Browsing by Author "Yao, Yung-Chi"

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A curvature-tunable random laser
(2019-02-28)
Lee, Ya-Ju
;
Yeh, Ting-Wei
;
Yang, Zu-Po
;
Yao, Yung-Chi
;
Chang, Chen-Yu
The application of random lasers has been restricted due to the absence of a well-defined resonant cavity, as the lasing action mainly depends on multiple light scattering induced by intrinsic disorders of the laser medium to establish the required optical feedback that hence increases the difficulty in efficiently tuning and modulating random lasing emissions. This study investigated whether the transport mean free path of emitted photons within disordered scatterers composed of ZnO nanowires is tunable by a curvature bending applied to the flexible polyethylene terephthalate (PET) substrate underneath, thereby creating a unique light source that can be operated above and below the lasing threshold for desirable spectral emissions. For the first time, the developed curvature-tunable random laser is implemented for in vivo biological imaging with much lower speckle noise compared to the non-lasing situation through simple mechanical bending, which is of great potential for studying the fast-moving physiological phenomenon such as blood flow patterns in mouse ear skin. It is expected that the experimental demonstration of the curvature-tunable random laser can provide a new route to develop disorder-based optoelectronic devices.
A demonstration of solid-state white light-emitting electrochemical cells using the integrated on-chip plasmonic notch filters
(2016)
Lee, Ya-Ju
;
Lin, Chia-Ching
;
Lee, Hsiao-Chin
;
Yao, Yung-Chi
;
Sarma, Monima
In this study, we demonstrate solid-state white light-emitting electrochemical cells (LECs) using an integrated plasmonic notch filter to tailor the electroluminescence (EL) spectrum of non-doped blue-green emissive material. The plasmonic notch filter is composed of randomly distributed silver nanoparticles (Ag-NPs) embedded in the anode contact of indium tin oxide (ITO). This plasmonic notch filter strongly absorbs green light due to local surface plasmon (LSP) resonance of the Ag-NPs embedded in ITO. Thus, the emission green light of the solid-state LEC is strongly suppressed, leaving the blue and red light output to generate a white EL emission. Moreover, the duration of white EL can be maintained for a longer time under operation, which overcomes the issues regarding the short lifetime of white EL generated by the microcavity effect. In addition, the Ag-NPs can be readily fabricated by the thermal annealing of Ag film, which is compatible with current fabrication technologies typically used in light-emitting diode (LED) industry. Therefore, solid-state white LECs using an integrated on-chip plasmonic notch filter have great potential for applications in solid-state lighting.
Achieving graded refractive index by use of ZnO nanorods/TiO2 layer to enhance omnidirectional photovoltaic performances of InGaP/GaAs/Ge triple-junction solar cells
(2015-05-01)
Tsai, Meng-Tsan
;
Yang, Zu-Po
;
Jing, Ting-Shivan
;
Hsieh, Hsing-Hua
;
Yao, Yung-Chi
In this study, we theoretically and experimentally demonstrate high performance of antireflection (AR) coating composed of the ZnO nanorods (NRs) and TiO2 layers applied on InGaP/GaAs/Ge triple-junction solar cells. The high performance of this AR coating is due to the realization of a smooth gradient profile of refractive index fabricated by only two physical layers. First, due to the inherent inhomogeneous-nanoporous geometry along the surface normal, the ZnO NRs are reasonable to be considered as discrete multiple optical layers with low scattering loss, leading to the index smoothly increasing from air ambient toward down to the solar cells. Second, to compensate the gap of index between ZnO and solar cells, an additional TiO2 layer with index in between is hence necessary to insert under ZnO NRs, significantly further reducing the Fresnel reflection loss of the entire device. The ZnO NRs/TiO2 layer shows a low wavelength-averaged (solar spectrum weighted) reflectance of 6.00% (4.78%) over a wide spectral range of lambda=380-1800 nm, and exhibits a hydrophobic surface with a water contact angle of 128.2 degrees. At device level, we compared the photovoltaic performance of solar cells with and without AR coating, the short-circuit density (J(SC)) is enhanced by 31.8% and 23.8% for solar cells integrated with the ZnO NR5/TiO2 layer and conventional SiO2/TiO2 double-layer AR (DLAR) coating, respectively. Under a very large incident angle of solar illumination (theta=80 degrees), the ZnO NRs/TiO2 layer remains static with J(SC) enhancement of 35.2%, whereas the J(SC) enhancement of conventional DLAR coating drops down to 9.4%. In addition, the ZnO NRs/TiO2 layer barely affects the open-circuit voltage and fill factor of the solar device. Therefore, the proposed ZnO NR5/TiO2 layer with a smoother graded refractive index change is highly promising for the AR coating applications for the next-generation solar cells. (C) 2014 Elsevier B.V. All rights reserved.
