We study unique optical trapping phenomenon at glass/solution interface for gold nanoparticles with the diameter of 200 ~ 400 nm. With prolonged irradiation, gathering of many NPs forms a dynamically moving and fluctuating assembly like a flying group of birds in sky. We call this phenomenon as optical trapping and swarming, which extends to a few ten µm, much larger than the focal area of the 1064 nm trapping laser. We have elucidated the swarming dynamics and mechanism in view of optical binding and its expanded network, while here we design new experiments of the swarming toward a plasmonic machine, depending on laser polarization. The morphology and size of the Au NP swarming is intrinsically determined by optical, physical, and chemical parameters of NP, which is demonstrated by utilizing silica-coated gold nanoparticles. Further the swarming can be controlled by performing trapping and swarming on patterned glasses; one is on gold nanodisk pattern fabricated lithographically and the other is on polycaprolactone microchannel prepared by electrospinning writing method. The dumbbell-shaped morphology is switched from bidirectional to unidirectional, and its shape is modified. The dynamically fluctuating Au NPs can induce hydrodynamic flow in solution and give mechanical pressure to the surrounding. Also, the swarming NPs are heated by photo-absorption of the 1064 nm laser. The present findings indicates that the swarming gold nanoparticles can work as a plasmonic machine, and its systematic study will enable various designs of dynamic matter in the few ten micrometer domain.
In optical trapping of gold (Au) nanoparticles (NPs) at a glass/solution interface, initially NPs align in the manner of optical binding not only inside but also outside of the focused trapping site. Further gathering and assembling of more NPs lead to the formation of their dynamically moving and fluctuating assembly like a flying group of bees in air. Since we found this phenomenon, we have been systematically studying its dynamics and mechanism. It shows clear dependence on trapping laser polarization. The assembling and swarming expand linearly with the direction perpendicular to linear laser polarization, giving a dumbbell shape morphology, while a disk like one is prepared for circularly polarized laser. The morphology and the size of such assemblies can be controlled by tuning optical, physical, and chemical parameters based on the intrinsic surface plasmon resonance properties of Au NPs. We demonstrate new appearance of two swarms with ellipse and ring distribution of NPs by shifting the axial position of the trapping laser focus with respect to the interface. The contours of the dumbbell shape can be controlled by the incident and focusing angles of the trapping laser. Upon using the Au NPs with different size, shape, and structures, surprisingly unique dynamic behavior is observed. Further we introduce nanolithographically fabricated gold nanopattern as a glass interface, showing a new control way of the swarming morphology. These results show high potential of “optically evolved assembling and swarming” in the studies on nanophotonic machine.
In the present work, we study optical trapping of two kinds of trapping targets together, 20 μm and 1 μm PS MPs, at the solution surface. The resulting assembly has a unique structure, that 1 μm PS MPs form a belt surrounding to a single 20 μm PS MP as the body, and 3-dimensionally grows more than 50 μm in diameter even though the trapping laser focus size is only 1 μm. We further demonstrate that the optically prepared light scattering assembly can serve as a unique reconfigurable random lasing medium. As the assembly is prepared exclusively where and when trapping laser is irradiated, our study will offer a new tool for studying optically reconfigurable and tunable disordered photonics.
Polystyrene and silica nanoparticles are gathered and form a single disk-like assembly at glass/solution interface upon optical trapping with a focused laser and its size evolves much larger than the focal volume. In addition, linearly aligned aggregates of nanoparticles are prepared at specific edge sites of the assembly, which looks like horns. Transmission spectral and diffraction pattern measurements were carried out, confirming a correlation between the central arrangement of the nanoparticles and the horn formation. This dynamics and mechanism characteristic of optical trapping at interface is discussed from the viewpoint of optical propagation of the trapping laser. The assembly formation started at the focus where trapping laser light is scattered, and the trapping laser propagated trough the prepared assembly expanding its size.
We elucidated femtosecond laser trapping dynamics of Rayleigh particles by modifying the surface property of nanoparticles and changing the viscosity. Through bright field, dark field and backscattering images, we monitored ejection behavior of nanoparticles which takes place in the direction perpendicular to laser polarization and switched from one side to the other. For very hydrophobic nanoparticle, we succeeded in observing directly a transient assembly and confirmed that its formation is responsible to the directional ejection. Also the trapping dynamics was compared for femtosecond and continuous-wave lasers in highly viscous solvent, which gives a new viewpoint for understanding femtosecond laser trapping mechanism.
