Browsing by Author "Chang, Yia-Chung"

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Aspect-ratio-dependent ultra-low reflection and luminescence of dry-etched Si nanopillars on Si substrate
(2009-01-01)
Pai, Yi-Hao
;
Meng, Fan-Shuen
;
Lin, Chun-Jung
;
Kuo, Hao-Chung
;
Hsu, Shih-Hsin
The Si nanopillars with high aspect ratio were fabricated by dry-etching the thin SiO(2)-covered Si substrate with a rapidly self-assembled Ni nanodot patterned mask. Aspect-ratio-dependent ultra-low reflection and anomalous luminescence of Si nanopillars are analyzed for applications in all-Si based lighting and energy transferring systems. The Si nanopillars induce an ultra-low reflectance and refractive index of 0.88% and 1.12, respectively, at 435 nm due to the air/Si mixed structure and highly roughened surface. The reflectance can be < 10% with a corresponding refractive index of < 1.80 between 190 and 670 nm. Lengthening the Si nanopillars from 150 +/- 15 to 230 +/- 20 nm further results in a decreasing reflectance, corresponding to a reduction in refractive index by Delta n/n = 18% in the visible and near-infrared wavelength region. After dry-etching an Si wafer into Si nanopillars, the weak blue-green luminescence with double consecutive peaks at 418-451 nm is attributed to the oxygen defect (O(2-))-induced radiation, which reveals less relevance with the ultra-low-reflective Si nanopillar surface.
Calculation of quasi dispersion curves and quality factors of coupled resonator optical waveguides in photonic-crystal slabs
(2012-09-01)
Huang, Chih-Hsien
;
Li, Wei-Shuo
;
Wu, Jing-Nuo
;
Hsieh, Wen-Feng
;
Chang, Yia-Chung
We propose a stabilization method to numerically calculate the dispersion relations and quality factors of optically confined finite structures. For the coupled resonator optical waveguide (CROW) made in a photonic-crystal slab (PCS) used as an example, the dispersion curve is normally not well defined due to the appearance of discontinuities, which do not occur in a two-dimensional CROW with infinite slab height. Therefore, there is less effort devoted to the calculation of quasi dispersion curves of the CROW in a slab. The dispersion relation of the PCS CROW can only be obtained by theoretical fitting to the experimental data under the tight-binding approximation. Here, we demonstrate the use of a stabilization method to calculate the quasi dispersion relation of a PCS CROW accurately. From the stabilization graph, we can calculate the quality factor for an eigenfrequency and properly choose the size of the simulation cell to avoid coupling the CROW modes with the unconfined modes and to accurately calculate the dispersion curve of the PCS CROW using the plane-wave expansion method. The proposed method and results not only provide important information for designing practical photonic devices such as slow-light optical waveguides and nonlinear photonic devices for the PCS CROWs but also can be applied to compute the quality factors and resonance frequencies of microcavities or nanocavities. (c) 2012 Optical Society of America
Characterization of Nonlinear Optical Properties of Crystal RbGeCl(3)center dot x(H(2)O) in Infrared Region
(2009-08-01)
Tang, Li-Chuan
;
Liu, Li-Qiang
;
Chang, Yia-Chung
;
Yao, Jui-hsien
;
Huang, Jung-Yau
The nonlinear optical (NLO) property of hydrated rubidium germanium chloride (HRGC), RbGeCl(3)center dot x(H(2)O), is identified. Infrared absorbtion data support structural evidence that HRGC contain co-ordinated water molecules with strong hydrogen bond. The infrared spectrum indicated HRGC is transparent in most of the infrared region with only little influence from water. Calculations base on density functional theory shows that the band gap of the RbGeCl(3) (RGC) crystal is at least 3.84 eV, which is larger than that of the infrared (IR) NLO crystal CsGeCl(3). Single crystals of HRGC, sized up to 3 x 2 x 1cm(3), were grown in aqueous solution by a slow dehydrate technique. The synthetic, structural and optical propertiels of an off-centrosymmetric IR nonlinear optical (NLO) RbGeCl(3)center dot x(H(2)O) crystal were investigated experimentally. Powder second harmonic generation (PSHG) measurement indicated that the crystal structure of HRGC becomes off-centrosymmetric. Precise X-ray diffraction measurements showed that [100] family diffraction peaks split slightly. Unlike the RGC crystal structure whose space group is P2(1)(m) over bar, the HRGC crystal loses the inversion symmetry. Comparisons with known NLO material KH(2)PO(4) (KDP), indicate that HRGC's NLO susceptibility, x((2)), is about one third of that for KDP. The absorption edge of HRGC occured at 310 nm (approximate to 4.0ev), which indicated NLO HRGC crystal can have larger laser damage threshold. According to the Fourier transform infrared (FTIR) measurement, HRGC has a transparent region from 0.31 to 30 mu m, thus it can be applied to wider optical spectrum from ultraviolet, visible to mid-IR. (C) 2009 The Japan Society of Applied Physics
