Browsing by Author "Usman, Anwar"

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Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond Laser Pulses
(2014-12-11)
Chiang, Wei-Yi
;
Okuhata, Tomoki
;
Usman, Anwar
;
Tamai, Naoto
;
Masuhara, Hiroshi
The development in optical trapping and manipulation has been showing rapid progress, most of it is in the small particle sizes in nanometer scales, substituting the conventional continuous-wave lasers with high-repetition-rate ultrashort laser pulse train and nonlinear optical effects. Here, we evaluate two-photon absorption in optical trapping of 2.7 nm-sized CdTe quantum dots (QDs) with high-repetition-rate femtosecond pulse train by probing laser intensity dependence of both Rayleigh scattering image and the two-photon-induced luminescence spectrum of the optically trapped QDs. The Rayleigh scattering imaging indicates that the two-photon absorption (TPA) process enhances trapping ability of the QDs. Similarly, a nonlinear increase of the two-photon-induced luminescence with the incident laser intensity fairly indicates the existence of the TPA process.
Enhanced optical confinement of dielectric nanoparticles by two-photon resonance transition
(2017-01-01)
Kittiravechote, Aungtinee
;
Usman, Anwar
;
Masuhara, Hiroshi
;
Liau, Ian
;
應用化學系
Despite a tremendous success in the optical manipulation of microscopic particles, it remains a challenge to manipulate nanoparticles especially as the polarizability of the particles is small. With a picosecond-pulsed near-infrared laser, we demonstrated recently that the confinement of dye-doped polystyrene nanobeads is significantly enhanced relative to bare nanobeads of the same dimension. We attributed the enhancement to an additional term of the refractive index, which results from two-photon resonance between the dopant and the optical field. The optical confinement is profoundly enhanced as the half-wavelength of the laser falls either on the red side, or slightly away from the blue side, of the absorption band of the dopant. In contrast, the ability to confine the nanobeads is significantly diminished as the half-wavelength of the laser locates either at the peak, or on the blue side, of the absorption band. We suggest that the dispersively shaped polarizability of the dopant near the resonance is responsible to the distinctive spectral dependence of the optical confinement of nanobeads. This work advances our understanding of the underlying mechanism of the enhanced optical confinement of doped nanoparticles with a nearinfrared pulsed laser, and might facilitate future research that benefits from effective sorting of selected nanoparticles beyond the limitations of previous approaches.
Femtosecond laser trapping, assembling, and ejection dynamics of dielectric nanoparticles in solution
(2018-01-01)
Masuhara, Hiroshi
;
Chen, Jim Jui-Kai
;
Chiang, Wei-Yi
;
Usman, Anwar
;
Sugiyama, Teruki
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.
Femtosecond Pulse-Width Dependent Trapping and Directional Ejection Dynamics of Dielectric Nanoparticles
(2013-09-19)
Chiang, Wei-Yi
;
Usman, Anwar
;
Masuhara, Hiroshi
;
應用化學系
;
應用化學系分子科學碩博班
We demonstrate that laser pulse duration, which determines its impulsive peak power, is an effective parameter to control the number of optically trapped dielectric nanoparticles, their ejections along the directions perpendicular to polarization vector, and their migration distances from the trapping site. This ability to controllably confine and eject the nanoparticle is explained by pulse width-dependent optical forces exerted on nanoparticles in the trapping site and ratio between the repulsive and attractive forces. We also show that the directional ejections occur only when the number of nanoparticles confined in the trapping site exceeds a definite threshold. We interpret our data by considering the formation of transient assembly of the optically confined nanoparticles, partial ejection of the assembly, and subsequent filling of the trapping site. The understanding of optical trapping and directional ejections by ultrashort laser pulses paves the way to optically controlled manipulation and sorting of nanoparticles.
