Browsing by Author "Cheng, Pi-Ju"

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Carrier lifetime of heavily p-doped base in light-emitting transistors and transistor lasers
(2019-01-01)
Hsieh, Chi-Ti
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Cheng, Pi-Ju
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Lin, Chih-Hsien
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Chang, Shu-Wei
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光電工程學系
Light-emitting transistor (LET) and transistor laser (TL) can provide the high-speed electrical and optical modulations simultaneously, advancing light-emitting diodes and diode lasers. Still, between experimental data and rate-equation modeling, there are two-order-of-magnitude uncertainties on the carrier lifetimes of quantum wells (QWs) inserted in heavily p-doped bases of these devices. In view of the importance of this timescale on the modulation speed, we provide a comprehensive approach to calculate carrier lifetimes under such circumstances. We model the Hartree potential energy with self-consistent solutions of the Schrodinger's and Poisson's equations. The hole distribution is obtained from real-space density of states through multiband retarded Green functions, taking the outgoing-wave features of hole quasi-bound states into account. We then estimate the carrier lifetimes based on a multiband source-radiation approach including both bound-to-bound and bound-to-continuum components of spontaneous (SP) emissions. Under low surface carrier injections, a large Hartree potential is formed, and the valence band around the QW is strongly tilted. Both bound and quasi-bound valence states are present, and quasi-bound holes may tunnel out of QW and reemerge in the base. The SP spectrum from the QW in the heavily doped base is significantly larger than that from an undoped one due to preexisting holes. At the high injection level, the screening effect significantly reduces the Hartree potential and band bending. We also include the nonradiative Auger recombination to evaluate the total carrier lifetime. Overall carrier lifetimes and small-signal ones are estimated as hundred picoseconds at a doping density of 10(19)cm(-3) and might be even shorter in the case of heavier doping.
Chiral Second-Harmonic Generation from Monolayer WS2/Aluminum Plasmonic Vortex Metalens
(2020-04-08)
Guo, Wan-Ping
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Liang, Wei-Yun
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Cheng, Chang-Wei
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Wu, Wei-Lin
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Wang, Yen-Ting
Two-dimensional spiral plasmonic structures have emerged as a versatile approach to generate near-field vortex fields with tunable topological charges. We demonstrate here a far-field approach to observe the chiral second-harmonic generation (SHG) at designated visible wavelengths from a single plasmonic vortex metalens. This metalens comprises an Archimedean spiral slit fabricated on atomically flat aluminum epitaxial film, which allows for precise tuning of plasmonic resonances and subsequent transfer of two-dimensional materials on top of the spiral slit. The nonlinear optical measurements show a giant SHG circular dichroism. Furthermore, we have achieved an enhanced chiral SHG conversion efficiency (about an order of magnitude greater than the bare aluminum lens) from monolayer tungsten disulfide (WS2)/aluminum metalens, which is designed at the C-exciton resonance of WS2. Since the C-exciton is not a valley exciton, the enhanced chiral SHG in this hybrid system originates from the plasmonic vortex field-enhanced SHG under the optical spin-orbit interaction.
Circular Dichroism Control of Tungsten Diselenide (WSe2) Atomic Layers with Plasmonic Metamolecules
(2018-05-09)
Lin, Hsiang-Ting
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Chang, Chiao-Yun
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Cheng, Pi-Ju
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Li, Ming-Yang
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Cheng, Chia-Chin
Controlling circularly polarized (CP) states of light is critical to the development of functional devices for key and emerging applications such as display technology and quantum communication, and the compact circular polarization-tunable photon source is one critical element to realize the applications in the chip-scale integrated system. The atomic layers of transition metal dichalcogenides (TMDCs) exhibit intrinsic CP emissions and are potential chiroptical materials for ultrathin CP photon sources. In this work, we demonstrated CP photon sources of TMDCs with device thicknesses approximately 50 nm. CP photoluminescence from the atomic layers of tungsten diselenide (WSe2) was precisely controlled with chiral metamolecules (MMs), and the optical chirality of WSe2 was enhanced more than 4 times by integrating with the MMs. Both the enhanced and reversed circular dichroisms had been achieved. Through integrations of the novel gain material and plasmonic structure which are both low-dimensional, a compact device capable of efficiently manipulating emissions of CP photon was realized. These ultrathin devices are suitable for important applications such as the optical information technology and chip-scale biosensing.
