Browsing by Author "Lai, CM"

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Analysis of size distributions of type IIZnTe/ZnSe quantum dots
(2004-12-01)
Lee, J
;
Yang, CS
;
Chang, CT
;
Liu, JR
;
Chou, WC
The photoluminescence (PL) spectrum of the ZnTe/ZnSe quantum dot (QD) structure which has a type II band alignment. was investigated. A broader structure peaking at 2.180 eV together with the features of the ZnSe buffer layer located at 2.820 eV in the PL spectrum at 10 K were observed. The broadness of the PL attributes to the spatial inhomogeneity of the QD sizes. The PL spectrum was a normal distribution with a variance of 0.002 eV(2). However, the dot size distribution was a deformed Gaussian. Using only one set of data of dot size distribution measured by the Atomic Force Microscopy (AFM), the statistical estimates with standard errors were evaluated using the bootstrap methodology. The probability distribution was inferred by calculating the means, percentiles and correlation coefficients of the base diameter and the height of the QDs. We concluded that the dot distributions were indeed a deformed Gaussian and the correlation coefficient of the diameter and the height of the QDs was 0.49 +/- 0.01. The uniformity of the dot size distribution was poor. To infer the dot size distribution from the PL spectrum, we numerically solved the Schrodinger equation by elaborating on the orthogonal periodic functions (OPF) approximation for the type II lens shaped QDs. The convergence of the probability density was discussed in a great detail. We found that the band edge discontinuities inside the dot acts as a barrier with DeltaE(v) = 315 meV for the conduction band and as a well with DeltaE(v) = 735 meV for the valence band. In the AFM measurement, the offset in dot height is 14 Angstrom and the aspect ratio is 0.04. We employed the Schrodinger equation to correlated the PL spectrum (intensity versus optical energy) to the AFM data (number of QDs versus dot size) and the agreement was excellent. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Bipolaron-induced third-order optical nonlinearity in conjugated conducting polymers
(1996-12-15)
Lai, CM
;
Meng, HF
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A mechanism, due to the presence of bipolarons, for large resonant chi((3)) in doped cis-polyacetylene and other conjugated conducting polymers is presented. Under optical pumping, the electronic occupations of the two intragap energy levels of the bipolarons will vary, resulting in a shift in their energies, which in turn causes a dramatic change in the absorption spectrum. Even for moderate doping concentrations of bipolarons, chi((3)) is found to be as high as 10(-8) esu.
Bipolaron-induced third-order optical nonlinearity in conjugated conducting polymers
(1996-12-15)
Lai, CM
;
Meng, HF
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A mechanism, due to the presence of bipolarons, for large resonant chi((3)) in doped cis-polyacetylene and other conjugated conducting polymers is presented. Under optical pumping, the electronic occupations of the two intragap energy levels of the bipolarons will vary, resulting in a shift in their energies, which in turn causes a dramatic change in the absorption spectrum. Even for moderate doping concentrations of bipolarons, chi((3)) is found to be as high as 10(-8) esu.
Bond length order parameter of conjugated polymers
(1999-11-01)
Lai, CM
;
Meng, HF
;
物理研究所
;
Institute of Physics
It is usually assumed that the ground state lattice configuration of polyacetylene is a dimerized chain. We found that the energy can be further reduced by an overall chain length contraction, which leads to a new bond length order parameter, in addition to the well-known bond alternation order parameter. For quasi-particles like solitons and polarons, these two order parameters are coupled with the electron wave functions and vary in space. We generalize the Takayama-Liu-Maki equations in the continuum limit to incorporate this new order parameter. Significant modifications are found for the polaron solution. Polyacene, a ladder polymer, is also studied with the inclusion of the bond length order parameter. The ground state is predicted to be a non-alternating structure, with zero energy gap and unusually high electric conductivity. (C) 1999 Elsevier Science B.V. All rights reserved.
