Browsing by Author "Ahn, Hyeyoung"

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Anisotropic exciton relaxation in nanostructured metal (Zn and F16Zn)-phthalocyanine
(2015-02-09)
Ahn, Hyeyoung
;
Liou, Wei-Hyun
;
Chen, Huang-Ming Philip
;
Hsu, Chia-Hung
;
光電工程學系
We report ultrafast excited state dynamics of zinc phthalocyanine and zinc hexadecafluoro phthalocyanine thin films which have nanorod-like structures. Excitons in the singlet states undergo multi-exponential relaxation processes to the ground state and the singlet lifetime within a few tens of picoseconds is attributed to the diffusion-limited exciton annihilation process. Diffusive migration of the singlet excitons shows the anisotropic lifetimes depending on the polarization of probe beam. Similar anisotropy is observed in the X-ray diffraction data which exhibits long-range alignment of molecular columns along the long axis of nanorod, whereas disordered arrangement in lateral direction to the axis of nanorod. (C) 2015 Optical Society of America
Annealing-induced phase transition in zinc phthalocyanine ultrathin films
(2016-11-01)
Ahn, Hyeyoung
;
Chu, Ting-Chang
;
光電工程學系
;
Department of Photonics
We report the evidence of post-annealing-induced phase transition in zinc phthalocyanine (ZnPc) ultrathin (similar to 20 nm) films upon annealing at 200 degrees C. The signatures of phase transition are observed in their morphological, structural, and ultrafast spectroscopic properties. X-ray diffraction measurement shows that alpha-phase nanospheres transform to beta-phase nanorods that have a high degree of crystallinity with an excellent ordering along the c-direction in a monoclinic arrangement. Tight packing of molecules in the beta-crystallites allows a red-shift and intensity gain of the lowest-energy subband in the Q-band and subsequently increases the electronic coupling in both intercolumn and intracolumn paths. (C) 2016 Optical Society of America
Background and Photoexcited Carrier Dependence of Terahertz Radiation from Mg-Doped Nonpolar Indium Nitride Films
(2010)
Ahn, Hyeyoung
;
Yeh, Yi-Jou
;
Hong, Yu-Liang
;
Gwo, Shangjr
;
光電工程學系
We report terahertz (THz) generation from Mg-doped nonpolar (a-plane) InN (a-InN:Mg). While the amplitude and polarity of the THz field from Mg-doped polar (c-plane) InN depend on the background carrier density, the p-polarized THz field from a-InN: Mg has background carrier-insensitive intensity and polarity, which can be attributed to carrier transport in a polarization-induced in-plane electric field. A small but apparent azimuthal angle dependence of the THz field from a-InN:Mg shows the additional contribution of the second-order nonlinear optical effect. Meanwhile, in this study, we did not observe the contribution of the intrinsic in-plane electric field which is significant for high stacking fault density nonpolar InN. (C) 2010 The Japan Society of Applied Physics
Broadband Multifunctional Plasmonic Logic Gates
(2018-07-04)
Sang, Yungang
;
Wu, Xiaojia
;
Raja, Soniya S.
;
Wang, Chun-Yuan
;
Li, Haozhi
Plasmonic logic gates provide a promising platform to realize diffraction-unlimited all-optical logic circuits. However, previous plasmonic logic gates suffer from narrow bandwidth and high loss. Here, broadband plasmonic logic gates are proposed and demonstrated using a unique design based on curved silver gratings. These logic gates can maintain a high intensity contrast ratio (29 dB) under front optical illumination. And, the broadband feature is achieved due to the feature of zero optical path difference from different positions of the curved grating to the output port. All the seven fundamental logic gates are demonstrated. In addition, different types of logic gates can be realized using the same device configuration. The possibility of multifunctionality makes these logic gates easy for device fabrication and system integration.
