Browsing by Author "Tang, Jau"

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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.
A highly efficient graphene oxide absorber for Q-switched Nd:GdVO(4) lasers
(2011-11-11)
Wang, Yong Gang
;
Chen, Hou Ren
;
Wen, Xiao Ming
;
Hsieh, Wen Feng
;
Tang, Jau
We demonstrated that graphene oxide material could be used as a highly efficient saturable absorber for the Q-switched Nd:GdVO(4) laser. A novel and low-cost graphene oxide (GO) absorber was fabricated by a vertical evaporation technique and high viscosity of polyvinyl alcohol (PVA) aqueous solution. A piece of GO/PVA absorber, a piece of round quartz, and an output coupler mirror were combined to be a sandwich structure passive component. Using such a structure, 104 ns pulses and 1.22 W average output power were obtained with the maximum pulse energy at 2 mu J and a slope efficiency of 17%.
A highly efficient graphene oxide absorber for Q-switched Nd:GdVO4 lasers
(2011-11-11)
Wang, Yong Gang
;
Chen, Hou Ren
;
Wen, Xiao Ming
;
Hsieh, Wen Feng
;
Tang, Jau
We demonstrated that graphene oxide material could be used as a highly efficient saturable absorber for the Q-switched Nd:GdVO4 laser. A novel and low-cost graphene oxide (GO) absorber was fabricated by a vertical evaporation technique and high viscosity of polyvinyl alcohol (PVA) aqueous solution. A piece of GO/PVA absorber, a piece of round quartz, and an output coupler mirror were combined to be a sandwich structure passive component. Using such a structure, 104 ns pulses and 1.22 W average output power were obtained with the maximum pulse energy at 2 mu J and a slope efficiency of 17%.
Antibunching Single-Photon Emission and Blinking Suppression of CdSe/ZnS Quantum Dots
(2009-10-01)
Yuan, C. T.
;
Yu, Pyng
;
Ko, H. C.
;
Huang, J.
;
Tang, Jau
We demonstrated that by property coupling to silver nanoprisms, single CdSe/ZnS semiconductor quantum dots (QDs) exhibited suppressed blinking behavior, an enhanced fluorescence intensity (similar to 2.5 fold), increased radiative decay rates (similar to 12.5,fold), and antibunching single-photon emission. All these modifications significantly promote the overall performance of the proposed single-photon sources based on colloidal semiconductor QDs.
Blinking Suppression and Anti-Bunching of Quantum Dots as Single-Photon Sources
(2010-01-01)
Yu, Pyng
;
Yuan, Chi-Tsu
;
Ko, Hsien-Chien
;
Huang, Jane
;
Tang, Jau
We report here our study on suppression of fluorescence blinking of colloidal CdSe/ZnS quantum dots (QDs) for potential applications as high performance single-photon sources. Blinking is an interesting property for single QDs, but it is undesirable for their applications. We have demonstrated that by coupling these QDs to adjacent silver nanoprisms, we could not only completely suppress the blinking but also enhance the fluorescence quantum yield, and also increase the fluorescence decay rates. These single QDs also exhibit anti-bunching behavior which is a signature for a single-photon emitter. In addition, we have also achieved blinking suppression by embedded the QDs in agarose gel. The electrostatic environment around QDs due to negatively charged fibers of gel might strongly affect the extent of blinking suppression. The mechanisms of blinking suppression will be discussed in the frame work of diffusion-controlled electron transfer model.
Blinking suppression of colloidal CdSe/ZnS quantum dots by coupling to silver nanoprisms
(2009-06-15)
Yuan, C. T.
;
Yu, Pyng
;
Tang, Jau
;
光電工程學系
;
Department of Photonics
Fluorescence blinking of quantum dots (QDs) is a common but annoying behavior for practical applications as fluorescent markers or single photon sources. We demonstrate here, by coupling colloidal CdSe/ZnS QDs to silver nanoprisms, fluorescence blinking behavior can be suppressed. For most of coupled QDs, two emissive states with different fluorescence intensities and lifetime were observed. We suggest that energy transfer could play an important role in suppressing blinking behavior.
Blinking suppression of single quantum dots in agarose gel
(2010-01-04)
Ko, H. C.
;
Yuan, C. T.
;
Lin, S. H.
;
Tang, Jau
;
應用化學系
Fluorescence blinking is commonly observed in single molecule/particle spectroscopy, but it is an undesirable feature in many applications. We demonstrated that single CdSe/ZnS quantum dots in agarose gel exhibited suppressed blinking behavior. In addition, the long-time exponential bending tail of the power-law blinking statistics was found to be influenced by agarose gel concentration. We suggest that electron transfer from the light state to the dark state might be blocked due to electrostatic surrounding of gel with inherent negatively charged fibers.