We conduct the optical trapping and assembling of polystyrene particles at the glass/solution interface by utilizing tightly focused 1064 nm laser of high power. Previously we reported that this leads to form the assembly sticking out horns consisting of single row of aligned particles through light propagation. Here, we demonstrate the laser power dependence of this phenomenon. With increasing the laser power, the particles are started to distribute around the focal spot and form the assembly larger than focal spot. The shape of the assembly becomes ellipse-like and the color at the central part of the assembly in transmission images is changed. This indicates that the assembly structure is changed, and trapping laser is started to propagate through the adjoining particles leading to horn formation. Strong laser power is necessary to elongate the horns and to align them straightly. We expect that this study will offer a novel experimental approach for assembling and crystallization of nanoparticles and molecules exclusively by optical trapping.
We present the laser power dependent behavior of optical trapping assembling of 208-nm polystyrene (PS) nanoparticles at the solution surface layer. The assembling dynamics is examined by reflection microspectroscopy as well as transmission and backscattering imaging. The transmission imaging shows that the laser irradiation at the solution surface layer forms a nanoparticle assembly, whose diameter becomes large with the increase in the laser power. The backscattering image of the assembly gives structural color, meaning that nanoparticles are periodically arranged over the whole assembly region. In reflection microspectroscopy, one band appears at long wavelength and is gradually shifted to the short wavelength with the irradiation. After the blue shift, the reflection band is located at the shorter wavelength under the laser irradiation at the higher power. We discuss these spectral changes from the viewpoint of the inter-particle distance determined by the dynamic balance between attractive optical force and repulsive electrostatic force among nanoparticles.
Rayleigh scattering correlation microspectroscopy is developed and applied to study diffusion dynamics of some
nanospheres in water. It was clearly found that the diffusion constant of gold nanoparticles decreased with increasing
excitation laser power at the excitation wavelength of higher absorption cross section. This behavior was explained in
terms of a coupling between laser trapping by the scattering excitation laser itself and laser heating of the particle. In the
case of non-absorbing nanospheres such as silica and polystyrene, the excitation power dependence can be ascribed only
to the laser trapping. Experimental setup is introduced, theoretical formulation is described, and future development of this measurement is considered.
We present laser trapping behaviors of 200 nm-polystyrene particles in D2O solution and at its surface using a focused
continuous-wave laser beam of 1064 nm. Upon focusing the laser beam into the solution surface, the particles are
gathered at the focal spot, and their assembly is expanded to the outside and becomes much larger than the focal volume.
The resultant assembly is observed colored under halogen lamp illumination, which is due to a periodic structure like a
colloidal crystal. This trapping behavior is much different compared to the laser irradiation into the inside of the solution
where a particle-like assembly with a size similar to that of the focal volume is prepared. These findings provide us new
insights to consider how radiation pressure of a focused laser beam acts on nanoparticles at a solution surface.
We successfully demonstrate crystallization and crystal rotation of L-alanine in D2O solution using a focused laser beam
of 1064 nm with right- or left-handed circularly polarization. Upon focusing each laser beam into a solution/air interface
of the solution thin film, one single crystal is generally formed from the focal spot. The necessary time for the
crystallization is systematically examined against polarization and power of the trapping laser. The significant difference
in the average time is observed between two polarization directions at a relatively high laser power, where the left-handed
circularly polarized laser takes 3 times longer than the right-handed one. On the other hand, the prepared crystal
is stably trapped and rotated at the focal point by circularly polarized lasers after the crystallization, and the rotation
direction is completely controlled by the polarization of the trapping laser. The mechanisms for the crystallization and
the crystal rotation are discussed in terms of trapping force and rotation torque of circularly polarized lasers acting on the
liquid-like clusters and its bulk crystal, respectively.
We present a comparison of optical trapping of 50-nm-sized silica nanospheres, suspended in water medium, by
femtosecond laser pulses and by continuous wave laser beam. With bright field microscopic imaging, we demonstrated
that intensity of scattering light at the focal area under fs-pulse mode is much higher than that under cw mode. This result
offers a basic interpretation that trapping efficiency of nanometer-sized particles by the ultrashort laser pulses is higher
than that by the cw mode at the same laser power. We interpret this finding by means of impulsive peak power of the
femtosecond laser pulses.
We present laser trapping behavior of individual smectic 4'-n-pentyl-4-cyanobiphenyl liquid crystalline micro-droplet
dispersed in heavy water; in particular, laser trapping-induced molecular reconfiguration of the optically trapped droplet
when the laser trapping power is above a definite threshold. The reconfiguration undergoes throughout the inside of the
droplets even though their size is larger than the focal spot, and the threshold laser power depends on the droplet size.