Femtosecond Trapping Efficiency Enhanced for Nano-sized Silica Spheres
(2012)
Usman, Anwar
;
Chiang, Wei-Yi
;
Masuhara, Hiroshi
;
應用化學系
;
Department of Applied Chemistry
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.
Formation Mechanism and Fluorescence Characterization of a Transient Assembly of Nanoparticles Generated by Femtosecond Laser Trapping
(2019-11-14)
Chiang, Wei-Yi
;
Chen, Jim Jui-Kai
;
Usman, Anwar
;
Kudo, Tetsuhiro
;
Xia, Kangwei
Femtosecond (fs) laser trapping of dielectric nanoparticles displays a unique trapping and directional ejection phenomenon, which is never observed in continuous-wave (cw) laser trapping. We studied the fs laser trapping and considered its dynamics and mechanism in terms of gathering, association, and ejection of nanoparticles. By tuning solution viscosity through adding ethylene glycol, the trapping behavior by fs and cw lasers are examined and compared with each other. The viscous solution slows down the diffusional and rotational movement of nanoparticles and increases the chance of nanoparticle assembly in the trapping site, which eventually leads to the novel ejection. We took advantage of fluorescence measurement to confirm the formation of the nanoparticle assembly during fs laser trapping. A red emission irregularly appeared in the fluorescence supports the transient assembly formation in the molecular level.
Glycine Crystallization in Solution by CW Laser-Induced Microbubble on Gold Thin Film Surface
(2012-03-01)
Uwada, Takayuki
;
Fujii, Sho
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Sugiyama, Teruki
;
Usman, Anwar
;
Miura, Atsushi
We have developed a novel laser-induced crystallization method utilizing local heat-induced bubble/water interface. Continuous laser beam of 1064 nm is focused on a gold nanoparticles thin film surface covered with glycine supersaturated aqueous solution. Light absorption of the film due to localized plasmon resonance caused local heating at the focal position and produced a single thermal vapor microbubble, which generated thermal gradient followed by convection flow around the bubble and eventually induced glycine crystallization and growth. The crystallization mechanism is discussed by considering gathering and accumulating molecules around the bubble/water interface assisted by convection flow and temperature jump.
Laser Trapping-Induced Reconfiguration of Individual Smectic Liquid Crystal Micro-Droplet Showing Size-Dependent Dynamics
(2012)
Usman, Anwar
;
Chiang, Wei-Yi
;
Uwada, Takayuki
;
Masuhara, Hiroshi
;
應用化學系
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.
Monomeric and Dimeric Erbium(III) Complexes: Crystal Structure and Photoluminescence Studies
(2011-02-01)
Kusrini, Eny
;
Saleh, Muhammad I.
;
Usman, Anwar
;
應用化學系
;
應用化學系分子科學碩博班
The crystal structure and photoluminescence (PL) studies of the monomeric and dimeric Er(III) complexes with two different chelating ligands (anthracene-9-carboxylic acid, 9-ACA; pentaethylene glycol, EO5; and picric acid, HPic) are reviewed. The Er(III) metal ion was coordinated to the attached ligands in eight- and nine-coordination number. The dimeric [Er(2)(9-AC)(6)(DMF)(2)(H(2)O)(2)] complex shows the presence of deprotonated 9-AC anions with the negatively charged oxygen atoms bridged two Er(III) ions leads to a great coordinative flexibility via three possibilities of coordination modes, i.e. monodentate, chelation bidentate, chelating-bridging tridentate, where 9-AC = anthracene-9-carboxylate anion. The monomeric [Er(Pic)(2)(EO5)](Pic) complex displays the important flexible structure of the acyclic EO5 ligand and the role of Pic anions act as bidentate and monodentate chelations. The PL spectra of both Er(III) complexes show a broad band with the center peak position being dependent on the attached aromatic ligands. The heavier lanthanide complexes show the difference in structures, coordination geometry environment, and luminescence properties compared to the lighter lanthanide complexes. The energy transfer process in the complexes could be optimized with maximize the overlap between the emission spectrum of donor atom and absorption spectrum of acceptor atom. The monomeric [Er(Pic)(2)(EO5)](Pic) and dimeric [Er(2)(9-AC)(6)(DMF)(2)(H(2)O)(2)] complexes, where EO5 = pentaethylene glycol, Pic = picrate anion, and 9-AC = anthracene-9-carboxylate anion, have been structurally investigated. The 9-AC anions with the negatively charged oxygen atoms have bridged two Er(III) ions leads to a great coordinative flexibility via three possibilities of coordination modes, i.e. monodentate, chelation bidentate, chelating-bridging tridentate. The pseudo-cyclic conformation of the acyclic EO5 ligand after the formation of complex was observed. The heavier lanthanides and Pic anion have a great important role in formation of complex. The photoluminescence spectra of both Er(III) complexes display a broad band with the center peak position being dependent on the attached aromatic ligands. The heavier lanthanide complexes show the difference in structures, coordination geometry environment, and luminescence properties compared to the lighter lanthanide complexes. The energy transfer process in the complexes could be optimized with maximize the overlap between the emission spectrum of donor atom and absorption spectrum of acceptor atom.