Continuous-wave high-power (320 mW) single mode operation of electronic vertically coupled InAs/GaAs quantum dot narrow-ridge-waveguide lasers
(2005-12-01)
Wang, JS
;
Lin, G
;
Hsiao, RS
;
Yang, CS
;
Lai, CM
High-power 3 mu m-wide narrow-ridge-waveguide lasers with ten stacks of electronic vertically coupled InAs/GaAs quantum dots in the active region were demonstrated. Unlike that from conventional uncoupled InAs quantum dot lasers, a narrow lasing spectrum was observed because the carriers tunneled in the vertical direction. Continuous-wave operation in single lateral mode yielded a kink-free output power of 320 mW with an efficiency of 0.46 W/A , and a sensitivity of lasing wavelength to temperature of 0.28 nm/K.
Evaluating the impact of spherical aberration on sub-0.2-micron contact/via hole patterning
(2001)
Chou, SY
;
Lou, JC
;
Lai, CM
;
Liang, FJ
;
Chen, LJ
Several super resolution techniques, such as phase-shifting mask (PSM) and off-axis illumination (OAI), have been reported to extend the resolution limit and increase the depth-of-focus ( DOF) of optical lithography. However, these techniques provide less immunity to spherical aberration than the conventional approaches like chrome binary mask and low coherent illumination. Best focus position shift is the most well known anomalous phenomenon resulted from spherical aberration. In this paper, the origin of best focus shift is explained in pictorial and analytical forms. The phenomenon is evaluated by observing the exposure-defocus windows of sub-0.2mum hole patterns from an 18 % transmission rim-type attenuated PSM combined with several types of illumination. Under high coherent illumination, severe focus shift was observed in sparse patterns as strong phase-shifting effect is applied. For dense hole patterns,, OAI results in abrupt focus position variation at specific pattern pitch. The experimental results show that spherical aberration would induce best focus shift, distortion of process windows, loss of DOF, and shrink-age of iso/dense process window overlap. Two approaches were proposed to suppress the impact of spherical aberration. One is introducing proper amount of phase bias in attenuated PSM to adjust the wave aberration in the lens. The other more feasible method is using a customized illumination. A synthesized illumination aperture was proposed to compensate the focus shift. Excellent lithographic performance was obtained in the experiment from this method.
Molecular-beam-epitaxy growth of high-quality InGaAsN/GaAs quantum well lasers emitting at 1.3 mu m
(2004-11-01)
Wang, JS
;
Hsiao, RS
;
Lin, G
;
Lin, KF
;
Liu, HY
Molecular-beam-epitaxy growth of high structural and optical-quality InGaAsN/GaAs quantum wells (QW) has been investigated. The material quality can be improved significantly by using low-temperature growth to suppress the phase separation. High-performance ridge-waveguide InGaAsN/GaAs single QW lasers emitting at 1.3 mum have been demonstrated. Infinite-cavity-length threshold-current density of 400 A /cm(2), internal quantum efficiency of 96%. and a slope efficiency of 0.67 W/A for a cavity length L=1 mm were obtained. A TO46 packaging laser shows sin-le lateral-mode kink-free output power of more than 200 mW with a maximum total wallplug efficiency of 29% at room temperature under continuous wave (cw) operation. Moreover. 1.3 Am InGaAsN/GaAs QW vertical-cavity surface-emitting lasers with a threshold current density lower than 2 KA/cm(2) at room temperature have been achieved. We obtained multimode cw output power and slope efficiency in excess of 1 mW and 0.15 W/A. respectively. (C) 2004 American Vacuum Society.
Resonant ultrafast nonlinear optics of bulk semiconductors under electric-field modulation
(1998-05-15)
Lai, CM
;
Meng, HF
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
In bulk direct-band-gap semiconductors the ground state of a (photoexcited) electron-hole system is normally an exciton gas, instead of an electron-hole plasma, as long as the carrier density is not too large. We demonstrate theoretically that, in the presence of an electric-field modulation, the energy of the plasma is decreased due to the induced charge-density fluctuation. As a results, the plasma becomes the ground state at much lower carrier densities, implying that the critical carrier density for the transition between these two states can be reduced. This transition provides a mechanism of nonlinear optics for bulk direct-band-gap semiconductors with picosecond relaxation time, large nonlinearity, and low pump intensity.