Characterization of Nonlinear Optical Properties of Crystal RbGeCl3 center dot x(H2O) in Infrared Region
(2009-08-01)
Tang, Li-Chuan
;
Liu, Li-Qiang
;
Chang, Yia-Chung
;
Yao, Jui-hsien
;
Huang, Jung-Yau
The nonlinear optical (NLO) property of hydrated rubidium germanium chloride (HRGC), RbGeCl3 center dot x(H2O), is identified. Infrared absorbtion data support structural evidence that HRGC contain co-ordinated water molecules with strong hydrogen bond. The infrared spectrum indicated HRGC is transparent in most of the infrared region with only little influence from water. Calculations base on density functional theory shows that the band gap of the RbGeCl3 (RGC) crystal is at least 3.84 eV, which is larger than that of the infrared (IR) NLO crystal CsGeCl3. Single crystals of HRGC, sized up to 3 x 2 x 1cm(3), were grown in aqueous solution by a slow dehydrate technique. The synthetic, structural and optical propertiels of an off-centrosymmetric IR nonlinear optical (NLO) RbGeCl3 center dot x(H2O) crystal were investigated experimentally. Powder second harmonic generation (PSHG) measurement indicated that the crystal structure of HRGC becomes off-centrosymmetric. Precise X-ray diffraction measurements showed that [100] family diffraction peaks split slightly. Unlike the RGC crystal structure whose space group is P2(1)(m) over bar, the HRGC crystal loses the inversion symmetry. Comparisons with known NLO material KH2PO4 (KDP), indicate that HRGC's NLO susceptibility, x((2)), is about one third of that for KDP. The absorption edge of HRGC occured at 310 nm (approximate to 4.0ev), which indicated NLO HRGC crystal can have larger laser damage threshold. According to the Fourier transform infrared (FTIR) measurement, HRGC has a transparent region from 0.31 to 30 mu m, thus it can be applied to wider optical spectrum from ultraviolet, visible to mid-IR. (C) 2009 The Japan Society of Applied Physics
Characterization of Si nanorods by spectroscopic ellipsometry with efficient theoretical modeling
(2008-04-01)
Hsu, Shih-Hsin
;
Liu, En-Shao
;
Chang, Yia-Chung
;
Hilfiker, James N.
;
Kim, Young Dong
Spectroscopic ellipsometry (SE) is applied to characterize Si columnar nanostructures. By employing effective medium approximation (EMA) theory, Si nanorods are treated as a graded layer with each sub-layer modeled as a mixture of Si and voids with varying porosity fraction. In addition, the rigorous coupled-wave analysis and finite-element Green's function method were used in modeling Si nanorods as a stack of disks with varying diameters and thicknesses, and the calculations are in satisfactory agreement with the measurement results. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics
(2011-03-04)
Chang, Chia-Hua
;
Yu, Peichen
;
Hsu, Min-Hsiang
;
Tseng, Ping-Cheng
;
Chang, Wei-Lun
As silicon photovoltaics evolve towards thin-wafer technologies, efficient optical absorption for the near-infrared wavelengths has become particularly challenging. In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near-infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on micro-grooved silicon substrates using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared by functioning as impedance matching layers with effective refractive indices gradually varying from 1 to 1.3. Materials with such unique refractive index characteristics are not readily available in nature. As a result, the solar cell with combined textures achieves over 90% external quantum efficiencies for a broad wavelength range of 460-980 nm, which is crucial to the development of advanced thin-substrate silicon solar cells.
Compact microdisk cavity laser with type-II GaSb/GaAs quantum dots
(2011-01-31)
Hsu, K. S.
;
Chiu, T. T.
;
Lin, Wei-Hsun
;
Chen, K. L.
;
Shih, M. H.