Carrier dynamics in InN nanorod arrays
(2012-01-16)
Ahn, Hyeyoung
;
Yu, Chih-Cheng
;
Yu, Pyng
;
Tang, Jau
;
Hong, Yu-Liang
In this report, we investigated ultrafast carrier dynamics of vertically aligned indium nitride (InN) nanorod (NR) arrays grown by molecular-beam epitaxy on Si(111) substrates. Dominant band filling effects were observed and were attributed to a partial bleaching of absorption at the probe wavelengths near the absorption edge. Carrier relaxation in nanorod samples was strongly dependent on the rod size and length. In particular, a fast initial decay was observed for carriers in NRs with a small diameter (similar to 30 nm), the lifetime of which is much shorter than the carrier cooling time, demonstrating the substantial surface-associated influence on carrier relaxation in semiconductor nanostructures. (C)2012 Optical Society of America
Chiral Second-Harmonic Generation from Monolayer WS2/Aluminum Plasmonic Vortex Metalens
(2020-04-08)
Guo, Wan-Ping
;
Liang, Wei-Yun
;
Cheng, Chang-Wei
;
Wu, Wei-Lin
;
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.
Efficient Defect Healing of Transition Metal Dichalcogenides by Metallophthalocyanine
(2018-08-29)
Ahn, Hyeyoung
;
Huang, Yu-Chiao
;
Lin, Chang-Wei
;
Chiu, Yi-Lun
;
Lin, Erh-Chen
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention as alternatives to graphene with semiconducting band gaps. Mono- or few-layer TMDCs can be prepared by various methods, but regardless of the fabrication methods [such as mechanical exfoliation and chemical vapor deposition (CVD)], TMDCs contain many structural defects, which significantly affect their physical properties and limit their performance in applications. Metallophthalocyanines (MPcs) are organic semiconductors, and as dopants, they are capable of modulating the optical and electrical properties of other semiconducting materials. Here, we report that besides the ability to modulate the optoelectronic properties of 2D TMDCs, MPc molecules can be used to heal defects and improve the physicochemical properties of TMDCs. Doping of planar MPc molecules to TMDCs is achieved by a simple solution dip-coating method and results in a significant improvement in the optical properties and thermal responses of CVD-grown TMDCs, even comparable to those of mechanically exfoliated counterparts. Study of carrier dynamics shows that the adsorption of MPc on the TMDC surface leads to the complete suppression of the mid-gap defect-induced absorption in TMDCs. Furthermore, MPc molecules with a large lateral size are found to effectively reduce the point defects in mechanically exfoliated TMDCs introduced during the preparation process. Our results not only clarify the optoelectronic modulation mechanism of chemical doping but also offer a simple method to control the nanosized defects in 2D TMDCs.
Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics
(2015-07-01)
Wang, Chun-Yuan
;
Chen, Hung-Ying
;
Sun, Liuyang
;
Chen, Wei-Liang
;
Chang, Yu-Ming
The development of ultrasmooth, macroscopic-sized silver (Ag) crystals exhibiting reduced losses is critical to fully characterize the ultimate performance of Ag as a plasmonic material, and to enable cascaded and integrated plasmonic devices. Here we demonstrate the growth of single-crystal Ag plates with millimetre lateral sizes for linear and nonlinear plasmonic applications. Using these Ag crystals, surface plasmon polariton propagation lengths beyond 100 mm in the red wavelength region are measured. These lengths exceed the predicted values using the widely cited Johnson and Christy data. Furthermore, they allow the fabrication of highly reproducible plasmonic nanostructures by focused ion beam milling. We have designed and fabricated double-resonant nanogroove arrays using these crystals for spatially uniform and spectrally tunable second-harmonic generation. In conventional 'hot-spot'-based nonlinear processes such as surface-enhanced Raman scattering and second-harmonic generation, strong enhancement can only occur in random, localized regions. In contrast, our approach enables uniform nonlinear signal generation over a large area.