We propose that the reconfiguration mechanism involves optical reorientation at the focal volume competing with the
droplet-liquid interfacial anchoring effect, leading to symmetry breaking throughout the inside of the optically confined
droplet. With this mechanism, we qualitatively described the existence of the threshold power and the dependence of the
threshold upon the droplet size.
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 D2O 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.
We present direct observation of particle transfer and assembling upon laser irradiation under a microscope. We
employed gold nanoparticles (60 nm) dispersed in water as optical markers and studied laser trapping and accompanying
phenomenon by wide-field Rayleigh scattering microscopy. At the focal spot of the near IR laser, laser trapping of gold
was observed. Simultaneously, we observed that the particle migration toward the focal spot from all the directions
within several tens micrometer. We consider that thermocapillary effect due to laser heating can assist the particle
migration from far away, resulting in concentration increase not only at the focal point but also near the surrounding area.
Injection and delivery of small amount reagent in aqueous solution for cell chip was performed utilizing regeneratively
amplified femtosecond laser system. In our new trial, the reagent integrated on a solid strip are released and delivered to
targeted cells with the femutosecond laser-induced impulsive-force. The reagent was fixed in poly(vinyl alcohol) or
polystyrene film on a glass-substrate strip. When a single pulsed femtosecond laser was focused in the solution, the film
near the focal point was fragmented and the reagent was dispersed in 45-μm φ area at 50 μm from the surface of the
reagent strip. As examples cardiomyocyte beating cells of P19CL6 were bombed with epinephrine and acetylcholine,
and as a result the beating ratio of the cells were quickly stimulated and suppressed, respectively. The results
demonstrate that the present method is a promising key nano/micro technology for diagnosis and drug discovery.
We demonstrate preparations of zinc porphyrin nanoparticles by reprecipitation method and their spectroscopic
analysis by dark-field light scattering microspectroscopy. The size distribution of the prepared nanoparticles was 80-150
nm. By using dark-field illumination the nanoparticles could be observed as bright points in dark background and could
be examined by their Rayleigh scattering spectra at single particle level. The spectra differed from particle to particle,
which would be ascribed to their size and crystalline phase difference. Thus we have performed this single particle
spectroscopic technique to remove the ambiguity about the spectroscopic information owing to distributions of particles
and to improve the space selectivility. In addition, we have successfully demonstrated the detection of amine molecules
in water at single particle level. These results indicate that the detection technique using the single porphyrin
nanoparticles can be applied to chemical and biological sensors with nanometer scale.
Several kinds of manipulation of biological cells were performed utilizing regeneratively amplified femtosecond laser
system. When single-shot pulse of an amplified Ti: Sapphire femtosecond laser pulse is focused on a culture medium,
shockwave and cavitation bubble are generated with little heating. An impulsive force resulting in these phenomena was
applied to pttern specific cells form a culture substrate. Furthermore, laser trapping of cells was realized using high-repetition
rate pulses from the laser oscillator. Although the cell was trapped stably when the laser power was less than
100 mW, the cell was burst above the threshold laser power. The bursting would be due to heating inside cell, on which
the laser was focused and multiphoton absorption was induced. On the bases of these results, we propose a new
methodology to pattern biological cells, which is speedy and flexible when compared with previous micropatterning
methods.
PC12 cells, which are derived from a rat pheochromocytoma, were independently patterned utilizing an impulsive force
resulting in impulsive shockwave and cavitation bubble generation by focused femtosecond laser irradiation. Since the
PC12 cells respond reversibly to nerve growth factor by induction of the neuronal phenotype, we can assess an influence
that the impulsive force gives to the bioactivity in term of the cell differentiation. The patterned cells were accumulated
on an intact dish and cultured for 3 days. The behavior of appearance and cell differentiation was observed by multipoint
time-lapse system. On bases of these results, it was proved that the biological activity of the cell is unaffected by the
femtosecond laser patterning.
We have prepared nanoparticles of five organic dye molecules by laser ablation of their microcrystals in poor solvents
using the third harmonics of nanosecond Nd3+:YAG laser as an excitation light source. Their colloidal solutions were
stable for longer than 1 week without any surfactants. The mean size was almost common to all the dyes (about 50 nm)
and its distribution was narrow, which was confirmed by SEM observation. By applying electrophoretic deposition, the
homogeneous thin film of quinacridone nanoparticles was fabricated on an indium-tin-oxide electrode. It was
demonstrated that the films with different grain size and crystalline phase can be fabricated arbitrarily by using different
nanoparticles. Laser ablation is also useful for crystallization of organic molecules, which was demonstrated for a
representative organic nonlinear optical material.