Microdisk lasers with active region made of type-II GaSb/GaAs quantum dots on the GaAs substrate have been demonstrated. A microdisk cavity with diameter of 3.9 mu m was fabricated from a 225-nm-thick GaAs layer filled with GaSb quantum dots. Lasing at wavelengths near 1000 nm at 150 K was achieved for this microdisk. A high threshold characteristic temperature of 77 K was also observed. It is found that the lasing wavelength matches closely with the first-order whispering-gallery mode of the cavity as obtained from the finite-element method simulation. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3543839]
Efficiency Enhancement of GaAs Photovoltaics Employing Antireflective Indium Tin Oxide Nanocolumns
(2009-04-27)
Yu, Peichen
;
Chang, Chia-Hua
;
Chiu, Ching-Hua
;
Yang, Chin-Sheng
;
Yu, Jue-Chin
Highly-oriented indium tin oxide nano-columns are prepared by glancing-angle deposition with nitrogen. The tapered column profiles, which function as a graded-refiractive-index layer, offer superior antireflective characteristics. The nano-structured material serves as the conductive antireflective layer for GaAs solar cells, demonstrating a viable efficiency-boosting strategy for next-generation photovoltaics.
Finite-difference time-domain analysis for the dynamics and diffraction of exciton-polaritons
(2015-10-01)
Chen, Minfeng
;
Chang, Yia-Chung
;
Hsieh, Wen-Feng
;
光電工程學系
;
Department of Photonics
We adopted a finite-difference time-domain (FDTD) scheme to simulate the dynamics and diffraction of exciton-polaritons, governed by the coupling of polarization waves with electromagnetic waves. The polarization wave, an approximate solution to the Schrodinger\'s equation at low frequencies, essentially captures the exciton behavior. Numerical stability of the scheme is analyzed and simple examples are provided to prove its validity. The system considered is both temporally and spatially dispersive, for which the FDTD analysis has attracted less attention in the literature. Here, we demonstrate that the FDTD scheme could be useful for studying the optical response of the exciton-polariton and its dynamics. The diffraction of a polariton wave from a polaritonic grating is also considered, and many sharp resonances are found, which manifest the interference effect of polariton waves. This illustrates that the measurement of transmittance or reflectance near polariton resonance can reveal subwave-length features in semiconductors, which are sensitive to polariton scattering. (C) 2015 Optical Society of America
First Principles Calculations of Linear and Second-Order Optical Responses in Rhombohedrally Distorted Perovskite Ternary Halides, CsGeX(3) (X = Cl, Br, and I)
(2009-11-01)
Tang, Li-Chuan
;
Chang, Yia-Chung
;
Huang, Jung-Yau
;
Lee, Ming-Hsien
;
Chang, Chen-Shiung
Systematic studies based on first-principles calculations of second. order optical susceptibilities as well as the dielectric function of CsGeX(3) (X = Cl Br, and I, CGX) are presented The relationship between structural properties and optoelectronic responses are examined The structural factors Delta alpha, and d(Ge), d(x) are proposed to describe the degree of distortion from an ideal perovskite structure. Delta alpha and dGe increase when halide anions are changed from Cl to I, while halide anion displacement, dx, decreases. The structural distortion effect on these rhombohedral CGX crystals is analyzed by first-principles calculations The dielectric function and the second harmonic generation (SHG) response coefficient also increase with increasing Delta alpha and d(Ge). The direct band gaps (E(c)) of CsGeX3 ail occur at the R-point, Delta E(R) The experimental band gaps of CGX crystals become smaller. i e, E(G)(CGI) (= 3 67eV) > E(G)(CGB) (=2 32 eV) > E(G)(CGI) (= 1 53eV), as Delta alpha and el increase, i e. d(Ge)(CGC) < d(Ge)(CGR) < d(GB)(CGI) Partial density of states (PDOS) analysis revealed that the valence band maximum (VBM) and conduction band minimum (CBM) are mainly contributed by the p-orbitals of germanium. The calculated magnitudes of chi((2))(ijk) are close to some reported experimental values near the band gap (C) 2009 The Japan Society of Applied Physics
First Principles Calculations of Linear and Second-Order Optical Responses in Rhombohedrally Distorted Perovskite Ternary Halides, CsGeX3 (X = Cl, Br, and I)
(2009-11-01)
Tang, Li-Chuan
;
Chang, Yia-Chung
;
Huang, Jung-Yau
;
Lee, Ming-Hsien
;
Chang, Chen-Shiung