High optical nonlinearity in low-dimensional halide perovskite polycrystalline films
(2020-08-17)
Liang, Wei-Yun
;
Liu, Fangzhou
;
Lu, Yu-Jung
;
Popovic, Jasminka
;
Djurisic, Aleksandra
The nonlinear optical properties of low-dimensional polycrystalline halide perovskite films consisting of ethylammonium (EA) and butylammonium (BA) cations are investigated using Z-scan technique. Across the band-edge, two-dimensional (BA)(2)PbI4 exhibits a transition from saturable absorption (SA) to reverse-SA and its nonlinear absorption and nonlinear refractive index are much smaller than those of bulk counterparts. Meanwhile, EAPbI(3) with one-dimensionality of the inorganic structure shows the SA behavior both above and below band-edge and the estimated nonlinear optical parameters of polycrystalline EAPbI(3) are comparable to those of single-crystalline ones, attributed to high dielectric contrast between the inorganic and organic elements in one-dimensional structures. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Monolayer Semiconductor Surface-Emitting Lasers Using 2D Dark Plasmonic Cavities
(2018-01-01)
Wang, Chun-Yuan
;
Shi, Jinwei
;
Raja, Soniya S.
;
Chen, Chun-An
;
Zhang, Xin-Quan
We demonstrated a low-threshold lasing from a monolayer gain medium which is made possible due to 2D feedback mechanism via strong exciton-SPP coupling in both real space and a wide range of momentum space.
Observation of coherent acoustic phonon in titanyl phthalocyanine thin solid films
(2015-08-01)
Ahn, Hyeyoung
;
Su, Shiu-Ho
;
光電工程學系
;
Department of Photonics
Ultrafast exciton dynamics is investigated in titanyl phthalocyanine (TiOPc) micrograin films in this study. Exponential exciton relaxation and oscillatory responses are observed in transient reflectivity measurements of the films. Laser-induced coherent acoustic waves (LCAW) are proposed to be responsible for this oscillation. Despite their fast attenuation with the increase in temperature, LCAWs are successfully detected at room temperature through probing at a large oblique angle near the low-energy absorption edge of the Q-Band. From the oscillation period of the LCAW, the sound velocity is estimated to be similar to 3.1 x 10(3) m/s, and an acoustic echo arising from the boundary between the films and substrate is also observed. (C) 2015 The Japan Society of Applied Physics
Observation of strain-induced phonon mode splitting in the tetragonal hybrid halide perovskite
(2017-11-01)
Ahn, Hyeyoung
;
Huang, Hsin
;
Hsieh, Meng-I
;
Li, Ming-Hsien
;
Chen, Peter
We report the structural and low-frequency vibrational properties of methylammonium lead-iodide (MAPbI(3)) perovskites studied by terahertz spectroscopy. The characteristic low-frequency mode splitting is observed in room-temperature tetragonal MAPbI(3) films grown on Si, and the frequencies of split modes are found to coincide with those of orthorhombic phases. The strain arising from the symmetry and lattice mismatch against the substrates is responsible for the local distortion of inorganic cages and results in the mode-splitting. The use of a mesoporous TiO2 interlayer effectively reduces the strain at the perovskite-substrate interface and induces larger grain growth and the improvement of structural ordering in the tetragonal MAPbI(3) films. (C) 2017 The Japan Society of Applied Physics
Photoluminescence from InN Nanorod Arrays with a Critical Size
(2013-06-01)
Ahn, Hyeyoung
;
Liu, Yu-Sheng
;
Chang, Ke-Yang
;
Gwo, Shangjr
;
光電工程學系
In this report, we investigated the rod size dependence of photoluminescence (PL) from vertically aligned indium nitride (InN) nanorod arrays grown on Si(111) substrates. Abnormal temperature dependence of the PL peak energy and the PL bandwidth was observed for InN nanorods with a critical diameter, which is of the same order of the surface electron accumulation layer (similar to 20 nm). Exceptionally large activation energy of the nanorods with the critical diameter implies that holes within these narrow nanorods need to surpass the band bending energy near the surface in order to recombine with electrons accumulated in the surface layer. (C) 2013 The Japan Society of Applied Physics
Plasmonic enhancement and control of optical nonlinearity in monolayer WS2
(2018-01-01)
Liang, Wei-Yun
;
Ahn, Hyeyoung
;
Sang, Yungang
;
Wang, Yanrong
;
Shi, Jinwei
A giant SHG enhancement is achieved from monolayer-WS2 incorporated onto 2D plasmonic metasurfaces (Ag-nanogroove-grating) in resonance with the excitonic resonance of monolayer-WS2. An optical encoding technique is demonstrated based on plasmonic manipulation of SHG polarization.
