Browsing by Author "Wu, Chung-Lun"

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Catalytically solid-phase self-organization of nanoporous SnS with optical depolarizability
(2016)
Cheng, Chih-Hsien
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Chi, Yu-Chieh
;
Wu, Chung-Lun
;
Lin, Chun-Jung
;
Tsai, Ling-Hsuan
The catalytic solid-phase synthesis of self-organized nanoporous tin sulfide (SnS) with enhanced absorption, manipulative transmittance and depolarization features is demonstrated. Using an ultralow radio-frequency (RF) sputtering power, the variation of the orientation angle between the anodized aluminum oxide (AAO) membrane and the axis of the sputtered ion beam detunes the catalytically synthesized SnS from nanorod to nanoporous morphology, along the sidewall of the AAO membrane. The ultraslow catalytic sputtering synthesis on the AAO at the RF plasma power of 20 W and the orientation angle of 0 degrees regulates the porosity and integrality of nanoporous SnS, with average pore diameter of 80-150 nm. When transferring from planar to nanoporous structure, the phase composition changes from SnS to SnS2-Sn2S3, and the optical bandgap shrinks from 1.43 to 1.16 eV, due to the preferred crystalline orientation, which also contributes to an ultralow reflectance of <1% at 200-500 nm when both the transmittance and the surface scattering remain at their maxima. The absorption coefficient is enhanced by nearly one order of magnitude with its minimum of >5 x 10(4) cm(-1) at the wavelength between 200 and 700 nm, due to the red-shifting of the absorption spectrum to at least 100 nm. The catalytically self-organized nanoporous SnS causes strong haze and beam divergence of 20 degrees-30 degrees by depolarized nonlinear scattering at the surface, which favors the solar energy conversion with reduced surface reflection and enhanced photon scattering under preserved transmittance.
Efficient wavelength conversion with low operation power in a Ta2O5-based micro-ring resonator
(2017-12-01)
Wu, Chung-Lun
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Huang, Jen-Yang
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Ou, Ding-Hsin
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Liao, Ting-Wei
;
Chiu, Yi-Jen
The Ta2O5-based micro-ring resonator with an unloaded quality factor of 182,000 has been demonstrated to realize efficient nonlinear wavelength generation. The propagation loss of the resonator is 0.5 cm(-1), and the buildup factor of the ring resonator is estimated to be similar to 50. With a high buildup factor of the ring structure, the four-wave-mixing (FWM) conversion efficiency of -30 dB is achieved in the resonator with a pump power of 6 mW. Based on power-dependent FWM results, the nonlinear refractive index of Ta2O5 is estimated to be 1.4x10-14 cm(2)/W at a wavelength of similar to 1550 nm. The demonstration of an enhanced FWM process in the Ta(2)O(5-)based micro-ring cavity implies the possibility of realizing FWM-based optical parametric oscillation in a Ta2O5-based micro-ring resonator. (C) 2017 Optical Society of America
Nonstoichiometric SiC Bus/Ring Waveguide Based All-Optical Data Format Follower and Inverter
(2016-05)
Su, Sheng-Pin
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Wu, Chung-Lun
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Cheng, Chih-Hsien
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Huang, Bo-Ji
;
Wang, Huai-Yung
Nonstoichiometric silicon carbide (SiC) bus/ring waveguide resonator based all-optical data-format follower and inverter is demonstrated to perform ultrafast dual functional wavelength and data-format conversion. The buried Si quantum dots (Si-Qps) with strong quantum confinement effect effectively result in a significant change on the nonlinear refractive index of 3.14 X 10(-1)3 cm(2)/W, which induces a red shift of 0.07 nm on the resonant wavelength comb of the nonstoichiometric SiC ring waveguide resonator. By injecting the 12 Gbit/s optical pulsed return-to-zero on-off keying (PRZ-OOK) data stream into the ring waveguide at a notched wavelength of 1551.08 nm, the induced red-shift on resonant wavelengths allows versatile all-optical data processing functions, including data wavelength conversion and data format following/inversion, on the data patterns transferred to the probe. The eye-opening diagram analysis at 12 Gbit/s reveals that the data transferred to the probe can provide a signal-to-noise ratio of 9.4 dB, and a receiving power penalty is degraded by 2.6 dB as compared to that of the pump data.