Adjusting Color Temperature of Electroluminescence from White Light-Emitting Electrochemical Cells by Laser-Scanned Annealing of Nanoparticles
(2017-01-01)
Lee, Hsiao-Chin
;
Chiang, Chien-Ming Fan
;
Wu, Po-Yi
;
Yao, Yung-Chi
;
Sarma, Monima
Color temperature of electroluminescence from white light-emitting electrochemical cells can be adjusted by locally laser-scanned thermal annealing on a semi-continuous thin Ag film embedded between indium tin oxide anode and glass substrate. The proposed technique offers programmable versatility to control white electroluminescence and is compatible with display manufacturing technologies.
Bending-induced tunable threshold in random laser
(2019-01-01)
Lee, Ya-Ju
;
Yeh, Ting-Wei
;
Yang, Zu-Po
;
Yao, Yung-Chi
;
Chang, Chen-Yu
We investigate the case that the transport mean free path of emitted photons within the disordered scatterers, which is composed by the ZnO nanowires, is tunable by a curvature bending applied on the flexible polyethylene terephthalate substrate underneath, thereby creating a unique light source which is able to be operated above and below the lasing threshold for desirable spectral emissions. (C) 2019 The Author(s)
Coherent and Polarized Random Laser Emissions from Colloidal CdSe/ZnS Quantum Dots Plasmonically Coupled to Ellipsoidal Ag Nanoparticles
(2017-02)
Yao, Yung-Chi
;
Yang, Zu-Po
;
Hwang, Jung-Min
;
Su, Hai-Ching
;
Haung, Jing-Yu
This study demonstrates the capability of controlling optical anisotropy of lasing emissions by manipulating the coupling strength spatially and spectrally between the oscillated electric field of emitted light and the localized surface plasmon (LSP) resonance for a random lasing medium composed of colloidal CdSe/ZnS quantum dots (QDs) and ellipsoidal silver nanoparticles (Ag NPs). Distinctive from the amplified spontaneous emission generally observed on colloidal CdSe/ZnS QDs, it has been found that lasing emissions of the revealed system exhibit clear interference features (coherent optical feedbacks) with low-threshold characteristics, mainly attributed to enhanced light scatterings and optical gains arisen in peripheral surfaces of ellipsoidal Ag NPs. Importantly, the relative orientation, between the oscillated electric field of emitted light from colloidal CdSe/ZnS QDs and the ellipse axis of Ag NPs, plays a critical role in selective excitation of LSP resonances leading to laser emissions with preferential optical polarizations. The achievement of spatial and spectral coupling of LSP resonance with colloidal QDs, respectively, by spin coating thin QDs film on Ag NPs and control of the Ag NP geometry makes the present system one of the promising candidates of colloidal QDs-based random lasers to achieve coherent and polarized lasing emissions.
Enhanced external quantum efficiency in GaN-based vertical-type light-emitting diodes by localized surface plasmons
(2016-03-03)
Yao, Yung-Chi
;
Hwang, Jung-Min
;
Yang, Zu-Po
;
Haung, Jing-Yu
;
Lin, Chia-Ching
Enhancement of the external quantum efficiency of a GaN-based vertical-type light emitting diode (VLED) through the coupling of localized surface plasmon (LSP) resonance with the wave-guided mode light is studied. To achieve this experimentally, Ag nanoparticles (NPs), as the LSP resonant source, are drop-casted on the most top layer of waveguide channel, which is composed of hydrothermally synthesized ZnO nanorods capped on the top of GaN-based VLED. Enhanced light-output power and external quantum efficiency are observed, and the amount of enhancement remains steady with the increase of the injected currents. To understand the observations theoretically, the absorption spectra and the electric field distributions of the VLED with and without Ag NPs decorated on ZnO NRs are determined using the finite-difference time-domain (FDTD) method. The results prove that the observation of enhancement of the external quantum efficiency can be attributed to the creation of an extra escape channel for trapped light due to the coupling of the LSP with wave-guided mode light, by which the energy of wave-guided mode light can be transferred to the efficient light scattering center of the LSP.