Systematic studies based on first-principles calculations of second. order optical susceptibilities as well as the dielectric function of CsGeX3 (X = Cl Br, and I, CGX) are presented The relationship between structural properties and optoelectronic responses are examined The structural factors Delta alpha, and d(Ge), d(x) are proposed to describe the degree of distortion from an ideal perovskite structure. Delta alpha and dGe increase when halide anions are changed from Cl to I, while halide anion displacement, dx, decreases. The structural distortion effect on these rhombohedral CGX crystals is analyzed by first-principles calculations The dielectric function and the second harmonic generation (SHG) response coefficient also increase with increasing Delta alpha and d(Ge). The direct band gaps (E-c) of CsGeX3 ail occur at the R-point, Delta E-R The experimental band gaps of CGX crystals become smaller. i e, E-G(CGI) (= 3 67eV) > E-G(CGB) (=2 32 eV) > E-G(CGI) (= 1 53eV), as Delta alpha and el increase, i e. d(Ge)(CGC) < d(Ge)(CGR) < d(GB)(CGI) Partial density of states (PDOS) analysis revealed that the valence band maximum (VBM) and conduction band minimum (CBM) are mainly contributed by the p-orbitals of germanium. The calculated magnitudes of chi((2))(ijk) are close to some reported experimental values near the band gap (C) 2009 The Japan Society of Applied Physics
Lasing action and extraordinary reduction in long radiative lifetime of type-II GaSb/GaAs quantum dots using circular photonic crystal nanocavity
(2015-08-31)
Hsu, Kung-Shu
;
Hung, Wei-Chun
;
Chang, Chih-Chi
;
Lin, Wei-Hsun
;
Shih, Min-Hsiung
We demonstrated the lasing action and remarkable reduction in long radiative lifetimes of type-II GaSb/GaAs quantum dots using a circular photonic-crystal nano-cavity with high Purcell factors. The associated enhancement in carrier recombination was surprisingly high and could even surpass type-I counterparts in similar conditions. These phenomena reveal that the type-II sample exhibited extremely low nonradiative recombination so that weak radiative transitions were more dominant than expected. The results indicate that type-II nanostructures may be advantageous for applications which require controllable radiative transitions but low nonradiative depletions. (C) 2015 AIP Publishing LLC.
Optical cavity modes of a single crystalline zinc oxide microsphere
(2013-02-11)
Moirangthem, Rakesh Singh
;
Cheng, Pi-Ju
;
Chien, Paul Ching-Hang
;
Buu Trong Huynh Ngo
;
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
Optical characterization of CO2-laser-ablated Si-rich SiOx
(2007-04-09)
Lin, Gong-Ru
;
Lin, Chun-Jung
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Chang, Yia-Chung
;
光電工程學系
;
Department of Photonics
Anomalous absorption and the corresponding change in the optical band gap of a CO2-laser-ablated Si-rich SiO2 (SiOx) film are studied. The optical band gap energy of as-grown nonstoichiometric SiOx is slightly reduced by increasing Si-Si bonds as compared to quartz. After rapid thermal annealing using a CO2 laser, the dehydrogenation of SiOx film further increases the Si-Si bonding states and redshifts the optical band gap by 1 eV. Laser ablation is initiated at a laser intensity of > 7.5 kW/cm(2), leaving numerous luminescent centers that are related to neutral oxygen vacancy defects, increasing the absorption coefficient and related optical band gap energy, and reducing the refractive index in partially annealed SiOx. (c) 2007 American Institute of Physics.
Optimization of High-Q Coupled Nanobeam Cavity for Label-Free Sensing
(2015-10-01)
Yaseen, Mohammad Tariq
;
Yang, Yi-Chun
;
Shih, Min-Hsiung
;
Chang, Yia-Chung
;
光電工程學系
We numerically and experimentally investigated the lateral coupling between photonic crystal (PhC) nanobeam (NB) cavities, pursuing high sensitivity and figure of merit (FOM) label-free biosensor. We numerically carried out 3D finite-difference time-domain (3D-FDTD) and the finite element method (FEM) simulations. We showed that when two PhC NB cavities separated by a small gap are evanescently coupled, the variation in the gap width significantly changes the coupling efficiency between the two coupled NB cavities and the resulting resonant frequencies split. Experimentally, we fabricated laterally-coupled PhC NB cavities using (InGaAsP) layer on the InP substrate. For sensing, we showed that the laterally coupled PhC NB cavities sensor exhibits higher sensitivity than the single PhC NB cavity. The higher sensitivity of laterally coupled PhC NB cavities is due to the strong evanescent coupling between nearby PhC NB cavities, which depends on the gap width and it is attributed to the large confinement of the electromagnetic field in the gap (air or liquid). As a result of the lateral coupling, both even (symmetric) and odd (asymmetric) modes exist. We show that even modes are more sensitive than odd modes. In addition, higher-order modes exhibit higher sensitivity. Hence, we characterized and examined the fabricated PhC NB cavity as a label-free biosensor, and it exhibits high figure of merit due to its high Q-factor. This illustrates a potentially useful method for optical sensing at nanoscale.
