Browsing by Author "Sun, Chi-Kuang"

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Acoustic Velocity and Optical Index Birefringence in A-Plane ZnO Thin Films
(2011-02-01)
Chen, Yi-Hsin
;
Wen, Yu-Chieh
;
Liu, Wei-Rein
;
Hsieh, Wen-Feng
;
Sun, Chi-Kuang
The longitudinal acoustic velocity in the direction normal to the c-axis and the birefringence of refractive index of a-plane zinc oxide (ZnO) thin films were measured by ultrafast transient reflection measurement. Through Fourier analysis, the experimental data show that the acoustic velocity is about 6000 m/s and the measured refractive index near the band gap, which is higher than the previously reported results by 20 similar to 30%.
Air-core microstructure fiber for terahertz radiation waveguiding
(2007)
Lu, Ja-Yu
;
Yu, Chin-Ping
;
Chang, Hung-Chung
;
Chen, Hung-Wen
;
Li, Yu-Tai
We demonstrate a simple and low-loss THz microstructure fiber for broadband THz waveguiding, which is constructed by using the highly flexible and readily available materials. Substantially low attenuation constant less than 0.01 cm(-1) has been achieved. (c) 2006 Optical Society of America
Bipolar Cascade Superluminescent Diodes at the 1.04-mu m Wavelength Regime
(2009-03-01)
Guol, Shi-Hao
;
Wang, Jr-Hung
;
Wu, Yu-Huei
;
Lin, Wei
;
Yang, Ying-Jay
In this study, we investigate the performance of GaAs-based bipolar cascade superluminescent diodes with different cavity lengths. The device operates around the important bio-optical therapeutic 1.04-mu m wavelength window. The introduction of tunnel junctions tends to minimize the nonuniform carrier distribution between distinct multiple quantum-wells (QWs), which is a problem in conventional SLDs, whose electroluminescent spectra are governed by the center wavelength of QWs near the p-side. Our devices exhibit nice electrical characteristics of low leakage current and overcome the limitation of nonuniform carrier distribution, thereby presenting a promising prospect for the near infrared white-light sources.
Direct Backward Third Harmonic Generation in Nanostructures
(2010-01-01)
Chang, Chieh-Feng
;
Chen, Hsing-Chao
;
Chen, Miin-Jang
;
Liu, Wei-Rein
;
Hsieh, Wen-Feng
We theoretically and experimentally demonstrated that direct backward third harmonic generation (THG) waves can be comparable in magnitude with forward THG waves in nanostructures, such as ZnO thin films and nanoparticles of CdSe and Fe3O4. (C) 2010 Optical Society of America
Direct backward third-harmonic generation in nanostructures
(2010-03-29)
Chang, Chieh-Feng
;
Chen, Hsing-Chao
;
Chen, Miin-Jang
;
Liu, Wei-Rein
;
Hsieh, Wen-Feng
Direct-backward third harmonic generation (DBTHG) has been regarded as negligible or even inexistent due to the large value of wave-vector mismatch. In the past, BTHG signals were often interpreted as back-reflected or back-scattered forward-THG (FTHG). In this paper, we theoretically and experimentally demonstrate that backward third harmonic waves can be directly generated, and that their magnitude can be comparable with FTHG in nanostructures. Experimental data of DBTHG from ZnO thin films, CdSe quantum dots and Fe(3)O(4) nanoparticles agree well with simulation results based on the Green's function. An integral equation was also derived for fast computation of DBTHG in nano films. Our investigation suggests that DBTHG can be a potentially powerful tool in nano-science research, especially when combined with FTHG measurements. (C) 2010 Optical Society of America
Investigation on spectral loss characteristics of subwavelength terahertz fibers
(2007-05-01)
Chen, Hung-Wen
;
Li, Yu-Tai
;
Pan, Ci-Ling
;
Kuo, Jeng-Liang
;
Lu, Ja-Yu
By measuring the spectral loss characteristics of subwavelength-diameter terahertz fibers, our study supports the recent theory proposed by M. Sumetsky [Opt. Lett. 31, 870 (2006)] that diameter-variation-induced radiation is a dominant loss mechanism for subwavelength fibers in the low- (<1%) core-fraction-power regime. This physical mechanism limits the lowest guidable frequency in a subwavelength fiber. (c) 2007 Optical Society of America.