Enhanced Performance of GaN-based Ultraviolet Light Emitting Diodes by Photon Recycling Using Graphene Quantum Dots
(2017-08-02)
Lin, Tzu-Neng
;
Santiago, Svette Reina Merden
;
Yuan, Chi-Tsu
;
Chiu, Kuo-Pin
;
Shen, Ji-Lin
ablation. The synthesized GQDs can improve the optical and electrical properties of InGaN/InAlGaN UV light emitting diodes (LEDs) remarkably. An enhancement of electroluminescence and a decrease of series resistance of LEDs were observed after incorporation of GQDs on the LED surface. As the GQD concentration is increased, the emitted light (series resistance) in the LED increases (decreases) accordingly. The light output power achieved a maximum increase as high as 71% after introducing GQDs with the concentration of 0.9 mg/ml. The improved performance of LEDs after the introduction of GQDs is explained by the photon recycling through the light extraction from the waveguide mode and the carrier transfer from GQDs to the active layer.
Enhancing UV-emissions through optical and electronic dual-function tuning of Ag nanoparticles hybridized with n-ZnO nanorods/p-GaN heterojunction light-emitting diodes
(2016)
Yao, Yung-Chi
;
Yang, Zu-Po
;
Hwang, Jung-Min
;
Chuang, Yi-Lun
;
Lin, Chia-Ching
ZnO nanorods (NRs) and Ag nanoparticles (NPs) are known to enhance the luminescence of light-emitting diodes (LEDs) through the high directionality of waveguide mode transmission and efficient energy transfer of localized surface plasmon (LSP) resonances, respectively. In this work, we have demonstrated Ag NP-incorporated n-ZnO NRs/p-GaN heterojunctions by facilely hydrothermally growing ZnO NRs on Ag NP-covered GaN, in which the Ag NPs were introduced and randomly distributed on the p-GaN surface to excite the LSP resonances. Compared with the reference LED, the light-output power of the near-band-edge (NBE) emission (ZnO, lambda = 380 nm) of our hybridized structure is increased almost 1.5-2 times and can be further modified in a controlled manner by varying the surface morphology of the surrounding medium of the Ag NPs. The improved light-output power is mainly attributed to the LSP resonance between the NBE emission of ZnO NRs and LSPs in Ag NPs. We also observed different behaviors in the electroluminescence (EL) spectra as the injection current increases for the treatment and reference LEDs. This observation might be attributed to the modification of the energy band diagram for introducing Ag NPs at the interface between n-ZnO NRs and p-GaN. Our results pave the way for developing advanced nano-structured LED devices with high luminescence efficiency in the UV emission regime.
Flexible random lasers with tunable lasing emissions
(2018-06-14)
Lee, Ya-Ju
;
Chou, Chun-Yang
;
Yang, Zu-Po
;
Thi Bich Hanh Nguyen
;
Yao, Yung-Chi
In this study, we experimentally demonstrated a flexible random laser fabricated on a polyethylene terephthalate (PET) substrate with a high degree of tunability in lasing emissions. Random lasing oscillation arises mainly from the resonance coupling between the emitted photons of gain medium (Rhodamine 6G, R6G) and the localized surface plasmon (LSP) of silver nanoprisms (Ag NPRs), which increases the effective cross-section for multiple light scattering, thus stimulating the lasing emissions. More importantly, it was found that the random lasing wavelength is blue-shifted monolithically with the increase in bending strains exerted on the PET substrate, and a maximum shift of similar to 15 nm was achieved in the lasing wavelength, when a 50% bending strain was exerted on the PET substrate. Such observation is highly repeatable and reversible, and this validates that we can control the lasing wavelength by simply bending the flexible substrate decorated with the Ag NPRs. The scattering spectrum of the Ag NPRs was obtained using a dark-field microscope to understand the mechanism for the dependence of the wavelength shift on the exerted bending strains. As a result, we believe that the experimental demonstration of tunable lasing emissions based on the revealed structure is expected to open up a new application field of random lasers.
