Browsing by Author "Hofkens, Johan"

Now showing 1 - 7 of 7

Dynamic Coupling of Optically Evolved Assembling and Swarming of Gold Nanoparticles with Photothermal Local Phase Separation of Polymer Solution
(2020-07-30)
Aibara, Issei
;
Huang, Chih-Hao
;
Kudo, Tetsuhiro
;
Bresoli-Obach, Roger
;
Hofkens, Johan
Optical trapping of gold nanoparticles (Au NPs) at a glass/solution interface initially generates a periodically aligned structure of a few NPs along the direction perpendicular to a linearly polarized laser. When the number of NPs was increased, this alignment was expanded to the outside of the irradiated focus, forming a single large assembly where the Au NPs dynamically fluctuated like a swarming of bees. The morphology was dumbbell-shape, consisting of two swarms at both sides of the focus, and its size reached about 10 mu m. This optically evolved assembling and swarming was studied in poly(N-isopropylacrylamide) (PNIPAM) solution, where liquid-liquid phase separation (LLPS) was induced by photothermal heating of the trapped Au NPs forming a microdroplet of highly concentrated PNIPAM. Dynamic coupling of the NPs assembling and swarming with the droplet formation of PNIPAM leads to cooperative optical evolution, through which the assembly was embedded in the single microdroplet.
Fast-tracking of single emitters in large volumes with nanometer precision
(2020-09-14)
Louis, Boris
;
Camacho, Rafael
;
Bresoli-Obach, Roger
;
Abakumov, Sergey
;
Vandaele, Johannes
Multifocal plane microscopy allows for capturing images at different focal planes simultaneously. Using a proprietary prism which splits the emitted light into paths of different lengths, images at 8 different focal depths were obtained, covering a volume of 50x50x4 mu m(3). The position of single emitters was retrieved using a phasor-based approach across the different imaging planes, with better than 10 nm precision in the axial direction. We validated the accuracy of this approach by tracking fluorescent beads in 3D to calculate water viscosity. The fast acquisition rate (>100 fps) also enabled us to follow the capturing of 0.2 mu m fluorescent beads into an optical trap. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Femtosecond Laser Trapping Dynamics of Nanoparticles: A Single Transient Assembly Formation Leading to Their Directional Ejection
(2018-06-28)
Chiang, Wei-Yi
;
Usrnan, Anwar
;
Sugiyama, Teruki
;
Hofkens, Johan
;
Masuhara, Hiroshi
We investigated femtosecond laser trapping dynamics of silica nanoparticles with different hydrophobic surface properties. We demonstrated that the hydrophobic surface on the silica nanoparticles facilitates mutual association of the nanoparticles in the optical trapping site. Such association of optically trapped nanoparticles is a prerequisite to induce their directional ejection away from the trapping site. The directional ejection of the optically trapped nanoparticles is most probably due to asymmetric three-dimensional ejecting forces generated by the electromagnetic interaction between transient assembly in the focal spot and the incident pulses. These findings provide important insights into the directional ejection of nanoparticles from the trapping site in the femtosecond laser trapping, and this physicochemical phenomenon is controlled by both the trapping laser and material properties.
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.
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.
Photo-induced electrodeposition of metallic nanostructures on graphene
(2020-05-28)
Xia, Kangwei
;
Chiang, Wei-Yi
;
de la Rosa, Cesar Javier Lockhart
;
Fujita, Yasuhiko
;
Toyouchi, Shuichi
Graphene, a single atomic layer of sp(2)hybridized carbon, is a promising material for future devices due to its excellent optical and electrical properties. Nevertheless, for practical applications, it is essential to deposit patterned metals on graphene in the micro and nano-meter scale in order to inject electrodes or modify the 2D film electrical properties. However, conventional methods for depositing patterned metals such as lift-off or etching leave behind contamination. This contamination has been demonstrated to deteriorate the interesting properties of graphene such as its carrier mobility. Therefore, to fully exploit the unique properties of graphene, the controlled and nano-patterned deposition of metals on graphene films without the use of a sacrificial resist is of significant importance for graphene film functionalization and contact deposition. In this work, we demonstrate a practical and low-cost optical technique of direct deposition of metal nano-patterned structures without the need for a sacrificial lift-off resist. The technique relies on the laser induced reduction of metal ions on a graphene film. We demonstrate that this deposition is optically driven, and the resolution is limited only by the diffraction limit of the light source being used. Patterned metal features as small as 270 nm in diameter are deposited using light with a wavelength of 532 nm and a numerical aperture of 1.25. Deposition of different metals such as Au, Ag, Pd, Pb and Pt is shown. Additionally, change in the Fermi level of the graphene film through the nano-patterned metal is demonstrated through the electrical characterization of four probe field effect transistors.
Surface plasmon resonance effect on laser trapping and swarming of gold nanoparticles a an interface
(2020-09-14)
Huang, Chih-Hao
;
Kudo, Tetsuhiro
;
Bresoli-Obach, Roger
;
Hofkens, Johan
;
Sugiyama, Teruki
Laser trapping at an interface is a unique platform for aligning and assembling nanomaterials outside the focal spot. In our previous studies, Au nanoparticles form a dynamically evolved assembly outside the focus, leading to the formation of an antenna-like structure with their fluctuating swarms. Herein, we unravel the role of surface plasmon resonance on the swarming phenomena by tuning the trapping laser wavelength concerning the dipole mode for Au nanoparticles of different sizes. We clearly show that the swarm is formed when the laser wavelength is near to the resonance peak of the dipole mode together with an increase in the swarming area. The interpretation is well supported by the scattering spectra and the spatial light scattering profiles from single nanoparticle simulations. These findings indicate that whether the first trapped particle is resonant with trapping laser or not essentially determines the evolution of the swarming. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement