Browsing by Author "Chen, Wen-Yen"

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An agent-based metric for quality of services over wireless networks
(2008-10-01)
Chen, Yaw-Chung
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Chen, Wen-Yen
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資訊工程學系
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Department of Computer Science
In a wireless LAN environment, clients tend to associate with the nearest access point (AP) which usually provides the strongest signal. However, this does not guarantee that users will receive the best quality of service (QoS) if the population sharing the network capacity were not considered. In other words, within the same access point, the more the population is, the less bandwidth each user will share, and the worse the quality of service will be. In this paper, we proposed an anticipative agent assistance (AAA) which is an agent-based metric for evaluating and managing the resource information of the wireless access points, computing the potential AP list, and providing clients with resource information of APs. We also propose a novel QoS feedback mechanism which allows users to promptly adjust the service quality with AAA according to the throughput and delay requirements. We evaluate the performance of our proposed method using the ns-2 simulator. Numerical results show that AAA achieves: (1) reduce the transmission delay, (2) increase the throughput, (3) improve the network utilization, (4) accommodate more users to access the networks, and (5) achieve load-balancing. Our metric is implementation feasible in various IEEE WLAN environments. (C) 2008 Published by Elsevier Inc.
An agent-based metric for quality of services over wireless networks
(2006)
Chen, Yaw-Chung
;
Chen, Wen-Yen
;
資訊工程學系
;
Department of Computer Science
In a wireless LAN environment, wireless stations with the strongest received signal can not be guaranteed to have the best quality of service if the population sharing the network capacity was not considered. In other words, within the same access point, the more the population, the less the shared bandwidth, and the worse the quality of service will be. In this paper, we proposed an Anticipative Agent Assistance which is an agent-based metric for evaluating and managing the resource of the wireless access points, computing the potential A-P list, and providing clients with resource information of APs. We also propose a novel QoS feedback mechanism which allows users to promptly adjust the service quality with AAA according to the throughput and delay requirements. We evaluate the performance of our proposed method using the ns-2 simulator. Numerical results show that AAA help reduce the transmission delay, increase the throughput, improve the network utilization, accommodate more users, and provide load-balancing.
Enhancing luminescence efficiency of InAs quantum dots at 1.5 mu m using a carrier blocking layer
(2006-07-31)
Hsieh, Tung-Po
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Chiu, Pei-Chin
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Chyi, Jen-Inn
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Chang, Hsiang-Szu
;
Chen, Wen-Yen
The authors report an effective way to enhance the optical efficiency of InAs quantum dots (QDs) on GaAs emitting at the wavelength of 1.5 mu m. It is found that the loss of holes from QDs to their proximity via the high indium composition InGaAs overgrown layer, which is necessary for achieving long wavelength emission, is the origin of photoluminescence intensity degradation at high temperature. Inserting a 4 nm thick Al0.45Ga0.55As layer, acting as a carrier blocking layer, into the GaAs capping matrix can improve the room temperature photoluminescence peak intensity by five and two times for the ground and first excited states, respectively.