Kinetics of J-aggregation of thiacarbocyanine (THIATS) has been investigated by measuring time variation of fluorescence spectrum under the solvent evaporation process of THIATS solution. Fluorescence spectrum of the THIATS J-aggregates changes following the increase of concentration, reflecting the formation and ripening process of J-aggregates. These THIATS J-aggregates are trapped and gathered at the focal point by focusing near-infrared (NIR) laser beam into the solution. Two-photon excited fluorescence from the focal spot is concurrently detected with the same trapping laser beam. Fluorescence spectral change is accelerated by focusing NIR laser beam. This result could be attributed that J-aggregates with higher polarizability are preferably formed in the focal spot of NIR laser. Furthermore, we have succeeded in the deposition of J-aggregates on a glass substrate by optical trapping.
We had already developed several series of fluoropolymers, FPRs and FUGUs, having a partially fluorinated monocyclic structure and having acidic hydroxyl group, which acts as dissolution unit into alkaline solution. Then we have optimized these polymers for top-coat as the developer-soluble type in the 193nm immersion lithography. However the hydrophobicity of these polymers were a little poor due to its hydroxyl group. So we thought that the introduction of water repellent moiety into the these polymers structure is effective to improve the their hydrophobicity though the increase of water repellent unit in the polymer leads to lower dissolution rate in developer. To introduce as much as possible of hydrophobicity unit, we selected FUGU as platform, which has larger dissolution rate in developer than that of FPRs, We copolymerized FUGU with higher water-repellent component and obtained three copolymers, FUGU-CoA, FUGU-CoB, and FUGU-CoC. In this paper, we described characteristics and evaluation of these polymers. Most of these polymer showed an improvement of hydrophobicity, in particular FUGU-CoB had excellent hydrophobicity due to introduction bulky containing-fluorine group. In this study, we also investigated the interaction between the water and various polymers by using QCM method. The difference between FUGU and water repellent polymers for swelling behavior to water became clear by analysis of diffusion coefficient. We found that our new co-polymers have excellent diffusion coefficient than FUGU which was confirmed by QCM method used to evaluate water permeability and water diffusion in the materials.
Micro-patterning of proteins has been attracted much attention as a potential technique to realize bio-microdevice. In this work, as a new method to realize non-destructive micro-patterning of proteins, laser transfer printing for a um-sized protein crystal was developed by utilizing focused femtosecond laser. The micro-patterning was performed to transfer the protein crystal which was adhered on a source substrate to a target substrate which was underlaid on the source substrate. An 800-nm femtosecond laser was focused in a water between the source and target substrates on an inverted microscope with a 100x objective lens. When the laser focal point was scanned at the position with distance of a few um far from the source substrate, the protein crystals were detached by a shockwave and cavitation bubble generation at the circumstance of the focal point and transferred to the target substrate forming a line pattern. The line width of the protein crystal was a few tens um with the scanning speed of 90 μm/sec. Furthermore, multi-patterning of several kinds of protein crystals was realized by this method. The pattering resolution is comparable or better than that by another multi-material transfer printing, such as ink jet printing, micro-printing, and laser direct writing.
When near-infrared laser was focused in colloidal silver, not only optical trapping of silver nanoparticles, but also hyper-Rayleigh scattering were observed at the laser focus. By addition of NaCl and rhodamine 6G in the colloidal silver, hyper-Rayleigh scattering was enhanced and hyper-Raman scattering was also observed. These nonlinear optical responses showed temporal fluctuation in spite of the continuous laser irradiation. Experimental results suggested that aggregates which have a high activity for nonlinear optical responses were trapped and/or produced by a focusing near-infrared laser. Hyper-Raman signal, whose scattering cross section is much lower than normal Raman scattering, could be obtained easily by focusing a cw-YAG laser in colloidal silver including analytes. It was demonstrated that optical trapping of colloidal silver is a powerful technique to obtain the nonlinear optical responses.
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.
Laser manipulation system combining microfluidic and microimaging devices was developed, in which a cell sorting in a transparent microchip was successfully demonstrated. The microchip containing two microchambers was prepared with laser microfabrication, in which a solution containing yeast cells was injected as a sample. In the microchip, a cell transfer from one chamber to another one was performed by using single, fixed trapping laser beam. Furthermore, to realize an efficient cell sorting, the trapping laser bam was split into two by a polarizing beam splitter and each beam was modulated independently; one trapping beam was used to trap individual cells and to move them, which is freely controlled by a mouse pointer, and another was used to store the selected yeast cells with its liner scanning. In this method, the cells on a locus of the scanned beam were isolated to transfer in the microchip. From these results, it is concluded that shortening of the cell sorting time in microchip by a few time was realized by using dual-beam laser manipulation.