Circular Polarized Emission of Tungsten Diselenide (WSe2) Atomic Layers with Plasmonic Metasurface
(2018-01-01)
Lin, Hsiang-Ting
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Chang, Chiao-Yun
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Cheng, Pi-Ju
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Li, Ming-Yang
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Cheng, Chia-Chin
A circularly polarized light source with ultrathin geometry were demonstrated by integrating the WSe2 atomic layers with plasmonic chiral metasurfaces. The intrinsic chiral emission of WSe2 was manipulated by chiral metasurfaces to achieve emission circular dichroism more than 25%.
Circularly-polarized lasing of ultraviolet plasmonic gammadion nanocavity
(2019-01-01)
Cheng, Pi-Ju
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Lai, Ming-Sheng
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Hsiao, Yu-Hao
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Yu, Cheng-Li
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Chang, Chiao-Yun
In this work, we present the model of plasmonic chrial nanolasers composed of aluminum-coated gallium-nitride (GaN) gammadions, which may lase with a high degree of circular polarization at room temperatures. Using the finite-element method, we examine resonant modes of the four-fold rotationally symmetric cavities of gammadions whose resonant frequencies lie in the gain spectrum of GaN. We find a degenerate doublet of resonant modes which can couple to plane waves in the far-field zone above gammadions. Their near-field profiles exhibit localized distribution in the arms of gammadions and a Fabry-Perot standing-wave pattern along the post. In practice, fabrication imperfections would inevitably spoil the four-fold rotation symmetry of gammadions. Typical perturbation could lift the degeneracy of doublet and leads to mixing of the two degenerate modes which may still output signals with observable handedness above gammadions. Considering a gammadion cavity with a single elongated arm, we show that the magnitude of dissymmetry factor of its resonant mode can be larger than unity. Our calculations are consistent with the experimental results, indicating that the right-handed gammadion cavities lase with a magnitude of dissymmetry factors near 1 at a wavelength of 364 nm. The dimensionless effective mode volume scaled by the cube of effective wavelength is 2.62, reflecting a modal distribution remarkably confined in the plasmonic structures and the capability of enhancing the spontaneous-emission rate noticeably. These chiral nanolasers with an ultrasmall footprint could be potentially utilized as future circularly-polarized photon source at the chip level.
Cladding Effect on Hybrid Plasmonic Nanowire Cavity at Telecommunication Wavelengths
(2013-05-01)
Cheng, Pi-Ju
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Weng, Chen-Ya
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Chang, Shu-Wei
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Lin, Tzy-Rong
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Tien, Chung-Hao
In this paper, we investigate the effect of cladding layers on a hybrid plasmonic nanowire cavity. A decent confinement factor and moderate modal loss can be achieved for lasing. Within a certain index range of the cladding layer, the fundamental hybrid plasmonic mode is well confined inside the narrow gain region and becomes the most promising lasing mode. Our results also indicate that the mirror loss can be reduced if the refractive index of the cladding layer is properly chosen. An estimated cold-cavity quality factor of 87, corresponding to a threshold gain of about 1072 cm(-1), is achievable at a cavity length of 10.13 mu m for the lasing mode.
Comparison of wavefront coding optical system using two conjugate phase masks among cubic, sinusoidal, and tangent phase masks
(2017-01-01)
Chen, Shao-Pai
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Cheng, Pi-Ju
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Hsieh, Chi-Ti
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Chang, Shu-Wei
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光電工程學系
Wavefront coding is an effective technique for extending the depth of field. We verified the usefulness of the method of generating symmetrical point spread function by using two conjugate phase masks. According to the results of this study, this method is effective at a small defocus.