Exciton fine structures and energy transfer in single InGaAs quantum-dot molecules
(2009)
Lin, Hsuan
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Wang, Sheng-Yun
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Lin, Chia-Hsien
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Chang, Wen-Hao
;
Cheng, Shun-Jen
We present a spectroscopic study of single quantum-dot molecules (QDMs) formed by two closely stacked In(0.5)Ga(0.5)As/GaAs layers. It was found that the interdot coupling and directional energy transfer between the two adjacent dots can be controlled by temperature tuning. Direct and indirect excitons, as well as charged excitons in single QDMs were classified and identified by excitation-power dependent, excitation-energy dependent and polarization-resolved microphotoluminescence measurements. With the increasing temperature, the direct-exciton intensity decreases while the indirect-exciton intensity increases. A rate equation model considering phonon mediated processes has been developed. The directional energy transfer in QDMs is explained in terms of the phonon-assisted tunnelling of hole between the two adjacent dots. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Growth of sparse arrays of narrow GaN nanorods hosting spectrally stable InGaN quantum disks
(2012-07-16)
Chen, Yen-Ting
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Tsai, Wen-Che
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Chen, Wen-Yen
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Hsiao, Ching-Lien
;
Hsu, Hsu-Cheng
Wafer-scale production of single InGaN quantum disks (QD) in-a-nanorod array with small rod diameter (> 9 nm) and low rod-density (< 10(8) cm(-2)) has been achieved without extensive processing steps. Excitation power-dependent mu PL spectrum of single QD reveals multi-excitonic peak with 0.75 meV blue-shift for 3 orders of magnitude increasing power, indicating the present system is spectrally stable and nearly free of quantum-confined Stark effects, due possibly to the strain relaxation induced by free surface of small rod diameters. The fully polarized emissions, a high working temperature (180 K), low rod density and good alignment, render this system promising as a potential quantum photon source. (C) 2012 Optical Society of America
Low-density quantum dots embedded in photonic-crystal nanocavities for single-photon generations
(2007)
Chang, Wen-Hao
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Chen, Wen-Yen
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Chang, Hsiang-Szu
;
Hsieh, Tung-Po
;
Chyi, Jen-Inn
An efficient single-photon source based on low-density InGaAs quantum dots in a photonic-crystal nanocavity is demonstrated. The single-photon source features the effects of photonic band gap, yielding a single-mode spontaneous emission coupling efficiency as high as beta=92% and a linear polarization degree up to p=95%. This appealing performance makes it well-suited for practical implementation of polarization-encoded schemes in quantum cryptography.
Nonresonant carrier transfer in single InGaAs/GaAs quantum dot molecules
(2008-06-01)
Chang, Wen-Hao
;
Lin, Hsuan
;
Wang, Sheng-Yun
;
Lin, Chia-Hsien
;
Cheng, Shun-Jen
We present a spectroscopic study of single quantum dot molecules (QDMs) formed by two closely stacked In(0.5)Ga(0.5)As layers. The exciton fine structures as well as direct and indirect excitonic species associated with QDMs were identified by power dependent and polarization resolved microphotoluminescence measurements. As the temperature was increased, a directional energy transfer between the direct and indirect excitons in single QDMs was observed. A rate-equation model was developed to explain our data. We show that a phonon-assisted nonresonant tunneling of the hole between the two adjacent dots is responsible for such directional energy transfers in QDMs.
Origins of nonzero multiple photon emission probability from single quantum dots embedded in photonic crystal nanocavities
(2009-04-20)
Chang, Hsiang-Szu
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Chen, Wen-Yen
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Hsu, Tzu-Min
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Hsieh, Tung-Po
;
Chyi, Jen-Inn
This work explores the origins of nonzero multiphoton emission probability for quantum dots embedded in photonic crystal nanocavities using different excitation energies to inject excitons into either the GaAs barrier or the quantum-dot excited state. The detected multiphoton events are established to arise from both the recapture of excitons and background emissions from the wetting layer tail states. The exciton emission is analyzed using rate-equation calculations, which suggest that multiphoton emission is dominated by the recapture effect.
Single photon emission from an InGaAs quantum dot precisely positioned on a nanoplane
(2007-02-12)
Hsieh, Tung-Po
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Chyi, Jen-Inn
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Chang, Hsiang-Szu
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Chen, Wen-Yen
;
Hsu, Tzu Min
This work demonstrates single photon emissions from a site-controlled quantum dot (QD) grown on a self-constructed nanoplane. The size of the nanoplane on the micron-sized multifacet structure is accurately controlled by a low surface reducing rate (similar to 16 nm/min). Single QD spectral lines were resolved and identified. The antibunching behavior reveals that single photons are emitted from the positioned QD. (c) 2007 American Institute of Physics.