Browsing by Author "WU, KH"

Now showing 1 - 20 of 22

A PULSED XE ION LASER WITH A DOUBLE-DISCHARGE EXCITATION
(1990-07-01)
WU, KH
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
;
Department of Electrophysics
BIREFRINGENCE-INDUCED SPECTRAL FEATURES IN CONTINUOUS-WAVE AND PASSIVELY MODE-LOCKED TI-SAPPHIRE LASERS
(1992-11-01)
PAN, CL
;
HWANG, CD
;
KUO, JC
;
SHIEH, JM
;
WU, KH
We show that the transient spectrum of a continuous-wave Ti:sapphire laser during the initial stage of the buildup, which exhibits multiple clusters (of longitudinal modes), is due to the spectral windowing effect of the Brewster-angle-cut Ti:sapphire crystal, which acts as a birefringent filter in the laser cavity. This effect is also responsible for certain spectral features of a passively mode-locked Ti:sapphire/DDI laser.
BUILDUP DYNAMICS OF THE SPECTRUM AND THE AVERAGE OUTPUT POWER OF A HOMOGENEOUSLY BROADENED CONTINUOUS-WAVE TI-SAPPHIRE LASER
(1992-07-15)
PAN, CL
;
KUO, JC
;
HWANG, CD
;
SHIEH, JM
;
LAI, Y
The buildup dynamics of a homogeneously broadened continuous-wave Ti:sapphire laser is experimentally investigated for the first time to our knowledge. The average output power of the laser exhibits relaxation oscillation behavior initially and approaches the steady-state value in almost-equal-to 20-mu-s. Concurrently, the longitudinal-mode spectrum evolves from a multicluster spectral distribution (tentatively attributed to the spectral windowing effect of the Brewster-angle-cut Ti:sapphire crystal as a birefringent filter) to a single cluster at 787.5 nm. The spectral narrowing (which is due to gain competition in the homogeneously broadened medium), however, takes almost-equal-to 500-mu-s. Physical mechanisms that explain qualitatively the two distinctive time scales are presented.
CRITICAL-CURRENT ANISOTROPY AND FLUX-PINNING IN YBCO(001) AND YBCO(110) SUPERCONDUCTING THIN-FILMS PREPARED BY KRF PULSED-LASER ABLATION
(1993)
JUANG, JY
;
WU, KH
;
WANG, SJ
;
LAI, TC
;
KAO, RT
EFFECT OF DYE CONCENTRATION ON PICOSECOND AND FEMTOSECOND CW PASSIVELY MODE-LOCKED TI-SAPPHIRE DDI LASERS
(1992-10-15)
PAN, CL
;
HWANG, CD
;
KUO, JC
;
SHIEH, JM
;
WU, KH
The role of the saturable absorber dye DDI in the cw passively mode-locked Ti:sapphire/DDI laser both with and without compensation prisms is investigated. This laser self-starts when the concentration of the dye is greater than almost-equal-to 2 X 10(-4) M for both picosecond and femtosecond pulses. With the prism pair, the laser generates Gaussian pulses 150 fs in duration. The corresponding spectrum is sech2 in shape with a FWHM of 8 nm. These parameters are independent of the DDI concentration. Without the prism pair, the laser generates picosecond pulses with the FWHM decreasing from 8 to 3 ps as the dye concentration is increased from 2.5 x 10(-4) to 1 X 10(-3) M. Concurrently the pulse shape changes from a Lorentzian to a nearly symmetric two-sided exponential shape. The picosecond spectra for different dye concentrations are also presented.
EFFECTS OF SUBSTRATE TREATMENTS ON THE GROWTH OF YBCO SUPERCONDUCTING THIN-FILMS ON MGO BY PULSED-LASER ABLATION
(1993-12-01)
WU, KH
;
LI, MH
;
JUANG, JY
;
UEN, TM
;
GOU, YS
Different treatments on single crystalline MgO(100) substrates have been performed to study the effects of the substrate conditions on the growth mechanisms and superconducting properties of YBCO thin films deposited in-situ by pulsed laser ablation process. The nucleation and growth of YBCO thin films on these substrates during deposition have been studied by using the in-situ resistance measurement method. The characteristics of as-deposited films have also been investigated by X-ray diffraction analysis (XRD), temperature dependent resitivity measurement (R - T), temperature dependent critical current density measurement (J(c) - T), scanning electron mircoscopy (SEM), and scanning tunneling mircoscopy (STM). It has been found that, while the zero-resistance transition temperature (T(co)) and the overall crystallographic orientation of the films are only affected slightly, the surface morphology and grain structure of the films have shown strong dependencies on the substrate conditions. Recognizing that for YBCO grown on MgO, due to the large lattice mismatch between the film and substrate, the film growth is semicoherent in nature, the observed results are consistently explained by attributing to the interplays between nucleation and grain coalescence processes. The dramatic differences in superconducting transport properties exhibited by films obtained with the conditions studied are also reported to reinforce the arguments proposed.