Single mode 1.3 mu m InGaAsN/GaAs quantum well vertical cavity surface emitting lasers grown by molecular beam epitaxy
(2004-12-01)
Hsiao, RS
;
Wang, JS
;
Lin, KF
;
Wei, L
;
Liu, HY
The growth by molecular-beam-epitaxy of high-quality 1.3 mum InGaAsN/GaAs quantum wells (QW) intra-cavity contacted vertical cavity surface emitting lasers (VCSELs) was demonstrated. Low-temperature growth, which suppresses the phase separation significantly improves material quality in the active region. Room-temperature continuous wave (RT-CW) single mode output power of 0.75 mW with an initial slope efficiency of 0.17 W/A and a side mode suppression ratio of 40 dB at a lasing wavelength of as long as 1304 nm were obtained.
Squeezed light and its photocurrent fluctuations
(1996)
Meng, HF
;
Lai, CM
;
物理研究所
;
Institute of Physics
Translationally invariant polarons in conjugated conducting polymers
(1997-05-15)
Meng, HF
;
Lai, CM
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A simple effective Hamiltonian for the electron-phonon coupling in the conjugated polymers is derived from the Su-Schrieffer-Heeger model with quantized lattice motions. Phonons correspond to lattice quantum fluctuations around the dimerized positions. Both acoustic and optical modes are considered. The energy-momentum relation for the polaron is calculated variationally, with the aid of Lee-Low-Pines transformation. The discrete translational invariance of an ideal conjugated polymer is. explicitly enforced. Analytical expression for the polaron effective mass m* is obtained. m* turns out to be, in general, much larger than the conduction-band effective mass.
Translationally invariant polarons in conjugated conducting polymers
(1997-05-15)
Meng, HF
;
Lai, CM
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
A simple effective Hamiltonian for the electron-phonon coupling in the conjugated polymers is derived from the Su-Schrieffer-Heeger model with quantized lattice motions. Phonons correspond to lattice quantum fluctuations around the dimerized positions. Both acoustic and optical modes are considered. The energy-momentum relation for the polaron is calculated variationally, with the aid of Lee-Low-Pines transformation. The discrete translational invariance of an ideal conjugated polymer is. explicitly enforced. Analytical expression for the polaron effective mass m* is obtained. m* turns out to be, in general, much larger than the conduction-band effective mass.
Ultrafast nonlinear optics due to electron-hole plasma in bulk semiconductors
(1997-09-01)
Meng, HF
;
Lai, CM
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
It is theoretically demonstrated that, in the presence of an electric potential landscape, the ground-state energy of the electron-hole plasma can be substantially lowered against the ground-state energy of the excitons. The electron-hole plasma is found to be unstable at lower carrier densities. The critical carrier density for the transition between these two states can be reduced. This reduction provides a new mechanism of nonlinear optics for bulk direct band-gap semiconductors, with picosecond relaxation time, large nonlinearity, and requiring low pump intensity.
Unsupervised fuzzy neural network structural active pulse controller
(2001-04-01)
Hung, SL
;
Lai, CM
;
土木工程學系
;
Department of Civil Engineering
Applying active control systems to civil engineering structures subjected to dynamic loading has received increasing interest. This study proposes an active pulse control model, termed unsupervised fuzzy neural network structural active pulse controller (UFN-SAP controller), for controlling civil engineering structures under dynamic loading. The proposed controller combines an unsupervised neural network classification (UNC) model, an unsupervised fuzzy neural network (UFN) reasoning model, and an active pulse control strategy. The UFN-SAP controller minimizes structural cumulative responses during earthquakes by applying active pulse control forces determined via the UFN model based on the clusters, classified through the UNC model, with their corresponding control forces. Herein, we assume that the effect of the pulses on structure is delayed until just before the next sampling time so that the control force can be calculated in time, and applied. The UFN-SAP controller also averts the difficulty of obtaining system parameters for a real structure for the algorithm to allow active structural control. Illustrative examples reveal significant reductions in cumulative structural responses, proving the feasibility of applying the adaptive unsupervised neural network with the fuzzy classification approach to control civil engineering structures under dynamic loading. Copyright (C) 2001 John Wiley & Sons, Ltd.