Plasmonic Enhancement and Manipulation of Optical Nonlinearity in Monolayer Tungsten Disulfide
(2018-10-01)
Shi, Jinwei
;
Liang, Wei-Yun
;
Raja, Soniya S.
;
Sang, Yungang
;
Zhang, Xin-Quan
Monolayer transition metal dichalcogenides (TMDCs) have large second-order optical nonlinearity owing to broken inversion symmetry in two-dimensional (2D) crystals. However, despite the strong light-matter coupling in monolayer TMDCs, their nonlinear responses are ultimately limited by subnanometer thickness. Here, a dramatic enhancement of the second-harmonic generation (SHG) is achieved from monolayer tungsten disulfide (WS2) incorporated onto a 2D silver (Ag) nanogroove grating with subwavelength pitch. By tuning surface plasmon mode and second-harmonic frequency in resonance with the C exciton in WS2, a large SHG enhancement factor (approximate to 400) and a large conversion efficiency (approximate to 2.0 x 10(-5)) can be obtained. Furthermore, the azimuthal angle dependence of polarized SHG from monolayer WS2 can be manipulated by the nanogroove plasmonic mode. Based on this property, a polarization-modulated optical encoding technique is demonstrated. The results suggest that 2D TMDC-plasmonic hybrid metasurface structures can provide an ideal integration platform for on-chip nonlinear photonics and plasmonics.
Plasmonic Green Nanolaser Based on a Metal-Oxide-Semiconductor Structure
(2011-10-01)
Wu, Chen-Ying
;
Kuo, Cheng-Tai
;
Wang, Chun-Yuan
;
He, Chieh-Lun
;
Lin, Meng-Hsien
Realization of smaller and faster coherent light sources is critically important for the emerging applications in nanophotonics and information technology. Semiconductor lasers are arguably the most suitable candidate for such purposes. However, the minimum size of conventional semiconductor lasers utilizing dielectric optical cavities for sustaining laser oscillation is ultimately governed by the diffraction limit (similar to(lambda/2n)(3) for three-dimensional (3D) cavities, where lambda is the free-space wavelength and n is the refractive index). Here, we demonstrate the 3D subdiffraction-limited laser operation in the green spectral region based on a metal oxide semiconductor (MOS) structure, comprising a bundle of green-emitting InGaN/GaN nanorods strongly coupled to a gold plate through a SiO(2) dielectric nanogap layer. In this plasmonic nanocavity structure, the analogue of MOS-type "nanocapacitor" in nanoelectronics leads to the confinement of the plasmonic field into a 3D mode volume of 8.0 x 10(-4) mu m(3) (similar to 0.14(lambda/2n)(3)).