Blocked Electron Transfer and Suppressed Blinking of Single CdSe/ZnS Quantum Dots in Agarose Gel
(2010)
Ko, H. C.
;
Yuan, C. T.
;
Lin, S. H.
;
Tang, Jau
;
應用化學系
Fluorescence blinking is an interesting phenomenon commonly observed in single molecule/particle spectroscopy and has attracted wide interest recently. Such behavior in quantum dots is undesirable for applications as fluorescence markers. In this report, we observed suppressed blinking for single CdSe/ZnS quantum dots mixed in agarose gel We attributed the suppression to an increase in activation energy so that the charge transfer is blocked from the neutral light state to the charged dark state as the gel concentration increases
Carrier dynamics in InN nanorod arrays
(2012-01-16)
Ahn, Hyeyoung
;
Yu, Chih-Cheng
;
Yu, Pyng
;
Tang, Jau
;
Hong, Yu-Liang
In this report, we investigated ultrafast carrier dynamics of vertically aligned indium nitride (InN) nanorod (NR) arrays grown by molecular-beam epitaxy on Si(111) substrates. Dominant band filling effects were observed and were attributed to a partial bleaching of absorption at the probe wavelengths near the absorption edge. Carrier relaxation in nanorod samples was strongly dependent on the rod size and length. In particular, a fast initial decay was observed for carriers in NRs with a small diameter (similar to 30 nm), the lifetime of which is much shorter than the carrier cooling time, demonstrating the substantial surface-associated influence on carrier relaxation in semiconductor nanostructures. (C)2012 Optical Society of America
Dynamic study on the transformation process of gold nanoclusters
(2014-11-07)
Ma, Xiaoqian
;
Wen, Xiaoming
;
Toh, Yon-Rui
;
Huang, Kuo-Yen
;
Tang, Jau
In this paper, the transformation process from Au-8 to Au-25 nanoclusters (NCs) is investigated with steady state fluorescence spectroscopy and time-resolved fluorescence spectroscopy at various reaction temperatures and solvent diffusivities. Results demonstrate that Au-8 NCs, protected by bovine serum albumin, transform into Au-25 NCs under controlled pH values through an endothermic reaction with the activation energy of 74 kJ mol(-1). Meanwhile, the characteristic s-shaped curves describing the formation of Au-25 NCs suggest this process involves a diffusion controlled growth mechanism.
Environmental effects on photoinduced electron transfer and fluorescence blinking of single semiconducting nanocrystals in various matrices
(2010-01-01)
Ko, H. C.
;
Yuan, C. T.
;
Yu, Ping
;
Huang, Jane
;
Tang, Jau
In this report we explore how the surrounding environments around a single semiconducting nancrystal affect the photoinduced electron transfer, charge trapping fluorescence lifetime and fluorescence blinking. Using the time-correlated single photon counting techniques combined with confocal microscopy, we investigated photoluminescence of single CdSe/ZnS quantum dots embedded in agarose gel and on conductive substrates as examples. Understanding of the underlying mechanisms would allow us to better control of the photoluminescence properties of nanoparticles and to improve their performance in biophotonics and optoelectronics applications such as fluorescence markers, single-photon sources, photovoltaics and quantum dot lasers.
Four-dimensional imaging of carrier interface dynamics in p-n junctions
(2015-01-09)
Najafi, Ebrahim
;
Scarborough, Timothy D.
;
Tang, Jau
;
Zewail, Ahmed
;
光電工程學系
The dynamics of charge transfer at interfaces are fundamental to the understanding of many processes, including light conversion to chemical energy. Here, we report imaging of charge carrier excitation, transport, and recombination in a silicon p-n junction, where the interface is well defined on the nanoscale. The recorded images elucidate the spatiotemporal behavior of carrier density after optical excitation. We show that carrier separation in the p-n junction extends far beyond the depletion layer, contrary to the expected results from the widely accepted drift-diffusion model, and that localization of carrier density across the junction takes place for up to tens of nanoseconds, depending on the laser fluence. The observations reveal a ballistic-type motion, and we provide a model that accounts for the spatiotemporal density localization across the junction.