Optical Reorientation and Trapping of Nematic Liquid Crystals Leading to the Formation of Micrometer-Sized Domain
(2011-06-23)
Usman, Anwar
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Uwada, Takayuki
;
Masuhara, Hiroshi
;
應用化學系
;
應用化學系分子科學碩博班
We report on observation of optical reorientation of homogeneous nematic liquid crystals (LCs) thin slab of 4'-penthyl-4-cyanobiphenyl induced by a tightly focused nearinfrared laser beam. We found that for laser beam with intensity higher than 56 MW/cm(2) the optical reorientation unusually transforms to a new metastable domain, which grows with laser irradiation time up to a dimension much larger than that of the focal spot. The optically reoriented LCs with refractive index mismatch compared to the surroundings can act as a micrometer-sized "ghost particle", leading to the generation of optical trapping and manipulation of the optically reoriented LCs. By using confocal Raman microspectroscopy, we show that the dichroic ratio in the confined focal volume changes upon the domain formation with an increase in the total Raman intensity, indicating that microscopic depolarization leading to deorientation or nematic isotropic phase transition and possible microscopic densification takes place by the tightly focused laser beam.
Optical trapping and polarization-controlled scattering of dielectric spherical nanoparticles by femtosecond laser pulses
(2012-04-15)
Usman, Anwar
;
Chiang, Wei-Yi
;
Masuhara, Hiroshi
;
應用化學系
;
應用化學系分子科學碩博班
We present optical trapping behavior of 50-nm-sized polystyrene beads, suspended in water medium, by femtosecond pulsed laser beam. In addition to a higher number of nanoparticles trapped at the focal spot by the ultrashort laser pulses compared with that by continuous-wave laser, the nanoparticles are scattered out of the focal spot by the laser pulses to the surrounding area. The scattered particles form a partially opened folding fan-shaped bright locus in two opposite directions, in an alternating manner, perpendicular to the laser polarization. To understand those phenomena, we analyzed radiation (gradient and scattering) force of femtosecond laser pulses and their temporal force exerted on the dielectric spherical nanoparticles by taking into account the impulsive peak power and the axial component of electric light field produced by high numerical aperture of objective lens. We show that the axial electric field is responsible for lateral components of the scattering and temporal forces, and hence, controls the scattering directions of the Rayleigh particles. These findings provide important information about the dynamic optical trapping of the Rayleigh particles by highly focused ultrashort laser pulses. (c) 2012 Elsevier B.V. All rights reserved.
