Browsing by Author "Lee, H. C."

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Analytic theory for betatron radiation from relativistic electrons in ion plasma channels with magnetic field
(2010-11-01)
Lee, H. C.
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Jiang, T. F.
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物理研究所
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Institute of Physics
We analytically solve the relativistic equation of motion for an electron in ion plasma channels and calculate the corresponding trajectory as well as the synchrotron radiation The relativistic effect on a trajectory is strong, i e, many high-order harmonic terms in the trajectory, when the ratio of the initial transverse velocity (v(x0)) to the longitudinal velocity (v(z0)) of the electron injected to ion plasma channels is high Interestingly, these high-order harmonic terms result in a quite broad and intense radiation spectrum, especially at an oblique angle, in contrast to an earlier understanding As the initial velocity ratio (v(x0) v(z0)) decreases, the relativistic effect becomes weak, only the first and second harmonic terms remain in the transverse and longitudinal trajectories, respectively, which coincides with the result of Esarey et al [Phys Rev E 65, 056505 (2002)] Our formalism also allows the description of electron's trajectory in the presence of an applied magnetic field Critical magnetic fields for cyclotron motions are figured out and compared with semiclassical results The cyclotron motion leads to more high-order harmonic terms than the trajectory without magnetic fields and causes an immensely broad spectrum with vastly large radiation amplitude for high initial velocity ratios (v(x0) v(z0)) The radiation from hard x-ray to gamma-ray regions can be generated with a broad radiation angle, thus available for applications 0 2010 American Institute of Physics [doi 10 1063/1 3496983]
Band structures of a slowly rotating dipolar Bose-Einstein condensate with a quantized vortex along a one-dimensional optical lattice
(2010-06-01)
Lee, H. C.
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Jiang, T. F.
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物理研究所
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Institute of Physics
We derive the effective Gross-Pitaevskii equation for a slowly rotating dipolar Bose-Einstein condensate (BEC) with a quantized vortex along a one-dimensional optical lattice and calculate its band structures. The band structure of a slowly rotating BEC in a lattice becomes interesting when dipole-dipole interaction (DDI) is involved. Under rotation, a dipolar rotating term emerges from the DDI potential. The dipolar rotating term makes a BEC with an attractive DDI more stable than one with a repulsive DDI. The dipolar rotating term changes and generalizes the definition for the type of BEC, which cannot be simply determined by an s-wave scattering length or an effective contact interaction term. The dipolar rotating term also makes the band structure fascinating and tunable. A so-called swallowtail band structure, i.e., a multi-valued solution due to nonlinear interaction, can either elongate or shrink as the band index increases, in contrast to a non-rotating dipolar BEC system with a monotonic dependence. With the dipolar rotating term, various band structures as well as an attractive BEC without collapse can be easily achieved. We demonstrate that a rotating dipolar BEC system subject to an optical lattice combines features of a crystal and a superfluid and promises wide applications.
Characteristics of membrane fouling in submerged membrane bioreactor under sub-critical flux operation
(2008)
Su, Y. C.
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Huang, C. P.
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Pan, Jill R.
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Lee, H. C.
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環境工程研究所
Recently, the membrane bioreactor ( MBR) process has become one of the novel technologies to enhance the performance of biological treatment of wastewater. Membrane bioreactor process uses the membrane unit to replace a sediment tank, and this can greatly enhance treatment performance. However, membrane fouling in MBR restricts its widespread application because it leads to permeate flux decline, making more frequent membrane cleaning and replacement necessary, which then increases operating and maintenance costs. This study investigated the sludge characteristics in membrane fouling under sub- critical flux operation and also assessed the effect of shear stress on membrane fouling. Membrane fouling was slow under sub- critical flux operation. However, as filamentous microbes became dominant in the reactor, membrane fouling increased dramatically due to the increased viscosity and polysaccharides. A close link was found between membrane fouling and the amount of polysaccharides in soluble EPS. The predominant resistance was the cake resistance which could be minimized by increasing the shear stress. However, the resistance of colloids and solutes was not apparently reduced by increasing shear stress. Therefore, smaller particles such as macromolecules ( e. g. polysaccharides) may play an important role in membrane fouling under sub- critical flux operation.
Control mechanism of atomic ionization and high-order harmonic generation assisted by attosecond pulses
(2012-12-01)
Lee, H. C.
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Jiang, T. F.