In Vivo Speckle Contrast Imaging Constructed by Threshold-tunable Random Lasers
(2019-01-01)
Yeh, Ting-Wei
;
Yang, Zu-Po
;
Yao, Yung-Chi
;
Chang, Chen-Yu
;
Tsai, Meng-Tsan
The application of random lasers has been restricted owing to the absence of well-defined resonant cavity, as the lasing action mainly depends on multiple light scatterings induced by intrinsic disorders of laser medium to establish the required optical feedbacks, and hence that increases the difficulty to efficiently tune and modulate random lasing emissions. In this study, we investigate the case that the transport mean free path of emitted photons within the disordered scatterers, which is composed by the ZnO nanowires, is tunable by a curvature bending applied on the flexible polyethylene terephthalate (PET) substrate underneath, thereby creating a unique light source which is able to be operated above and below the lasing threshold for desirable spectral emissions. The developed curvature-tunable random laser, for the first time, is implemented for in vivo biological imaging with much lower speckle noise when compared to the non-lasing situation through a simple mechanical bending, which is of great potential for studying the fast-moving physiological phenomenon like blood flow patterns in mouse's ear skins. As a result, we expect the experimental demonstration on the curvature-tunable random laser can open up a new route towards developing disorder based optoelectronic devices.
Laser-Scanned Programmable Color Temperature of Electroluminescence from White Light-Emitting Electrochemical Cells
(2016-11-23)
Lee, Hsiao-Chin
;
Chiang, Chien-Ming Fan
;
Wu, Po-Yi
;
Yao, Yung-Chi
;
Sarma, Monima
Recently, the control of correlated color temperature (CCT) of artificial solid-state white-light sources starts to attract more attention since CTs affect human physiology and health profoundly. In this work, we proposed and demonstrated a method that can widely tune the CCTs of electroluminescence (EL) from white-light-emitting electrochemical cells (LECs) by employing plasmonic filters. These integrated on-chip plasmonic filters are composed of semicontinuous thin Ag film or Ag nanoparticles (NPs) both included in the indium tin oxide anode contact, which have different characteristics of plasmonic resonant absorptions that can tune the EL spectra of white LECs. The CCTs of EL from white LECs integrated with semicontinuous thin Ag film and randomly distributed Ag NPs are 5778 and 2350 K, respectively. A commercially available laser scanning system was used to locally thermal anneal the semicontinuous thin Ag film to form the randomly distributed Ag NPs on the scanned areas. Hence, these two kinds of filters can be integrated on the same chip of white LEC, giving more freedom to control the CCTs of white EL and more potential applications. In addition, the laser scanning system used here is quite often used in display manufactures so that our proposed method can be immediately adopted by the light-emitting diode industry.
Monolithic integration of GaN-based light-emitting diodes and metal-oxide-semiconductor field-effect transistors
(2014-10-20)
Lee, Ya-Ju
;
Yang, Zu-Po
;
Chen, Pin-Guang
;
Hsieh, Yung-An
;
Yao, Yung-Chi
In this study, we report a novel monolithically integrated GaN-based light-emitting diode (LED) with metal-oxide-semiconductor field-effect transistor (MOSFET). Without additionally introducing complicated epitaxial structures for transistors, the MOSFET is directly fabricated on the exposed n-type GaN layer of the LED after dry etching, and serially connected to the LED through standard semiconductor-manufacturing technologies. Such monolithically integrated LED/MOSFET device is able to circumvent undesirable issues that might be faced by other kinds of integration schemes by growing a transistor on an LED or vice versa. For the performances of resulting device, our monolithically integrated LED/MOSFET device exhibits good characteristics in the modulation of gate voltage and good capability of driving injected current, which are essential for the important applications such as smart lighting, interconnection, and optical communication. (C) 2014 Optical Society of America
Monolithic integration of GaN-based light-emitting diodes and metal-oxide-semiconductor field-effect transistors: reply
(2018-01-22)
Lee, Ya-Ju
;
Yang, Zu-Po
;
Chen, Pin-Guang
;
Hsieh, Yung-An
;
Yao, Yung-Chi