Controllable Surface Plasmon Rasonance in Gold Nanocomposites for Organic Light-Emitting Diodes
(2011)
Chang, Chia-Ying
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Lin, Yung-Lun
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Cheng, Pi-Ju
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Cheng, Jung-An
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Chang, Shu-Mei
The water dispersible gold nanoparticles coordinated with various surfactants for surface plasmon enhancement were synthesized. Via modulating the refractive indexes of surfactants, the surface plasmon resonance wavelength can be varied between 520 and 570 nm. As a result, Au nanoparticles-incorporated PEDOT:PSS matrix layer revealed a 2.7x increase in fluorescence quantum yields from fluorescent dye, P2-A.
Coupled nanowire-based hybrid plasmonic nanocavities on thin substrates
(2014-11-28)
Cheng, Pi-Ju
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Chiang, Chih-Kai
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Chung, Yi-Cheng
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Tien, Chung-Hao
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Lin, Tzy-Rong
We theoretically analyze nanowire-based hybrid plasmonic nanocavities on thin substrates at visible wavelengths. In the presence of thin suspended substrates, the hybrid plasmonic modes, formed by the coupling between a metal nanowire and a dielectric nanowire with optical gain, exhibit negligible substrate-mediated characteristics and overlap better with the gain region. Consequently, the confinement factor of the guided hybrid modes is enhanced by more than 42%. However, the presence of significant mirror loss remains the main challenge to lasing. By adding silver coatings with a sufficient thickness range on the two end facets, we show that the reflectivity is substantially enhanced to above 50%. For a coating thickness of 50 nm and cavity length of about 4 mu m, the quality factor is above 100.
Criteria of backscattering in chiral one-way photonic crystals
(2016-01-01)
Cheng, Pi-Ju
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Chang, Shu-Wei
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光電工程學系
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Department of Photonics
Optical isolators are important devices in photonic circuits. To reduce the unwanted reflection in a robust manner, several setups have been realized using nonreciprocal schemes. In this study, we show that the propagating modes in a strongly-guided chiral photonic crystal (no breaking of the reciprocity) are not backscattering-immune even though they are indeed insensitive to many types of scatters. Without the protection from the nonreciprocity, the backscattering occurs under certain circumstances. We present a perturbative method to calculate the backscattering of chiral photonic crystals in the presence of chiral/achiral scatters. The model is, essentially, a simplified analogy to the first-order Born approximation. Under reasonable assumptions based on the behaviors of chiral photonic modes, we obtained the expression of reflection coefficients which provides criteria for the prominent backscattering in such chiral structures. Numerical examinations using the finite-element method were also performed and the results agree well with the theoretical prediction. From both our theory and numerical calculations, we find that the amount of backscattering critically depends on the symmetry of scatter cross sections. Strong reflection takes place when the azimuthal Fourier components of scatter cross sections have an order l of 2. Chiral scatters without these Fourier components would not efficiently reflect the chiral photonic modes. In addition, for these chiral propagating modes, disturbances at the most significant parts of field profiles do not necessarily result in the most effective backscattering. The observation also reveals what types of scatters or defects should be avoided in one-way applications of chiral structures in order to minimize the backscattering.
Crystal Study of N,N '-diphenyl-N,N '-bis(1-naphthyl)-1,1 '-biphenyl-4,4 '-diamine
(2010-06-01)
Cheng, Jung-An
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Cheng, Pi-Ju
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光電工程學系
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顯示科技研究所
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Department of Photonics
The structure of N,N'-diphenyl-N,N'-bis(1-naphthyl)- 1,1'-biphenyl-4,4'-diamine (NPB) with two molecules in the asymmetric unit, was solved and refined to a final R factor of 0.0434. The compound crystallized in the triclinic crystal system, space group P - 1, Z = 2, V = 1597.0(3) angstrom(3) with unit cell parameters a = 10.3077(10) angstrom, b = 11.3545(11) angstrom, c = 14.4778(14) angstrom, alpha = 82.339(2)degrees, beta = 77.657(2)degrees, and gamma = 75.526(2)degrees. The two molecules in the asymmetric unit differed in the orientation of their triarylamino groups. The biphenyl group in the central structure is coplanar, and the angle between the least-square planes of the two six-carbon rings of the biphenyl moiety is zero degree. The packing of the molecules in the lattice is mainly due to the naphthyl-to-naphthyl pi-pi interaction with a distance of 3.861 angstrom.
