Browsing by Author "Kao, Tsung-Sheng"

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Bridging the "green gap" of LEDs: giant light output enhancement and directional control of LEDs via embedded nano-void photonic crystals
(2016-01-01)
Tsai, Yu-Lin
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Liu, Che-Yu
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Krishnan, Chirenjeevi
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Lin, Da-Wei
;
Chu, You-Chen
Green LEDs do not show the same level of performance as their blue and red cousins, greatly hindering the solid-state lighting development, which is the so-called "green gap". In this work, nano-void photonic crystals (NVPCs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVPCs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boost the light extraction efficiency of LEDs with an enhancement of 78% but also collimate the view angle of LEDs from 131.5 degrees to 114.0 degrees. This could be because of the highly scattering nature of NVPCs which reduce the interference giving rise to Fabry-Perot resonance. Moreover, due to the threading dislocation suppression and strain relief by the NVPCs, the internal quantum efficiency was increased by 25% and droop behavior was reduced from 37.4% to 25.9%. The enhancement of light output power can be achieved as high as 151% at a driving current of 350 mA. Giant light output enhancement and directional control via NVPCs point the way towards a promising avenue of solid-state lighting.
Greatly Improved Carrier Injection in GaN-based VCSEL by Multiple Quantum Barrier Electron Blocking Layer
(2015)
Hsieh, Dan-Hua
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Tzou, An-Jye
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Lin, Da-Wei
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Kao, Tsung-Sheng
;
Lin, Chien-Chung
In this report, the fabrication and characteristics of III-nitride based vertical-cavity surface-emitting laser (VCSEL) with bulk AlGaN and AlGaN/GaN superlattice electron blocking layer (EBL) are observed experimentally and theoretically. The results have been revealed that laser performance is improved by using superlattice EBL. The output power and the slope efficiency are enhanced by the improvement of carrier injection into active region. And the reduction of threshold current density from 10 to 8 kA/cm2 is also observed. Theoretical calculation results suggest that the improved carrier injection efficiency can be mainly attributed to the partial release of the strain at the interface of last quantum barrier and superlattice EBL and hence the increase of electrons and holes effective barrier height.
High-Efficiency InGaN/GaN Core-Shell Nanorod Light-Emitting Diodes With Low-Peak Blueshift and Efficiency Droop
(2017-03)
Tzou, An-Jye
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Hsieh, Dan-Hua
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Hong, Kuo-Bin
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Lin, Da-Wei
;
Huang, Jhih-Kai
In this study, novel three-dimensional (3-D) nanoscale structures and methodology are demonstrated for application in high-efficiency core-shell nanorod (NR) light-emitting diodes (LEDs). The key to our successful growth of the structures is the introduction of passivation, which can be used to selectively grow active layers in desired structures. Through the fabrication methodology, core-shell NR green LEDs exhibiting large nonpolar active region and homogeneous indium distributions without a point tip shape were achieved. Stable light emission at a central wavelength of 518 nm was achieved as the injected current increased to more than 40 mA. The improved NR LEDs exhibited a stable luminescence emission wavelength (blueshift of 62 nm) and low-efficiency droop (18.1%) at 200 mA. Our scheme is scalable and compatible with current technologies, which provides a new perspective for developing high-performance, 3-D nanoscale optoelectronic devices.
Improved optical limiting performance of laser-ablation-generated metal nanoparticles due to silica-microsphere-induced local field enhancement
(2015-05-22)
Du, Zheren
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Chen, Lianwei
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Kao, Tsung-Sheng
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Wu, Mengxue
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Hong, Minghui
For practical application, optical limiting materials must exhibit a fast response and a low threshold in order to be used for the protection of the human eye and electro-optical sensors against intense light. Many nanomaterials have been found to exhibit optical limiting properties. Laser ablation offers the possibility of fabricating nanoparticles from a wide range of target materials. For practical use of these materials, their optical limiting performance, including optical limiting threshold and the ability to efficiently attenuate high intensity light, needs to be improved. In this paper, we fabricate nanoparticles of different metals by laser ablation in liquid. We study the optical nonlinear properties of the laser-generated nanoparticle dispersion. Silica microspheres are used to enhance the optical limiting performance of the nanoparticle dispersion. The change in the optical nonlinear properties of the laser-generated nanoparticle dispersion caused by silica microspheres is studied. It is found that the incident laser beam is locally focused by the microspheres, leading to an increased optical nonlinearity of the nanoparticle dispersion.
Improvement of perovskite photoluminescence characteristics by using a lithography-free metasurface
(2018-01-01)
Chen, Wenjun
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Sha, Jun
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Yan, Kanghong
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Luo, Ji
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Xu, Ruijia
Organic-inorganic metal halid perovskite is an emerging field of promising semiconductive material in optoelectronic devices applications. It has been a research topic of interest for scientists. To date, photoluminescence (PL) efficiency of perovskite is low and does not have any literature to report the solution. Here, we proposed and demonstrated an effective approach for the improvement of perovskite PL characteristics. The PL intensity and spectra bandwidth could be enhanced significantly by using Al-disk metasurface formed on perovskite surface. Such strategy can be exploited to enhance perovskite PL characteristics for developing high efficient light emitting diodes, solar cells, and photodetectors, etc.
Low Coherence Illumination of Flexible Perovskite Random Lasers
(2018-01-01)
Wang, Yu-Chi
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Hong, Yu-Heng
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Hong, Kuo-Bin
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Kao, Tsung-Sheng
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Lu, Tien-Chang
We demonstrated that the methylammonium metal-halide perovskites can be exploited as a random laser source for the advanced speckle-free imaging Furthermore, the lasing performance can be actively controlled via the mechanical modulation of supporting substrates.