High-power, passively mode-locked Nd:GdVO(4) laser using single-walled carbon nanotubes as saturable absorber
(2011-04-01)
Chen, Hou-Ren
;
Wang, Yong-Gang
;
Tsai, Chih-Ya
;
Lin, Kuei-Huei
;
Chang, Teng-Yao
We use a new (to our knowledge) fabrication method of a single-walled carbon nanotube (SWCNT) absorber without polymer to sustain high-power illumination. Using a series of saturable absorbers (SAs) incorporating different amounts of SWCNTs, we demonstrate mode-locking for a Nd:GdVO(4) laser in the 1 mu m spectral range. Continuous-wave mode-locking (CWML) pulses with a maximum output power of 3.6 W at 1063 nm and high noise extinction of 61 dB has been achieved to give the highest pulse peak power of 3.6 kW and pulse energy of 30 nJ under 15 W pumping. To our knowledge, this is the highest CWML output power with SWCNT-SAs reported. The measured nonlinear absorption of the SWCNT-SAs shows a modulation depth of similar to 3% with subpicosecond recovery time. (C) 2011 Optical Society of America
High-power, passively mode-locked Nd:GdVO4 laser using single-walled carbon nanotubes as saturable absorber
(2011-04-01)
Chen, Hou-Ren
;
Wang, Yong-Gang
;
Tsai, Chih-Ya
;
Lin, Kuei-Huei
;
Chang, Teng-Yao
We use a new (to our knowledge) fabrication method of a single-walled carbon nanotube (SWCNT) absorber without polymer to sustain high-power illumination. Using a series of saturable absorbers (SAs) incorporating different amounts of SWCNTs, we demonstrate mode-locking for a Nd:GdVO4 laser in the 1 mu m spectral range. Continuous-wave mode-locking (CWML) pulses with a maximum output power of 3.6 W at 1063 nm and high noise extinction of 61 dB has been achieved to give the highest pulse peak power of 3.6 kW and pulse energy of 30 nJ under 15 W pumping. To our knowledge, this is the highest CWML output power with SWCNT-SAs reported. The measured nonlinear absorption of the SWCNT-SAs shows a modulation depth of similar to 3% with subpicosecond recovery time. (C) 2011 Optical Society of America
Observation of Inverted Regime Electron Transfer in CdSe/ZnS QDs from pH-Sensitive Single-Particle and Ensemble Fluorescence Measurements
(2011-07-28)
Ko, H. C.
;
Yuan, C. T.
;
Lin, S. H.
;
Tang, Jau
;
應用化學系
In this report, we investigated the pH dependence of the photoluminescence of CdSe/ZnS quantum dots (QDs). We present experimental results and theoretical analysis of both the blinking behavior of single-QDs as well as the fluorescence intensity time trace from an ensemble of QDs in agarose gel fibers at different pH environments. Such a combined approach of confined QDs by single-particle and ensemble measurements has not been used previously. This study allows us to elucidate the electron transfer processes from the light state to the dark state. The observed increase in both the activation energy for the charge transfer and the free energy gap between the light and the dark states at an increased concentration of H(+) ions supported the Marcus inverted-regime, electron transfer.
Optical properties of gold particle-cluster core-satellite nanoassemblies
(2013)
Yu, Pyng
;
Wen, Xiaoming
;
Toh, Yon-Rui
;
Lee, Yu-Chieh
;
Tang, Jau
Better understanding of core-satellite nanostructures is of great interest to researchers owing to their unusual properties and is key to their promising applications. The well-developed techniques based on protein-nanoparticle interactions were adopted to produce core-satellite nanoassemblies of similar to 10 nm core of Au nanoparticles (AuNPs) covered by Au-25@BSA or Au-10@histidine nanoclusters (NCs). The photoexcited dynamics in the core-satellite nanoassemblies were studied using steady-state and time-resolved spectroscopic measurements. Fluorescence quenching in AuNP-AuNCs core-satellite nanostructures was observed and confirmed as static quenching. The AuNPs alter neither radiative decay nor nonradiative decay in both nanoclusters. This indicates that the electron/energy transfer within the NP-NCs core-satellite nanoassemblies is absent, in contrast to the results involving other larger NP-NPs core-satellite nanoassemblies.
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]
Quantum Confined Stark Effect in Au-8 and Au-25 Nanoclusters
(2013-02-21)
Wen, Xiaoming
;
Yu, Pyng
;
Toh, Yon-Rui
;
Tang, Jau
;
光電工程學系
The quantum confined Stark effect is investigated for the first time in bovine serum albumin (BSA) protected Au-8 and Au-25 nanoclusters. We observed a red-shift of 63 meV in Au-8 nanoclusters upon an increase in pH from 2.14 to 12.0. Such behavior could be well explained in terms of the presence of a linear polar component and a quadratic polarizable component. In contrast, Au-25 nanoclusters exhibit more complicated Stark shifts due to their specific core/semiring structure. A plateau of the Stark shift was observed in both absorption and fluorescence, showing an offset of 30 meV. The lifetime measurements confirm that the plateau is due to the screening effect of the semirings in Au-25@BSA. Moreover, the dual fluorescent bands of Au-25 nanoclusters exhibit two different Stark shifts of 79 and 52 meV, respectively. The experimental data indicate that the Stark shift in both Au-8@BSA and Au-25@BSA has a significant linear polar component due to their asymmetric structure. This study suggests that gold nanoclusters can become a potentially useful candidate in probing local electric fields and also in pH-sensing in nanoscale environment of biological systems.
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.