Magnitude-tunable sub-THz shear phonons in a non-polar GaN multiple-quantum-well p-i-n diode
(2012-05-14)
Chen, Chien-Cheng
;
Huang, Huei-Min
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
;
Sun, Chi-Kuang
Coherent transverse acoustic phonons are optically generated and detected through the piezoelectric coupling between the build-in electric fields and shear strains of a non-polar GaN multiple quantum wells embedded in a p-n junction. By optical transient transmission change measurement, the phonon frequency is observed to be 0.4 THz which corresponds to a wavelength of 12.5 nm, the periodicity of the multiple quantum wells, and the estimated phonon velocity corresponds to the transverse acoustic phonon velocity in GaN. Moreover, we can magnify the driving amplitude of the generated shear phonons by increasing the reverse bias of the p-i-n diode. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4718524]
Observation of Femtosecond Acoustic Anomaly in a Solid Liquid Interface
(2020-02-06)
Sun, Chi-Kuang
;
Yao, Yi-Ting
;
Shen, Chih-Chiang
;
Ho, Mu-Han
;
Lu, Tien-Chang
Here we report a femtosecond anomaly observed at a solid liquid interface: femtosecond acoustic echo being advanced by the application of a viscous liquid to the solid surface. Through extensive simulation and experiments, we attribute this new discovery to the effect introduced by the surface-adsorbed water layer, switching between properties more viscous than water and more solid-like than ice. Our study further indicates a new technology paradigm: the femtosecond acoustic system to probe the viscoelastic properties of the surface-adsorbed water layer, which is not achievable by any available current technology.
Photogeneration of coherent shear phonons in orientated wurtzite semiconductors by piezoelectric coupling
(2009-11-01)
Wen, Yu-Chieh
;
Ko, Tsung-Shine
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
;
Chyi, Jen-Inn
Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in a-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.
Resonance-enhanced dipolar interaction between terahertz photons and confined acoustic phonons in nanocrystals
(2008-03-03)
Liu, Tzu-Ming
;
Lu, Ja-Yu
;
Chen, Hung-Ping
;
Kuo, Chung-Chiu
;
Yang, Meng-Ju
Taking advantage of the specific core-shell charge separation structure in the CdSe/CdTe core-shell type-II nanocrystals, we experimentally observed and verified the existence of the resonance-enhanced dipolar interaction between terahertz photons and their corresponding confined acoustic phonons. From the measured terahertz transmission spectra, we found that the photon frequency of the terahertz resonant absorption is inversely proportional to the diameter D of the nanocrystals and agrees with that of dipolar active l=1, n=0 confined acoustic modes. The corresponding absorption cross section shows a D(4) dependence, supporting a charged simple-harmonic-oscillator model. These facts verify the occurrence of dipolar interaction between terahertz photons and confined terahertz acoustic phonons. (c) 2008 American Institute of Physics.
Resonant-Enhanced Dipolar Interaction between THz-Photons and Confined Acoustic Phonons in Nanocrystals
(2008)
Liu, Tzu-Ming
;
Lu, Ja-Yu
;
Chen, Hung-Ping
;
Kuo, Chung-Chiu
;
Lai, Chih-Wei
We proved the existence of resonant-enhanced dipolar interaction between THz-photons and confined acoustic phonons in nanocrystals. By a specific core-shell charge separation, the dipolar (l=1) confined acoustic modes was activated to absorb THz waves. (C) 2007 Optical Society of America
Resonant-enhanced dipolar interaction between THz-photons and confined acoustic phonons in nanostructures
(2008-01-01)
Liu, Tzu-Ming
;
Lu, Ja-Yu
;
Kuo, Chung-Chiu
;
Yang, Meng-Ju
;
Lai, Chih-Wei
By using a frequency-controlled narrow band THz source, a Fourier Transform Infrared (FTIR) spectroscopy system, and a frequency-controlled terahertz (THz) emitter, for the first time, we studied the THz photon absorption related to the THz confined acoustic vibrations in semiconductor nanocrystals. Through a specific charge separation in the CdSe/CdTe type-II nanocrystals and a piezoelectric coupling in the wurtzite CdSe nanocrystals, the THz photons can be resonantly coupled with (l=1) dipolar modes and the (l=0) breathing modes, respectively. Our results provide new mechanisms for low dimensional systems to convert a THz photon into a phonon of the same frequency.