Photostriction of strontium ruthenate
(2017-04-24)
Wei, Tzu-Chiao
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Wang, Hsin-Ping
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Liu, Heng-Jui
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Tsai, Dung-Sheng
;
Ke, Jr-Jian
Transition metal oxides with a perovskite crystal structure exhibit a variety of physical properties associated with the lattice. Among these materials, strontium ruthenate (SrRuO3) displays unusually strong coupling of charge, spin and lattice degrees of freedom that can give rise to the photostriction, that is, changes in the dimensions of material due to the absorption of light. In this study, we observe a photon-induced strain as high as 1.12% in single domain SrRuO3, which we attribute to a nonequilibrium of phonons that are a result of the strong interaction between the crystalline lattice and electrons excited by light. In addition, these light-induced changes in the SrRuO3 lattice affect its electrical resistance. The observation of both photostriction and photoresistance in SrRuO3 suggests the possibility of utilizing the mechanical and optical functionalities of the material for next-generation optoelectronics, such as remote switches, light-controlled elastic micromotors, microactuators and other optomechanical systems.
Pre-Chirped Pulse Excitation Enhanced Terahertz Radiation
(2016-03)
Lee, Chao-Kuei
;
Lin, Yuan-Yao
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Lin, Gong-Ru
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Wu, Chung-Lun
;
Wu, Tsung-Han
The enhanced broadband THz radiation excited by pre-chirping the femtosecond pumping laser pulse is explored from a photoconductive antenna made by the low-temperature molecular-beam-epitaxy (LT-MBE) grown Gallium Arsenide (GaAs). Nearly 3-dB enlargement on peak amplitude of THz radiation accompanied with broadened bandwidth is achieved with positively chirped pulse excitation. The laser with short pulsewidth assists the broadening of THz bandwidth, whereas the peak intensity of THz relies strictly on lengthened carrier lifetime. The enhancement of high-frequency radiation components at low pumping intensity also indicates the nature of power dependence in such a coherently controlled process. When photocurrent saturation occurs in the LT-MBE-grown GaAs under high-power pumping condition, the clipping effect on peak amplitude of current pulse lengthens the duration of current surge effect. Both the peak intensity and the frequency spectrum of the generated THz pulse concurrently declines. The physical mechanism related to pre-chirped pulse excitation dependent carrier dynamics is proposed to elucidate the spectral response as well as the power dependence in such a coherently controlled THz radiation process.
Strong optical nonlinearity of the nonstoichiometric silicon carbide
(2015-01-01)
Cheng, Chih-Hsien
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Wu, Chung-Lun
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Lin, Yung-Hsiang
;
Yan, Wen-Long
;
Shih, Min-Hsiung
Enhanced nonlinear refractive indices and absorption coefficients of nonstoichiometric SixC1-x with varying C/Si composition ratios from 0.51 to 1.83 grown by low-temperature plasma-enhanced chemical vapor deposition are demonstrated. When increasing the [CH4]/[CH4 + SiH4] fluence ratio from 70% to 92%, two Raman scattering signals at 795 and 970 cm(-1) contributed by transverse and longitudinal optical modes of Si-C bonds are simultaneously enhanced in stoichiometric and C-rich SixC1-x samples, whereas the Si-associated Raman peak at 520 cm(-1) decreases with increasing C/Si composition ratio. The C-rich SixC1-x film shows additional Raman scattering peaks at 1330 and 1580 cm(-1), which are individually contributed by the diamond-like C-C bonds and the G peak of the graphite-like C-C bonds, respectively. The nonlinear refractive index increases to 1 +/- 0.1 x 10(-11) cm(2) W-1 upon increasing the C/Si composition ratio up to 1.83, which is four orders of magnitude higher than that of bulk SiC. The increased C/Si composition ratio reduces the lattice constant and the effective mass of the SixC1-x film and enhances its nonlinear refractive index effectively. In comparison with the sp(3)-orbital C-C bond, the sp(2)-orbital C-C bond in SixC1-x films dominates the enhancement of nonlinear refractive index. By contrast, the nonlinear absorption coefficient changes sign and decreases magnitude when transferring the SixC1-x film from Si-rich to C-rich conditions. This sign reversal is primarily attributed to the conversion in the absorption mechanism from two-photon absorption to nonlinear saturable absorption. Unlike Si-Si bonds which favor two-photon absorption, the sp(2)-orbital C-C bonds with delocalized pi-electrons essentially enhance the saturable absorbance. Hence, a higher nonlinear refractive index and a higher saturable absorption coefficient can be obtained by enriching the sp(2)-orbital C-C bonds in C-rich SixC1-x films.