EFFECTS OF ZRO2 ON THE TEXTURING AND PROPERTIES OF MELT-PROCESSED Y1BA2CU3O7-DELTA
(1994-04-25)
JUANG, JY
;
WU, CL
;
WANG, SJ
;
CHU, ML
;
WU, KH
The addition of ZrO2 is found to be very effective as the nucleation sites to promote the oriented growth of Y1Ba2Cu3O7-delta (YBCO) during peritectic solidification. Extended regions of well-textured YBCO grains with superior superconducting properties of the YBCO bulk are readily obtained by a single-step melt process at much lower process temperatures. At 77 K, the platelike Josephson weak-link behaviors observed in pure melt-processed YBCO have been effectively eliminated, indicating that the coherency between the spherulitic superconducting YBCO grains may have been greatly improved by ZrO2 additions.
ENHANCEMENT OF CRITICAL-CURRENT DENSITY IN DIRECT-CURRENT-SPUTTERED TLBA2CA2CU3O9-/+DELTA SUPERCONDUCTING THIN-FILMS
(1995-02-13)
JUANG, JY
;
HORNG, JH
;
CHEN, SP
;
FU, CM
;
WU, KH
FABRICATION AND PROPERTIES OF DC-SPUTTERED TL-1223 SUPERCONDUCTING THIN-FILMS
(1995-06-01)
JUANG, JY
;
HORNG, JH
;
LIN, HC
;
WANG, SJ
;
FU, CM
A multi-step deposition and post annealing scheme has been developed for preparing nearly single phase, c-axis oriented TlBa2Ca2Cu3O9 (Tl-1223) superconducting thin films by dc-sputtering. The effects of the compensates used during post annealing on final phase formation suggest the crucial role played by thermodynamic conditions and further validates the viability of the present process scheme. In particular, films obtained by the present process have shown, for the first time in this system, a critical current density (Jc) above 10(6) A/cm(2) at 17 K with a zero- resistance transition temperature (Tco) above 110 K. The results are attributed to the improvement of film morphology, specifically a decreased density of weak-linking.
GROWTH AND PROPERTIES OF SUBMILLIMETER SINGLE GRAIN TL2BA2CA2CU3O10+X SUPERCONDUCTING THIN-FILMS
(1995-10-02)
LIN, HC
;
UEN, TM
;
LIU, CK
;
JUANG, JY
;
WU, KH
Distinct crystalline superconducting grains of Tl2Ba2Ca2Cu3O10+x(Tl-2223) with sizes up to 150 mu mx150 mu mX1 mu m were prepared by a single target dc-sputtering process followed by proper postannealing at temperatures around 910 degrees C. A reactive ion etching process was developed to pattern an similar to 10 mu m wide bridge with four contact leads directly on a single grain to study its superconducting transport properties. The etching process was found to have only minimal effect on the superconducting transition temperature T-c, which was typically around 110 K. Grains grown on MgO(100) and LaAlO3(100) substrates, although, displaying somewhat different in-plane orientations, all had critical current densities J(c), well above 10(6) A/cm2 at 77 K. The existence of only one grain boundary, however, can reduce the J(c) by orders of magnitude. (C) 1995 American Institute of Physics.
GROWTH AND PROPERTIES OF SUBMILLIMETER SINGLE GRAIN TL2BA2CA2CU3O10+X SUPERCONDUCTING THIN-FILMS
(1995-10-02)
LIN, HC
;
UEN, TM
;
LIU, CK
;
JUANG, JY
;
WU, KH
Distinct crystalline superconducting grains of Tl2Ba2Ca2Cu3O10+x(Tl-2223) with sizes up to 150 mu mx150 mu mX1 mu m were prepared by a single target dc-sputtering process followed by proper postannealing at temperatures around 910 degrees C. A reactive ion etching process was developed to pattern an similar to 10 mu m wide bridge with four contact leads directly on a single grain to study its superconducting transport properties. The etching process was found to have only minimal effect on the superconducting transition temperature T-c, which was typically around 110 K. Grains grown on MgO(100) and LaAlO3(100) substrates, although, displaying somewhat different in-plane orientations, all had critical current densities J(c), well above 10(6) A/cm2 at 77 K. The existence of only one grain boundary, however, can reduce the J(c) by orders of magnitude. (C) 1995 American Institute of Physics.