Optical trapping assembling of clusters and nanoparticles in solution by CW and femtosecond lasers
(2015-02-01)
Masuhara, Hiroshi
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Sugiyama, Teruki
;
Yuyama, Ken-ichi
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Usman, Anwar
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應用化學系
Laser trapping of molecular systems in solution is classified into three cases: JUST TRAPPING, EXTENDED TRAPPING, and NUCLEATION and GROWTH. The nucleation in amino acid solutions depends on where the 1064-nm CW trapping laser is focused, and crystallization and liquid-liquid phase separation are induced by laser trapping at the solution/air surface and the solution/glass interface, respectively. Laser trapping crystallization is achieved even in unsaturated solution, on which unique controls of crystallization are made possible. Crystal size is arbitrarily controlled by tuning laser power for a plate-like anhydrous crystal of L-phenylalanine. The a-or c-crystal polymorph of glycine is selectively prepared by changing laser power and polarization. Further efficient trapping of nanoparticles and their following ejection induced by femtosecond laser pulses are introduced as unique trapping phenomena and finally future perspective is presented.
Optical Trapping Dynamics of a Single Polystyrene Sphere: Continuous Wave versus Femtosecond Lasers
(2016-02)
Liu, Tsung-Han
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Chiang, Wei-Yi
;
Usman, Anwar
;
Masuhara, Hiroshi
;
應用化學系
Understanding of optical trapping dynamics of a single particle in the trapping site is important to develop its optical manipulation for molecular assembly and chemical application. For micrometer-sized Mie particles, similar trapping efficiency of the conventional continuous wave (cw) laser or high-repetition-rate femtosecond (fs) laser pulse train has been established [Dholakia et al., Opt. Express 2010, 18, 7554-7568], in contrast to higher efficiency of the laser pulses to trap dielectric Rayleigh particles. To further explore and clarify the switching phenomena of optical trapping efficiency with cw laser and fs laser pulse and to elucidate its nature, we study the immobilization dynamics of a single polystyrene sphere with 500 nm in diameter (which is comparable to focal beam size) in shallow potential well. By observing trapping events and immobilization time of the particle with a size in Lorenz-Mie regime, distinct from well-known Rayleigh particle and ray optics approximations, we found that immobilization time is only linearly related to the incident laser power <= 40 mW, and at higher laser powers cw laser is more efficient than fs laser pulses to immobilize the particle. This finding means that the dynamics of the particle in this size region is still affected by the strong transient force fields induced by high-repetition-rate ultrashort pulse train as usually observed for Rayleigh particles. This may provide an understanding that the dynamics of the target particle in the trapping site is size- and laser mode-dependent.
Photochemical Reaction of p-hydroxycinnamic-thiophenyl Ester in the Microcrystalline State
(2010-11-18)
Usman, Anwar
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Asahi, Tsuyoshi
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Sugiyama, Teruki
;
Masuhara, Hiroshi
;
Tohnai, Norimitsu
We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state We attributed the fluorescence spectral evolution of the microcrystal under UV irradiation to the photoinduced trans-to-cis isomerization The photocyclic behavior of the chromophore was demonstrated by cis-to-trans back reaction under a subsequent visible light irradiation In addition the [2 2] topochemical photocyclodimer was observed as another photoproduct It is considered that the cooperative photoisomerization is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal sui lace and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior of the crystal
Photochemical Reaction of p-hydroxycinnamic-thiophenyl Ester in the Microcrystalline State
(2010-11-18)
Usman, Anwar
;
Asahi, Tsuyoshi
;
Sugiyama, Teruki
;
Masuhara, Hiroshi
;
Tohnai, Norimitsu
We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state We attributed the fluorescence spectral evolution of the microcrystal under UV irradiation to the photoinduced trans-to-cis isomerization The photocyclic behavior of the chromophore was demonstrated by cis-to-trans back reaction under a subsequent visible light irradiation In addition the [2 2] topochemical photocyclodimer was observed as another photoproduct It is considered that the cooperative photoisomerization is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal sui lace and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior of the crystal
Picosecond Motional Relaxation of Nanoparticles in Femtosecond Laser Trapping
(2016-03-10)
Muramatsu, Masayasu
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Shen, Tse-Fu
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Chiang, Wei-Yi
;
Usman, Anwar
;
Masuhara, Hiroshi
Repetitive drag and release dynamics by impnlsive force is characteristic of optical trapping by femtosecond laser pulses. We studied the dynamics utilizing double pulse train and found that trapped polystyrene particles are ejected repetitively from the focal spot and its frequencies become less for longer interval of the pulse trains. The ejection changes drastically in a few-ps interval region, although particles cannot move appreciable distance in such a short time. It means that displacement of particles by a conventional diffusive motion is not dominant and another fast process has an important role in femtosecond pulse trapping. We also revealed that the silica nanoparticles shows a decay at few-ps, indicating that the picosecond decay is not due to a material property but considered to be a general dynamics. We propose that a picosecond relaxation process of inertia force of particles is important for understanding laser trapping dynamics by femtosecond laser pulses.