Photonic Crystal Circular Nanobeam Cavity Laser with Type-II GaSb/GaAs Quantum Rings as Gain Material
(2020-03-16)
Lin, Hsiang-Ting
;
Hsu, Kung-Shu
;
Chang, Chih-Chi
;
Lin, Wei-Hsun
;
Lin, Shih-Yen
The optical emission from type-II semiconductor nanostructures is influenced by the long carrier lifetime and can exhibit remarkable thermal stability. In this study, utilizing a high quality photonic crystal circular nanobeam cavity with a high quality factor and a sub-micrometer mode volume, we demonstrated an ultra-compact semiconductor laser with type-II gallium antimonide/gallium arsenide quantum rings (GaSb/GaAs QRs) as the gain medium. The lasing mode localized around the defect region of the nanobeam had a small modal volume and significant coupling with the photons emitted by QRs. It leads the remarkable shortening of carrier lifetime observed from the time-resolved photoluminescence (TRPL) and a high Purcell factor. Furthermore, a high characteristic temperature of 114K was observed from the device. The lasing performances indicated the type-II QRs laser is suitable for applications of photonic integrated circuit and bio-detection applications.
Plasmonic thermal emitter using perfect absorber made of metallic disk on SiO2
(2012)
Abbas, Mohammed Nadhim
;
Cheng, Cheng-Wen
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Chang, Yia-Chung
;
Shih, Min-Hsiung
;
Chen, Hung-Hsin
It is shown that the metallic disk structure can be used as an efficient narrow-band thermal emitter in the IR region. The absorption spectra of such structure are investigated both theoretically and experimentally. Calculations of thermal radiation properties of the metallic disk show that the metallic disk is a perfect emitter at a specific wavelength. The metallic disk exhibits only one significant localized surface plasmon polariton mode for both TM and TE polarizations simultaneously.
Sound Wave Propagation Anisotropy in Silver Nanoprisms: Characterization of Photoinduced Multiple Modes Using the Symmetric Molecular Dynamics Method
(2013-07-04)
Ng, Ming-Yaw
;
Yu, Pyng
;
Tang, Jau
;
Chang, Yia-Chung
;
光電工程學系
We present both theoretical and experimental studies of the sound velocity anisotropy and laser-induced multiple acoustic phonon oscillations in silver nanoprisms. A large-scale simulation method based on molecular dynamics and group theory for nanoprisms with C-3v symmetry was developed. We observed the most prominent planar modes and the less studied vertical modes and found that the anisotropic vibrational properties of silver nanoprisms strongly depend on the particle's aspect ratio (bisector length over thickness). For the case of a smaller aspect ratio, we identified these modes as the A, modes with a larger in phase radial atomic displacement These modes can be classified into three main types: breathing modes and totally symmetric modes which are planar modes and thickness-related vertical modes. Due to the strong coupling with laterally confined modes, a broad band of thickness-related modes is found (in contrast to a single sharp peak for thin films), which leads to a fast decaying component in the oscillation spectrum. The calculated results and the sound velocity anisotropy along different crystal axes are in good agreement with our transient optical absorption experiments.
Whispering gallery modes of a single crystalline zinc oxide microsphere at visible wavelengths
(2013-01-01)
Cheng, Pi-Ju
;
Moirangthem, Rakesh Singh
;
Chien, Paul Ching-Hang
;
Buu Trong Huynh Ngo
;
Chang, Shu-Wei
We present photoluminescence spectra of ultrasmall zinc oxide microspheres with sharp resonances of whispering gallery modes covering the whole visible window and compare with theoretical analyses. (C) 2013 Optical Society of America
Wide-angle polarization independent infrared broadband absorbers based on metallic multi-sized disk arrays
(2012-04-23)
Cheng, Cheng-Wen
;
Abbas, Mohammed Nadhim
;
Chiu, Chao-Wei
;
Lai, Kun-Ting
;
Shih, Min-Hsiung
Two-dimensional metallic broadband absorbers on a SiO2/Ag/Si substrate were experimentally studied. The absorptivity of such structure can be increased by tailoring the ratio of disk size to the unit cell area. The metallic disk exhibits a localized surface plasmon polariton (LSPP) mode for both TE and TM polarizations. A broadband thermal emitter can be realized because the LSPP mode is independent of the periodicities. By manipulating the ratios and disk sizes, a high-performance, wide-angle, polarization-independent dual band absorber was experimentally achieved. The results demonstrated a substantial flexibility in absorber designs for applications in thermal photovoltaics, sensors, and camouflage. (C) 2012 Optical Society of America