Plasmonic Metasurfaces for Nonlinear Optics and Quantitative SERS
(2016-08)
Gwo, Shangjr
;
Wang, Chun-Yuan
;
Chen, Hung-Ying
;
Lin, Meng-Hsien
;
Sun, Liuyang
Plasmonic metasurfaces consist of two-dimensional arrays of metallic nanoresonators (plasmonic "meta-atoms"), which exhibit collective and tunable resonance properties controlled by electromagnetic near-field coupling. These man-made surfaces can produce a range of unique optical properties unattainable with natural materials. In this review, we focus on the emerging applications of metasurfaces with precisely engineered plasmonic properties for nonlinear optics and surface-enhanced Raman spectroscopy (SERS). In practice, these applications are quite susceptible to material losses and structural imperfections, such as variations in size, shape, periodicity of meta-atoms, and their material states (crystallinity, impurity, and oxidation, etc.). In these aspects, conventional top-down lithographic techniques are facing major challenges due to inherent limitations in intrinsic material properties and material quality introduced during growth, synthesis, and fabrication processes, as well as achievable lithographic resolution. Moreover, they are prohibitively expensive and time-consuming for fabrication over large areas. Here, we show that colloidal silver crystals (millimeter-sized single-crystalline plates and thiolate-capped nanoparticles) synthesized by solution-based chemical methods are excellent material platforms for the fabrication of high-quality plasmonic metasurfaces. In, particular, both top-down (focused ion-beam milling) and bottom-up (centimeter-scale self-assembly) techniques can be exploited to generate uniform and precisely engineered colloidal metasurfaces for broadband tunable (across the full visible range) second-harmonic generation and quantitative SERS at the single-molecule level.
Realization of Metal-Insulator Transition and Oxidation in Silver Nanowire Percolating Networks by Terahertz Reflection Spectroscopy
(2014-01-08)
Tsai, Yao-Jiun
;
Chang, Chi-Ying
;
Lai, Yi-Chun
;
Yu, Pei-Chen
;
Ahn, Hyeyoung
Metal nanowires (NWs) enable versatile applications in printed electronics and optoelectronics by serving as thin and flexible transparent electrodes. The performance of metal NWs as thin electrodes is highly correlated to the connectivity of NW meshes. The percolation threshold of metal NW films corresponds to the minimum density of NWs to form the transparent, yet conductive metal NW networks. Here, we determine the percolation threshold of silver NW (AgNW) networks by using morphological analysis and terahertz (THz) reflection spectroscopy. From the divergent behavior of carrier scattering time and the increase of carrier backscattering factor, the critical NW density at which crossover from Drude to non-Drude behavior of THz conductivity occurs can be unambiguously determined for AgNW thin films. Furthermore, the natural oxidation of AgNWs which causes the gradual reduction of the connectivity of the AgNW network is also realized by the THz spectroscopy. The selective oxidation of NW-to-NW junctions weakens the ohmic contact, and for AgNWs near a critical density, it can even lead to metal insulator transition. The presented results offer invaluable information to accelerate the deployment of metal nanowires for next-generation electronics and optoelectronics on flexible substrates.
Spectral Dependence of Third-Order Nonlinear Optical Properties in InN
(2015)
Ahn, Hyeyoung
;
Lee, Min-Tse
;
光電工程學系
;
Department of Photonics
Spectral dependence of nonlinear optical absorption cross-section (sigma) near and below the bandgap (E-g) is measured for InN by using Z-scan technique. A drastic increase of sigma near E-g can be understood by the band-filling model.
Strong green photoluminescence from In(x)Ga(1-x)N/GaN nanorod arrays
(2009-09-28)
Hong, Chi-Chang
;
Ahn, Hyeyoung
;
Wu, Chen-Ying
;
Gwo, Shangjr
;
光電工程學系
We report intense green photoluminescence (PL) from vertically aligned indium gallium nitride (In(x)Ga(1-x)N) nanorod arrays. The formation of In(x)Ga(1-x)N/GaN-heterostructure nanorods increases the localization depth of the radially confined carriers (> 100 meV). Temperature dependent PL peak energy of InGaN nanorods shows the characteristic S-shaped behavior, indicating the prominent carrier trapping in band-tail states associated with the nonuniformity of In content. Time-resolved PL (TRPL) response decays biexponentially and the dominant slow decay component of TRPL for In(x)Ga(1-x)N nanorods is due to the transfer of excitons to the localized states before the radiative decay. (C) 2009 Optical Society of America