Polarization and Droplet Size Effects in the Laser-Trapping-Induced Reconfiguration in Individual Nematic Liquid Crystal Microdroplets
(2013-04-25)
Usman, Anwar
;
Chiang, Wei-Yi
;
Uwada, Takayuki
;
Masuhara, Hiroshi
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應用化學系
We experimentally demonstrate reordering throughout the inside of an individual bipolar nematic liquid-crystalline microdroplet optically trapped by a highly focused laser beam, when the laser powers are above a definite threshold. The threshold depends on the droplet size and laser polarization. A physical interpretation of the results is presented by considering the nonlocal orientations of the nematic liquid-crystal molecules in the droplets with the dimensions on the order of the focal spot diameter or larger. On the basis of the finite size approximation, we show that the dependence of threshold power on the droplet size is calculated to be in qualitative agreement with the experimental data.
Single femtosecond laser pulse-single crystal formation of glycine at the solution surface
(2013-03-01)
Liu, Tsung-Han
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Uwada, Takayuki
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Sugiyama, Teruki
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Usman, Anwar
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Hosokawa, Yoichiroh
We demonstrate femtosecond laser-induced crystallization of glycine from its supersaturated solution depending on laser tunable parameters (pulse energy and repetition rate) and focal position, and examine the crystallization probability, crystal morphology, and crystal polymorph. The generation of cavitation bubble through multiphoton absorption of water depends on input laser pulse energy and repetition rate, which strongly determine morphology and number of the obtained crystals. Significant increase in the crystallization probability is observed by irradiating the femtosecond laser pulses to the air/solution interface, and single pulse-induced single crystal formation is successfully achieved. The crystallization mechanism is discussed in view of inhomogeneous mechanical stress induced by cavitation bubble generation and molecular assembly characteristics of the surface. (C) 2012 Elsevier B.V. All rights reserved.
Size-Dependent Optical Properties of Grana Inside Chloroplast of Plant Cells
(2017-02-09)
Uwada, Takayuki
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Huang, Ling-Ting
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Hee, Ping-Yu
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Usman, Anwar
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Masuhara, Hiroshi
Well-packed thylakoids known as grana are one of the major functional sites for photosynthesis in algae and plants. Their highly ordered structures can be considered as a few hundred nanometer-sized particles having distinct scattering cross sections from other various macromolecular organizations inside plant cells. With this background we shOw that elastic light scattering imaging and microspectroscopy is an important tool for investigating structure and organisation of grana inside a single chloroplast in plant tells. We have demonstrated this noninvasive method to identify the distribution of grana in intact fresh leaf of robust and rapidly growing Egaria densa, which is also known as Anachris and among the most popular aquarium plants. The scattering efficiency spectra of their individual grana fairly resemble cooperative absorption spectra of porphyrins and carotenoids. We found that the electronic structure of the stacked thylakoids shows granum size-dependence, indicating that size of grana is one of the critical parameters in the regulation of the photochemical functions in the thylakoid.