We present some comments to the paper "Monolithic integration of GaN-based light-emitting diodes and metal-oxide-semiconductor field-effect transistors: comment" [Opt. Express 22, A1589 (2014)]. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Numerical Analysis on Polarization-Induced Doping III-Nitride n-i-p Solar Cells
(2015-02-01)
Lee, Ya-Ju
;
Yao, Yung-Chi
;
Yang, Zu-Po
;
光電系統研究所
;
Institute of Photonic System
We design and numerically evaluate a new type of III-nitride n-i-p solar cells whose p-and n-type regions with equal carrier concentration of 3 x 1018 cm(-3) are not generated by extrinsic impurity doping but by the so-called polarization-induced doping, which is induced by the graded InxGa(1-x)N layers of linearly increasing (from x = 0% to 30%) and decreasing (from x = 30% to 0%) indium composition to construct the conductive p-and n-type regions, respectively. Because of the identical and uniform polarization charges within each unit cell, a smooth spatial variation of the potential profile of the device is, hence, expected, which mitigates the energy band discontinuities at heterointerfaces and facilitates transportation and collection of photogenerated carriers with high efficiency. Most importantly, as the conductive n-and p-type regions are formed by electrostatic field ionization but not by the thermal activation, the concentration of field-induced carriers is independent of thermal freeze-out effects. Thus, the polarization-induced doping III-nitride n-i-p solar cells can provide stable power conversion efficiency, even when operated at low temperatures.
Plasmonically Induced Coherent and Polarized Random Laser Emissions in Colloidal CdSe/ZnS Quantum Dots with Ellipsoidal Ag Nanoparticles
(2017-01-01)
Yao, Yung-Chi
;
Yang, Zu-Po
;
Haung, Jing-Yu
;
Lee, Min-Hung
;
Lee, Ya-Ju
We demonstrate the capability of controlling the optical anisotropy of lasing emissions by manipulating the coupling between the oscillated electric field and the localized surface plasmon (LSP) resonance for a random lasing medium composed of colloidal CdSe/ZnS quantum dots (QDs) and ellipsoidal silver nanoparticles (Ag NPs). Distinctive from the amplified spontaneous emission (ASE) generally observed on the CdSe/ZnS QDs, it has been found that lasing emissions of the revealed system exhibit clear interference features with low-threshold characteristics, mainly attributed to enhanced light scatterings and optical gains arisen in the peripheral surfaces of ellipsoidal Ag NPs. Importantly, the relative orientation, between the oscillated electric field of emitted light from CdSe/ZnS QDs and the ellipse axis of Ag NPs, plays a critical role in selective excitation of LSP resonances leading to the laser emissions with preferential optical polarizations. That is further examined and validated by the finite-difference time-domain simulation. The achievement of spatial and spectral coupling of LSP resonance with colloidal QDs respectively make the present system one of the promising candidates of colloidal QDs-based random lasers to achieve coherent and polarized lasing emissions.
Tunable random lasing emissions by manipulating plasmonic coupling strengths on flexible substrates
(2018-01-01)
Yeh, Ting-Wei
;
Chou, Chun-Yang
;
Yang, Zu-Po
;
Nguyen Thi Bich Hanh
;
Yao, Yung-Chi
A random laser is a unique system in which an assembly of disordered scatters is distributed all over the gain medium to sustain the required optical feedbacks by multiple scatterings for the stimulation of lasing action. Due to the absence of well-defined resonance cavity or rigid alignment of optical elements, it is hence difficult to control the random lasing emissions. In this study, we experimentally demonstrated a flexible random laser fabricated on the polyethylene terephthalate (PET) substrate with a high degree of tunability in lasing emissions. Random lasing wavelength is blue-shifted monolithically with the increasing of bending strains exerted on the PET substrate, and the maximum shift of lasing wavelength of similar to 15 nm was achieved as a 50% bending strain is exerted on the PET substrate. Such observation is highly repeatable and reversible, and it validates that we are able to control the lasing wavelengths by simply bending the flexible substrate. The result herein shows a great advance for the applications of flexible optoelectronic devices, including wearable devices, ultrathin display, and health sensors.