microparticles due to simultaneous twophoton absorption. A polymer microparticle including dye molecules was optically trapped with femtosecond (800 nm, 100 fs) pulse pairs with time-interval Δt in water. Under the condition that Δt > the pulse duration, the particle was trapped with absorbing small number of photons via multiphoton process. The number of photons absorbed via simultaneous two-photon absorption increased by decreasing Δt to overlap the pulse pair, leading to the micromechanical motion of the particle. By tracking the position of the trapped particle, we confirmed that the particles were pushed in the direction of light propagation and the displacement increased with decreasing Δt.
We fabricated cadmium selenide nanoparticles using a pulsed laser-ablation scheme in superfluid helium. The fabricated nanoparticles showed photoluminescence blinking and a clear blueshifted emission whose linewidth was nearly equal to the homogeneous linewidth. These results demonstrate that our method can prepare quantum dots with a sharp resonance directly in superfluid helium, enabling the further implementation of resonant optical manipulation and optical trapping in cryogenic systems.
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.
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.
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.
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.
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