Tens of GHz Tantalum pentoxide-based micro-ring all-optical modulator for Si photonics
(2017-03-01)
Wu, Chung-Lun
;
Hsieh, Cheng-Hsuan
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Lin, Gong-Ru
;
Chi, Wen-Chun
;
Chiu, Yi-Jen
A tantalum pentoxide-based (Ta2O5-based) micro-ring alloptical modulator was fabricated. The refractive index inside the micro-ring cavity was modified using the Kerr effect by injecting a pumped pulse. The transmittance of the ring resonator was controlled to achieve all-optical modulation at the wavelength of the injected probe. When 12 GHz pulses with a peak power of 1.2 W were coupled in the ring cavity, the transmission spectrum of the Ta2O5 resonator was redshifted by 0.04 nm because of the Kerr effect. The relationship between the modulation depth and gap of the Ta2O5 directional coupler is discussed. An optimized gap of 1100 nm was obtained, and a maximum buildup factor of 11.7 with 84% modulation depth was achieved. The nonlinear refractive index of Ta2O5 at 1.55 mu m was estimated as 3.4 x 10(-14) cm(2)/W based on the Kerr effect, which is almost an order of magnitude higher than that of Si3N4. All results indicate that Ta2O5 has potential for use in nonlinear waveguide applications with modulation speeds as high as tens of GHz.
Transferring the bendable substrateless GaN LED grown on a thin C-rich SiC buffer layer to flexible dielectric and metallic plates
(2017-01-21)
Cheng, Chih-Hsien
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Huang, Tzu-Wei
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Wu, Chung-Lun
;
Chen, Mu Ku
;
Chu, Cheng Hung
By growing an epitaxial GaN LED on C-rich a-SiC buffer deposited SiO2/Si substrate, the simplified transfer to versatile flexible metallic/dielectric membranes is demonstrated. Both the high growth temperature at 1000 degrees C and the slow deposition rate played important roles in the meticulous MOCVD growth of n-GaN along the surface normal of the a-SiC at the very beginning. High substrate temperature facilitated the refinement of C-rich SiC buffer from amorphous to partially crystalline with (0004)-orientation, which effectively reduces the lattice mismatch between n-GaN and SiC at the interface so as to gradually improve the crystalline n-GaN regrowth. The substrateless GaN LED transferred to flexible copper plates showed reduced turn-on voltage of 2.6 V, enhanced output power of 370 mW, enlarged power-to-current slope of 1.24 W A(-1), increased external quantum efficiency of 45%, and reduced efficiency droop of 15% under a bias of 300 mA. The thermal conductivity of the transferred substrate affected the EL peak wavelength shift of the substrateless GaN LED on SiC buffer. Heating the GaN LED on flexible copper plate to 65 degrees C only reduced its power by 10% and red-shifted its wavelength by 1 nm under a bias at 100 mA; the Auger effect resulted in a degraded EQE of 39.1% and an enlarged EQE droop of 8.5%. Bending the surface diameter of the curvature of the copper plate to 1.2 cm decayed the output power by 12% and red-shifted the EL peak by 5 nm because of the lattice strain induced quantum confined Stark effect (QCSE). Such a bendable substrateless GaN LED transferred to flexible membrane with superior heat dissipation and bending tolerance is a desired lighting element for green photonics in this era.