Browsing by Author "Lee, Ya-Ju"

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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.
Elucidating the Physical Property of the InGaN Nanorod Light-Emitting Diode: Large Tunneling Effect
(2011-07-01)
Lee, Ya-Ju
;
Lee, Chia-Jung
;
Chen, Chih-Hao
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
The current-voltage (I-V) characteristics of the InGaN nanorod light-emitting diode (LED) are evaluated using nanoprobes installed on a field-emission scanning electron microscope. The saturated current of the InGaN nanorod LED (I(O) = 2 x 10(-9) A) is found to be orders of magnitude higher than those obtained by downscaling a conventional InGaN LED to a chip size of 300 x 300 mu m(2) (I(O1) = 1 x 10(-25) A; I(O2) = 1 x 10(-14) A). This observation is explained by the fact that the nanorod LED is associated with enhanced tunneling of injected carriers and, therefore, reduction of the defect-assisted leakage current and the diffusion-recombination process that normally occurs in InGaN LEDs.
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 extracted electroluminescence from light-emitting electrochemical cells by employing high-refractive-index substrates
(2017-12-01)
Jang, Yea-Fen
;
Lin, Tzu-Chun
;
Guo, Jhih-Yan
;
Chiang, Chien-Ming Fan
;
Wu, Ming-Lun
Solidstate lightemitting electrochemical cells (LECs) show several advantages over conventional organic lightemitting devices (OLEDs) such as simple device structure compatible with solution processes, low operation voltage and capability of utilizing inert cathode metals. However, device performance of LECs must be improved, e.g. enhancing light extraction, to meet the requirements for practical applications. Among the optical modes trapped in LECs, light trapped in substrate mode is easier to be extracted, e.g., by simply roughing the output surface. Therefore, increasing the percentage of substrate mode is beneficial in improving light extraction. In this work, the contributions of optical modes in LECs employing substrates with various refractive indices are analyzed. Higherrefractiveindex substrates are shown to trap more light in the substrates. Smaller refractive index difference between higher-refractive-index substrate and indium tin oxide (ITO) layer also increases the cutoff spectral range of light waveguided in ITO layer. Furthermore, light intensity in surface plasmon mode significantly reduces as the refractive index of the substrate increases. Reducing the percentage of surface plasmon mode facilitates light extraction since it requires more complicated methods for outcoupling light in this mode. With commercially available unpolished sapphire substrates, light output of LECs is enhanced by 56%. When a scattering layer was inserted between ITO and sapphire substrate, more light in substrate mode can be extracted and 71% enhancement in light output is realized. High external quantum efficiency up to 5.5% is consequently obtained in LECs based on a ruthenium complex. Such device efficiency is among the highest reported values for redemitting LECs and thus confirms that utilizing higher-refractive-index substrates would offer a simple and feasible approach to improve light output of LECs. In comparison to OLEDs, increased EL trapped in substrates of LECs mainly comes from surface plasmon mode rather than waveguide mode. (C) 2017 Elsevier B.V. All rights reserved.
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.
Graphene Quantum Dot Vertical Cavity Surface-Emitting Lasers
(2019-11-01)
Lee, Ya-Ju
;
Yeh, Ting-Wei
;
Zou, Chen
;
Yang, Zu-Po
;
Chen, Jia-Wen
Nonzero-bandgap graphene quantum dots (GQDs) are novel optical gain materials promising for solution-processed light sources with high cost efficiency and device performance. Currently, there have only been a few reports on the realization of GQDs-based lasers. Herein, we demonstrate for the first time room-temperature lasing emission with green gamut from GQDs in a vertical optical cavity composed of Ta2O5/SiO2 dielectric distributed Bragg reflectors (DBRs). The lasing is enabled by the unique design of the DBR, which not only provides a wide stopband spectrally overlapping with the emission of the GQDs but also allows high transmittance of optical excitation in the UV-light region. This demonstration is a clear evidence of the use of GQDs as optical gain materials and represents an important step forward toward their potential applications in wide-gamut laser displays and projectors.
High brightness GaN-based light-emitting diodes
(2007-06-01)
Lee, Ya-Ju
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
;
Wang, Shing-Chung
;
光電工程學系
This paper reviews our recent progress of GaN-based high brightness light-emitting diodes (LEDs). Firstly, by adopting chemical wet etching patterned sapphire substrates in GaN-based LEDs, not only could increase the extraction quantum efficiency, but also improve the internal quantum efficiency. Secondly, we present a high light-extraction 465-nm GaN-based vertical light-emitting diode structure with double diffuse surfaces. The external quantum efficiency was demonstrated to be about 40%. The high performance LED was achieved mainly due to the strong guided-light scattering efficiency while employing double diffuse surfaces.