Time-resolved interferometry, surface light scattering imaging, and optical microscopic imaging have been developed and applied to amorphous and multicrystalline films upon intense pulsed excitation. Nanosecond interferometry of neat polystyrene film gives interesting expansion dynamics followed by complete recovery. Femtosecond surface light scattering imaging reveals the roughing processes of Copper phthalocyanine (CuPc) films. Femtosecond optical microscopic imaging of a single anthracene microcrystal shows dynamics of its laser-induced fracture and destruction. All the morphological behaviors have been directly measured in the time domain by these newly developed pump-probe methods. Ablation of CuPc crystalline and amorphous films show novel dependence of etch profile on excitation pulse width; fs etch depth increases stepwise with laser fluence, while ns etch depth becomes gradually deep as fluence does. The results are discussed in view of how electronic excitation energy evolves to morphological change.
Laser ablation and etching of microcrystalline Cu- phthalocyanine thin films were examined by changing pulse duration (170 fs, 250 ps, 100 ns) of a 780 nm Ti:sapphire laser. Above fs (40 mJ/cm2) and ps (50 mJ/cm2) ablation thresholds, the etch depth becomes constant and is almost independent of laser fluence, and further increase in the fs fluence results in complete removal of the film. We name the unique ablation phenomenon discrete etching. On the other hand, the depth etched by ns laser excitation increases gradually with the fluence above its ablation threshold (80 mJ/cm2. In order to reveal the difference between the fs and ns etching behaviors, we measured directly excitation energy relaxation and surface morphology change with time-resolved absorption spectroscopy and time- resolved surface scattering imaging, respectively. The fs discrete etching phenomenon and its mechanism were considered in view of time evolutions from highly intense fs laser excitation to the step-wise etching. On the basis of the results, we propose an fs laser ablation model that ultrafast stress increase brings about mechanical disruption leading to the discrete etching behavior.
Photopolymers based on the triazeno chromophore group (-NequalsN-N<) have been developed. The absorption properties can be tailored for a specific irradiation wavelength (e.g. 308 cm XeCl laser). The photochemical exothermic decomposition yields high energetic gaseous products which are not contaminating the surface. The polymer can be structured with high resolution. No debris has been found around the etched corners. Maximum ablation rates of about 3 micrometers / pulse were achieved due to the dynamic absorption behavior (bleaching during the pulse). No physical or chemical modifications of the polymer surface could be detected after irradiation at the tailored absorption wavelength, whereas irradiation at different wavelengths resulted in modified (physical and chemical) surfaces. The etching of the polymer starts and ends with the laser pulse, shown by ns-interferometry, confirming that the acting mechanism is mainly photochemical. TOF-MS revealed fragments which are also totally compatible with a photochemical decomposition mechanism.
We have developed novel photopolymers based on the triazeno chromophore group. The absorption properties can be tailored for a specific irradiation wavelength. With the introduction of a photolabile group into the main chain of the polymer we expected a mechanisms which is mainly photochemical. This should result in high resolution etching with no thermal damage or chemical/physical modification to the material. The gaseous products of the photochemical decomposition were thought to assist the material removal, and to prevent the re-deposition of solid products which would contaminate the surface. We confirmed that the irradiation of the polymer at 308 nm resulted in high resolution etching. No debris has been found around the etched corners. Maximum ablation rates of about 3 (Mu) m/pulse were achieved due to the dynamic absorption behavior. No physical or chemical modifications of the polymer surface could be detected after irradiation at the tailored absorption wavelength, whereas irradiation at different wavelengths resulted in modified surfaces. The etching mechanism can be described as a laser induced microexplosion, revealed by ns-imaging. The etching of the polymer starts and ends with the laser pulse, shown by ns- interferometry, confirming that the acting mechanism is mainly photochemical at high fluences for our polymers, which can be used as high resolution laser dry etching resists.
Pyrene-doped poly(p-hydroxystyrene) (PHST) thin films prepared by spin- coating method are studied by time-resolved total-internal-reflection fluorescence spectroscopy. We observed inhomogeneity such as a concentration gradient of doped pyrene molecules and a gradient of polarity (hydrophobicity), and also the existence of isolated pyrene molecules. Stable ground-state dimers of pyrene are found in surface, bulk, and interface layers of a PHST film. These features cause the complicated rise and decay of fluorescence. These results also depend upon the preparing process such as the baking condition.
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