Spectral Loss Characteristics of Subwavelength THz Fibers
(2007)
Chen, Hung-Wen
;
Lu, Ja-Yu
;
Chen, Li-Jin
;
Li, Yu-Tai
;
Pan, Ci-Ling
From the spectral loss characteristics of subwavelength-diameter THz fibers, our study supports the recent theory proposed by M. Sumetsky that diameter-variation-induced-radiation is a dominant loss mechanism for subwavelength fibers which limits the lowest guidable frequency. (C) 2007 Optical Society of America
Terahertz air-core microstructure fiber
(2008-02-11)
Lu, Ja-Yu
;
Yu, Chin-Ping
;
Chang, Hung-Chung
;
Chen, Hung-Wen
;
Li, Yu-Tai
A low-loss terahertz air-core microstructure fiber is demonstrated for terahertz waveguiding. Substantially low attenuation constant less than 0.01 cm(-1) has been achieved and the guiding wavelength is found to be tunable by linear scaling the fiber size. The experimental results well agree with the simulation based on the finite-difference frequency-domain method, which interprets the guiding mechanism as the antiresonant reflecting waveguiding. The simulated modal pattern shows that most terahertz field is concentrated inside the central hollow air core and is guided without outside interference, which has high potential for guiding intense terahertz waves with minimized loss. (C) American Institute of Physics.
Terahertz scanning imaging with a subwavelength plastic fiber
(2008-02-25)
Lu, Ja-Yu
;
Chiu, Chui-Min
;
Kuo, Chung-Chiu
;
Lai, Chih-Hsien
;
Chang, Hung-Chung
The feasibility to perform terahertz imaging utilizing a terahertz subwavelength plastic fiber is investigated. Dour study shows that, With a low (<1%) fractional power inside the fiber score, the bending loss of the terahertz subwavelength fiber is acceptable to enable large area scanning without seriously sacrificing the signal-to-noise ratio of the acquired image. With a transmission geometry, this feasibility is demonstrated by direct two-dimensional scanning of the terahertz subwavelength fiber output end to image different biological samples and concealed substances. (C) 2008 American Institute of Physics.
THz Fiber Directional Coupler
(2007)
Chen, Hung-Wen
;
Lu, Ja-Yu
;
Chen, Li-Jin
;
Chiang, Po-Jui
;
Chang, Hung-Chun
We demonstrated a THz subwavelength fiber coupler for future millimeter wave applications. Unlike traditional optical fiber couplers, its coupling ratio is independent of the length of the coupling region because of the anti-symmetric mode cutoff. (C) 2007 Optical Society of America
THz interferometric imaging using subwavelength plastic fiber based THz endoscopes
(2008-02-18)
Lu, Ja-Yu
;
Kuo, Chung-Chiu
;
Chiu, Chui-Min
;
Chen, Hung-Wen
;
Hwang, Yuh-Jing
We demonstrate a new reflective imaging technique using continuous-wave THz fiber-endoscopy, in which the sample is placed behind the output of a THz subwavelength plastic fiber and the Fabry Perot interference is formed therein. 3D THz reflective images with a reasonable SNR as well as high lateral and subwavelength axial resolutions are acquired by moving the sample along the axial (z) direction and by 2D scanning the output end of the subwavelength plastic fiber without any focusing medium. By analyzing the axial-position dependent THz signals backward collected by the subwavelength plastic fiber, the THz reflection amplitudes and phases on the sample surface can be successfully reconstructed. (C) 2008 Optical Society of America.
THz interferometric imaging using subwavelength plastic fiber based THz endoscopes
(2008-01-01)
Lu, Ja-Yu
;
Kuo, Chung-Chiu
;
Chiu, Chui-Min
;
Chen, Hung-Wen
;
Pan, Ci-Ling
We demonstrate a continuous-wave THz fiber-endoscopy by utilizing low-loss THz subwavelength plastic fibers. The reconstructed 3D images not only reflect the depth variation of the object surface, but also reveal the molecular distribution of samples. (C) 2008 Optical Society of America