Browsing by Author "Tai, Po-Tse"

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2-D modeling of dual-mode acoustic phonon excitation of a triangular nanoplate
(2010-08-23)
Tai, Po-Tse
;
Yu, Pyng
;
Tang, Jau
;
光電工程學系
;
Department of Photonics
In this theoretical work, we investigated coherent phonon excitation of a triangular nanoplate based on 2-D Fermi-Pasta-Ulam lattice model. Based on the two-temperature model commonly used in description of laser heating of metals, we considered two kinds of forces related to electronic and lattice stresses. Based on extensive simulation and analysis, we identified two major planar phonon modes, namely, a standing wave mode related to the triangle bisector and another mode corresponding to half of the side length. This work elucidates the roles of laser-induced electronic stress and lattice stress in controlling the initial phase and the amplitude ratio between these two phonon modes. (C) 2010 Elsevier B.V. All rights reserved.
Photoinduced multimode coherent acoustic phonons of metallic nanoprisms and the effects of shape-induced anisotropic electronic stresses
(2011-05-14)
Tai, Po-Tse
;
Yu, Pyng
;
Tang, Jau
;
光電工程學系
;
Department of Photonics
In this work we reported experimental measurements of ultrafast structural dynamics in metallic nanoprisms induced by a femtosecond laser pulse. The main focus of this study of anisotropic heating in nanoprisms is about laser fluence effects on photoexcitation of two planar coherent acoustic phonon modes, namely, the breathing mode and the totally symmetric mode. We presented a combined two-temperature model and 2-D Fermi-Pasta-Ulam model to explain both the dependence of the initial phases and the mode weight on the excitation power. Our transient optical absorption data for both the initial fast monotonic decay and the subsequent coherent acoustic oscillations clearly indicate the presence of anisotropic thermal expansion in nanoprisms. (C) 2011 American Institute of Physics. [doi:10.1063/1.3590373]
Saturable absorber using single wall carbon nanotube-poly (vinylalcohol) deposited by the vertical evaporation technique
(2011-03-01)
Tai, Po-Tse
;
Pan, Shu Di
;
Wang, Yong-Gang
;
Tang, Jau
;
光電工程學系
We employed a vertical evaporation technique to fabricate saturable absorbers by embedding single wall carbon nanotubes (SWCNT) in polymer (vinylalcohol). Two fast recovery time constants, 250 fs and 1.13 ps, respectively, were measured for the absorbers using a transient absorption experimental setup. The saturation intensity of the absorber was found to be 300-400 mu J/cm(2) at 1060 nm, and a modulation depth as high as 3.5% was achieved. (C) 2010 Elsevier B.V. All rights reserved.
Selective acoustic phonon mode excitation of multi-mode silver nanoprisms
(2010-08-30)
Tai, Po-Tse
;
Yu, Pyng
;
Huang, Jane
;
Tang, Jau
;
光電工程學系
Nanoprisms are known to exhibit two major planar phonon modes in ultrafast experiments, namely, a stronger mode related to the bisector height and a weaker mode related to half of the edge length. In this work we demonstrated photoacoustic excitation of a specific acoustic phonon mode in silver nanoprisms using two properly timed pump pulses. Using this technique, we could selectively excite only the weak totally symmetric mode while suppressing the dominant breathing mode. Via direct observation of such a mode, which is hardly detectable directly with the conventional single pump pulse technique, we could elucidate the roles of hot electrons in photoinduced ultrafast structural dynamics in laser-heated nanoparticles. (C) 2010 Elsevier B.V. All rights reserved.