Full-Spectrum Analysis of Perovskite-Based Surface Plasmon Nanolasers
(2020-03-29)
Cheng, Pi-Ju
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Zheng, Qi-Yan
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Hsu, Chu-Yuan
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Li, Heng
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Hong, Kuo-Bin
We systematically studied the characteristics of hybrid perovskite-based surface plasmon nanolasers. If one changes the anion composition of perovskites, the emission wavelength can be easily tuned. We conducted in full-spectrum modeling that featured hybrid perovskite nanowires placed on different SiO2-coated metallic (Au, Ag, and Al) plates. The proposed nanocavities that supported plasmonic gap modes exhibited distinguished properties of nanolasers, such as low-transparency threshold-gain and low lasing threshold. The corresponding experimental results for the MAPbBr(3) nanolaser on Ag revealed the low-threshold operation. These superior features were attributed to enhanced light-matter interaction with strong coupling. Therefore, the proposed scheme, integrated with hybrid perovskite as gain material, provides an excellent platform for nanoscale plasmon lasing in the visible to near-infrared spectra.
High Circular Polarized Nanolaser with Chiral Gammadion Metal Cavity
(2020-05-12)
Yu, Cheng-Li
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Hsiao, Yu-Hao
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Chang, Chiao-Yun
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Cheng, Pi-Ju
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Lin, Hsiang-Ting
We demonstrate a circularly polarized laser with the metal-gallium-nitride gammadion nanocavities. The ultraviolet lasing signal was observed with the high circular dichroism at room temperature under pulsed optical pump conditions. Without external magnetism which breaks the time-reversal symmetry to favor optical transitions of a chosen handedness, the coherent outputs of these chiral nanolasers show a dissymmetry factor as high as 1.1. The small footprint of these lasers are advantageous for applications related to circularly polarized photons in future integrated systems, in contrast to the bulky setup of linearly-polarized lasers and quarter-wave plates.
High Q/V-m hybrid photonic-plasmonic crystal nanowire cavity at telecommunication wavelengths
(2015-01-01)
Chiang, Chih-Kai
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Chung, Yi-Cheng
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Cheng, Pi-Ju
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Wu, Chien-Wei
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Chang, Shu-Wei
We haw analyzed a hybrid photonic-plasmonic crystal nanocavity consisting of a silicon grating nanowire adjacent to a metal surface with a gain gap between. The hybrid plasmonic cavity modes are highly confined in the gap due to the coupling of photonic crystal cavity modes and surface plasmonic gap modes. Using the finite-element method, we numerically solve guided modes of the hybrid plasmonic waveguide at a wavelength of 1.55 jim. The modal characteristics such as wave guide confinement factors and modal losses of the fundamental hybrid plasmonic modes are explored as a function of the groove depth at various gap heights. After that, we show the band structure of the hybrid crystal modes, corresponding to a wide band gap of 17.8 THz. To effectively trap the optical modes, we introduce a single defect into the hybrid crystal. At a deep sub-wavelength defect length as small as 180 nm, the resonant mode exhibits a high quality factor of 566.5 and an ultrasmall mode volume of 0.00186 (ln)3 at the resonance wavelength of 1.55 gm. In comparison to the conventional photonic crystal nanowire cavity in the absence of metal surface, the figure of merit QIVm is enormously enhanced around 15 times. The proposed nanocavities open up the opportunities for various applications with strong light-matter interaction such as nanolasers and bio sensors.
High-Performance Plasmonic Nanolasers with a Nanotrench Defect Cavity for Sensing Applications
(2018-07-01)
Cheng, Pi-Ju
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Huang, Zhen-Ting
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Li, Jhu-Hong
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Chou, Bo-Tsun
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Chou, Yu-Hsun
Recent developments in small footprint plasmonic nanolasers show promise for active optical sensing with potential applications in various fields, including real-time and label-free biochemical sensing, and gas detection. In this study, we demonstrate a novel hybrid plasmonic crystal nanolaser that features a ZnO nanowire placed on Al grating surfaces with a nanotrench defect nanocavity. The lasing action of gain-assisted defect nanocavity overcomes the ohmic loss parasitically in the plasmonic nanostructures. Therefore, the plasmonic nanolaser exhibits an extremely small mode volume, a narrow linewidth Delta lambda, and a high Purcell factor that can facilitate the strong interaction between light and matter. This can be used as a refractive index sensor and is highly sensitive to local changes in the refractive indices of ambient materials. By careful design, the near-ultraviolet nanolaser sensors have significant sensing performances of glucose solutions, revealing a high sensitivity of 249 nm/RIU and high resolution, with a figure of merit of 1132, at the resonant wavelength of 373 nm.