High light-extraction GaN-based vertical LEDs with double diffuse surfaces
(2006-11-01)
Lee, Ya-Ju
;
Kuo, Hao-Chung
;
Lu, Tien-Chang
;
Wang, Shing-Chung
;
光電工程學系
High light-extraction (external quantum efficiency similar to 40%) 465-nm GaN-based vertical light-emitting diodes (LEDs) employing double diffuse surfaces were fabricated. This novel LED structure includes one top transmitted diffuse surface and another diffuse omnidirectional reflector (ODR) on the bottom of a LED chip. The diffusive ODR consists of a roughened p-type GaN layer, an indium-tin-oxide (ITO) low refractive index layer, and an Al layer. The surface of the p-type GaN-layer was naturally roughened while decreasing the growth temperature to 800 degrees C. After flip-bonding onto a Si substrate by AuSn eutectic metal and laser lift-off processes to remove the sapphire substrate, an anisotropic etching by dilute potassium hydroxide (KOH) was employed on the N-face n-GaN layer to obtain transmitted diffuse surfaces with hexagonal-cone morphology. The double diffused surfaces LEDs show an enhancement of 56% and 236% in light output power compared to single side diffused surface and conventional LEDs, respectively. The devices also show a low leakage current in the order of magnitude of 10(-8) A at -5 V and a calculated external quantum efficiency of about 40%. The high scattering efficiency of. double diffused surfaces could be responsible for the enhancement in the device light output power.
High output power density from GaN-based two-dimensional nanorod light-emitting diode arrays
(2009-04-06)
Lee, Ya-Ju
;
Lin, Shawn-Yu
;
Chiu, Ching-Hua
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
Here we propose and realize a scheme for making a direct contact to a two-dimensional nanorod light-emitting diode (LED) array using the oblique-angle deposition approach. And, more importantly, we demonstrate highly efficient electrical carrier injection into the nanorods. As a result, we show that at a 20 mA dc current injection, the light output power density of our nanorod LED array is 3700 mW cm(-2). More general, this contact scheme will pave the ways for making direct contacts to other kinds of nanoscale optoelectronic devices.
High output power density from GaN-based two-dimensional nanorod light-emitting diode arrays (vol 94, 141111, 2009)
(2009-05-25)
Lee, Ya-Ju
;
Lin, Shawn-Yu
;
Chiu, Ching-Hua
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
Improvement of quantum efficiency in green light-emitting diodes with pre-TMIn flow treatment
(2011-06-08)
Lee, Ya-Ju
;
Chen, Yi-Ching
;
Lu, Tien-Chang
;
光電工程學系
;
Department of Photonics
The effects of pre-trimethlyindium (TMIn) flow on the improved electrical characteristics and highly stable temperature properties of InGaN green light-emitting diodes (LEDs) are discussed. For the LED sample with a pre-TMIn flow treatment, the tunnelling of injected carriers associated with threading defects is significantly reduced, which promotes the diffusion-recombination of injected carriers, as well as the overall emission efficiency of the LED. In addition, the pre-TMIn flow treatment evidently reduces the dependence of external quantum efficiency on temperature and efficiency droop of green LEDs. As a result, we conclude that the pre-TMIn flow treatment is a promising scheme for the improvement of output performance of InGaN-based green LEDs.
Improving color saturation of blue light-emitting electrochemical cells by plasmonic filters
(2017-12-01)
Chiang, Chien-Ming Fan
;
Chang, Bo-Ren
;
Lee, Ya-Ju
;
Sarma, Monima
;
Yang, Zu-Po
In consideration of the advantages of light-emitting electrochemical cells (LECs), it is desired to develop the saturated blue LECs for LEC display, which is hindered by the features of broad emission spectrum and emission peak not short enough. In this study, we demonstrated a novel method to improve blue saturation of the sky-blue LECs by engineering its emission spectrum through the plasmonic filters. These plasmonic filters composed of randomly distributed silver nanoparticles (Ag-NPs) can absorb the green and red emission tail of the sky-blue LECs due to localized surface plasmon resonance (LSPR). The LSPR wavelengths of Ag-NPs are tuned by manipulating the effective refractive index of materials around Ag-NPs through the accurate control of the TiO2 thickness using atomic layer deposition technique. By integrating with the plasmonic filters, the CIE1931 coordinate of the blue LECs can approach to (0.14, 0.22), which is comparable to or even better than the reported bluest values of blue LECs. Combination with the green and red LECs, the color gamut increases from 34% (without filters) to 54% of National Television System Committee (NTSC) color gamut, corresponding to 1.6 times enhancement. In addition, the blue LECs integrated with plasmonic filters still have better efficiency than those of the reported bluest LECs. (C) 2017 Elsevier B.V. All rights reserved.
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.