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物理研究所
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Institute of Physics
Attosecond pulses combined with infrared laser constitute a powerful tool for controlling atomic photoionization and high-order harmonic generation (HHG). We apply the intense-field many-body S-matrix theory to solve such two-pulse excitation problems. The theory can give a clear explanation for the oscillation of ionization probability as a function of time delay between infrared field and attosecond pulses with central frequency lying below ionization threshold at moderate infrared intensities. The HHG assisted by such attosecond pulses is also interpreted. In addition to a known dramatic enhancement of HHG, a harmonic emission from rapid oscillation of bound-state population caused by the counter-rotating wave is presented.
Effect of double heterojunctions on the plasmon-phonon coupling in a GaAs/Al(0.24)Ga(0.76)As quantum well
(2008-12-01)
Lee, H. C.
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Sun, K. W.
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Lee, C. P.
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應用化學系
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電子工程學系及電子研究所
Quasi-two-dimensional (Q2D) Coulomb interactions between plasmons and phonons in a GaAs quantum well are studied numerically. Based on the dielectric continuum model, the plasmon-confined phonon coupling and plasmon-symmetric (+/-) interface phonon coupling are considered. By using renormalized phonon propagators, six branches involving plasmon-like and phonon-like modes are obtained and used to study the effect on the hot-carrier average energy-loss rate (AELR). Plasmon-like modes are demonstrated to enhance the AELR around the sheet carrier density of 10(11) cm(-2) due to a significant contribution of the plasmon-symmetric (+) interface phonon coupled mode. The importance of interface phonon modes on the plasmon-phonon coupling (PPC) is also indicated. If the interface phonon is not considered, the AELR enhancement due to the PPC will not appear. In addition, the carrier temperature is found to be important for the AELR enhancement. As the carrier temperature is as low as possible, the AELR enhances more remarkably because the effect of reabsorption of plasmon-like modes for hot carriers (hot-plasmon effect) can be greatly suppressed.
In0.11Ga0.89N-based p-i-n photodetector
(2009)
Su, Y. K.
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Lee, H. C.
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Lin, J. C.
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Huang, K. C.
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Lin, W. J.
The InGaN photodetector with a In composition of i-In0.11Ga0.89N active layer have been fabricated by MOCVD system. From the photoluminescence measurement, the FWHM of InGaN epitaxial layer was only 13.8 nm. The strong luminescence peak was located at wavelength of 399 nm. We also found that the FWHM of X-ray diffraction of GaN and InGan layer were 218 and 350 arcsec, respectively. Clear photo-responses are also observed in InGaN photodetector with 11% content at responsivity measurement. The cut-off range of spectral responsivity was occurred at 400 nm. The highest spectral responsivity of photodetector was 0.206 A/W at wavelength of 380 nm. The rejection ratio was approached to 103. The external quantum efficiency was about 67%. The current density-voltage (J-V) characteristic of i-InGaN p-i-n photodetector was measured. The dark and photo current density were 1.77x10(-4) A/cm(2) and 4.5x10(-2) A/cm(2) when biased at -3 V, respectively. At photovoltaic characteristics, the V-oc and J(sc) of the i-InGaN photodetector are 1.63 V and 3.1x10(-2) A/cm(2), respectively. The fill factor of i-InGaN p-i-n photodetector was about 37%. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Quantum kinetic equation for spin relaxation and spin Hall effect in GaAs
(2010-01-01)
Lee, H. C.
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Mou, C. -Y.
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物理研究所
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Institute of Physics
We present a general quantum kinetic theory of spin transport based on the Kadanoff- Baym equation (KBE), which we use to study dynamical spin processes in semiconductors right down to femtosecond and nanometer scales. In our application of KBE we describe the evolution of the non-equilibrium 2x2 matrix Green function for carrier spin, averaged over the thermal bath. Spin relaxation effects are treated within the Kadanoff-Baym Ansatz (KBA), while carrier interactions are treated within the random-phase model of screening. We track the detailed oscillation of the spin- polarized carrier state within the coherence time. Our general kinetic approach also allows description of the spin Hall effect when both impurity scattering and the Frohlich interaction are included in the KBE collision term. We find that the level of spin current is very sensitive to the density of impurities, and that the Frohlich interaction can generate a considerable spin current. Significantly, the Frohlich term leads to a unique type of oscillatory behaviour in the spin current that is independent of impurity scattering effects.