Incomplete immunity to backscattering in chiral one-way photonic crystals
(2015-04-20)
Cheng, Pi-Ju
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Tien, Chung-Hao
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Chang, Shu-Wei
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光電工程學系
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Department of Photonics
We show that the propagating modes in a strongly-guided chiral one-way photonic crystal are not backscattering-immune even though they are indeed insensitive to many kinds of scatters. Since these modes are not protected by the nonreciprocity, the backscattering does occur under certain circumstances. We use a perturbative method to derive criteria for the prominent backscattering in such chiral structures. From both our theory and numerical examinations, we find that the amount of backscattering critically depends on the symmetry of scatters. Additionally, for these chiral photonic modes, disturbances at the most intense parts of field profiles do not necessarily lead to the most effective backscattering. (C) 2015 Optical Society of America
Optical cavity modes of a single crystalline zinc oxide microsphere
(2013-02-11)
Moirangthem, Rakesh Singh
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Cheng, Pi-Ju
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Chien, Paul Ching-Hang
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Buu Trong Huynh Ngo
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Chang, Shu-Wei
A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 mu m are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors. (C) 2013 Optical Society of America
Plasmonic gap mode nanocavities at telecommunication wavelengths
(2014)
Cheng, Pi-Ju
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Weng, Chen-Ya
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Chang, Shu-Wei
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Lin, Tzy-Rong
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Tien, Chung-Hao
We analyze a plasmonic gap-mode Fabry-Perot nanocavity containing a metallic nanowire. The proper choice of the cladding layer brings about a decent confinement inside the active region for the fundamental and first-order plasmonic gap modes. We numerically extract the reflectivity of the fundamental and first-order mode and obtain the optical field inside the cavity. We also study the dependence of the reflectivity on the thickness of Ag reflectors and show that a decent reflectivity above 90 % is achievable. For such cavities with a cavity length approaching 1.5 pm, a quality factor near 150 and threshold gain lower than 1500 cm-1 are achievable.
Plasmonic gap-mode nanocavities with metallic mirrors in high-index cladding
(2013-06-03)
Cheng, Pi-Ju
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Weng, Chen-Ya
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Chang, Shu-Wei
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Lin, Tzy-Rong
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Tien, Chung-Hao
We theoretically analyze plasmonic gap-mode nanocavities covered by a thick cladding layer at telecommunication wavelengths. In the presence of high-index cladding materials such as semiconductors, the first-order hybrid gap mode becomes more promising for lasing than the fundamental one. Still, the significant mirror loss remains the main challenge to lasing. Using silver coatings within a decent thickness range at two end facets, we show that the reflectivity is substantially enhanced above 95 %. At a coating thickness of 50 nm and cavity length of 1.51 mu m, the quality factor is about 150, and the threshold gain is lower than 1500 cm(-1). (C) 2013 Optical Society of America
Surface Plasmon Mediated Luminescent Enhancement of Polymer LEDs Using Dual-functional Hole-Injection Layer
(2009)
Cheng, Pi-Ju
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Cheng, Jung-An
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Lin, Zong-Wei
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Tien, Chung-Hao
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光電工程學系
A hole-injection-layer in PLED with a plasmonic metal nanostructure was demonstrated. Dual-functional Au nanoparticles-incorporated PEDOT:PSS matrix layer revealed a 2.3x increase in fluorescence quantum yields from MEH-PPV, via dipolar surface plasmon resonance. With an additional spacer layer, the nanocompostie layer remarkably enhanced the PL intensity of MEH-PPV more than ten fold.