GROWTH OF PULSED-LASER DEPOSITED YBCO THIN-FILMS ON YSZ AND YSZ/SI SUBSTRATES
(1994-02-01)
JUANG, JY
;
KAO, RT
;
HSIEH, MC
;
LI, MH
;
CHU, ML
The growth mechanism of superconducting YBCO thin films deposited in - situ on both single crystalline YSZ and YSZ-buffered Si substrates as revealed by scanning tunneling microscopy (STM) are found to be very different. The results are attributed to the initial surface condition of the substrates.
INFLUENCE OF AERODYNAMIC AND ELECTRICAL-DISCHARGE HOMOGENEITY ON A TRANSVERSELY EXCITED CW CO2-LASER
(1987-01-01)
WU, KH
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
;
Department of Electrophysics
INSITU DEPOSITION OF SUPERCONDUCTING YBCO FILMS ON SI WITH CERAMIC BUFFER LAYERS
(1993-03-01)
JUANG, JY
;
WU, KH
;
LAI, TC
;
LEE, MH
;
HSEIH, MC
Systematic studies on the optimization of depositing superconducting YBCO thin films on Si substrates with YSZ buffer layers by pulsed laser ablation have been carried out. Nearly c-axis oriented YBCO films with T(co) almost-equal-to 85 K were obtained routinely on Si(100) substrates with in-situ deposition of YSZ layers on top. Preliminary measurements on the critical current density Jc of these films showed Jc > 2.0 x 10(4) A/cm2 at 77K and Jc > 1.4 x 10(6) A/CM2 at 20K. The significant improvements on Jc are attributed to the great reduction of microcracks and the crystallinity of YSZ layer, achieved by the in-situ annealing of the buffer layer prior to the YBCO deposition. The effects of oxygen partial pressure and substrate temperature during YSZ deposition on the crystallinity of YSZ layer as well as on the quality of subsequently deposited YBCO films will be discussed. In our most recent attempt the microcracks has been sucessfully eliminated by a process combining the in-situ annealing and slow cooling process.
INSITU GROWTH OF Y1BA2CU3O7-X SUPERCONDUCTING THIN-FILMS USING A PULSED NEODYMIUM YTTRIUM-ALUMINUM-GARNET LASER WITH CO2-LASER HEATED SUBSTRATES
(1991-03-11)
WU, KH
;
LEE, CL
;
JUANG, JY
;
UEN, TM
;
GOU, YS
By replacing the resistive heater with a CO2 laser to heat the substrates together with concentered oxygen stream blowing onto the substrate during deposition, high quality Y1Ba2Cu3O7-x thin films with nearly perfect c-axis orientation and T(c0) = 87 K were grown in situ without subsequent slow cooling in oxygen atmosphere or any further heat treatment. The rapid quenching, from typically 600-degrees-C to room temperature in less than 50 s of the as-deposited films, excludes the possibility of further oxygen incorporation during the cooling process and indicates that the in situ growth may have completed during deposition under proper deposition conditions.
MICROWAVE-ENHANCED SUPERCONDUCTIVITY AND TRANSPORT-PROPERTIES OF Y-BA-CU-O BICRYSTAL WEAK-LINKS
(1995-06-01)
FU, CM
;
JUANG, JY
;
CHEN, MF
;
WU, KH
;
UEN, TM
The effects of the tilt angle on microwave-enhanced transition temperatures and critical currents of YBCO bicrystal weak-links have been investigated. It was found that, with smaller grain boundary tilt angles (e.g. 24 degrees), the enhancement was small, displaying essentially the same behavior as that manifested in intragranular regions. By contrast, when the tilt angle was increased from 36.8 degrees to 45 degrees, noticeable Tc enhancement was found to occur even at low input microwave powers. The results were compared to the temperature dependent critical current, in order to delineate possible correlations between the two physical parameters.
OPERATIONAL CHARACTERISTICS OF A TE CW CO2-LASER WITH A DC AUXILIARY DISCHARGE
(1987-11-01)
WU, KH
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
;
Department of Electrophysics
OPTIMIZATION OF DEPOSITING Y1BA2CU3O7-DELTA SUPERCONDUCTING THIN-FILMS BY EXCIMER LASER ABLATION WITH CO2 LASER-HEATED SUBSTRATES
(1992-06-01)
WU, KH
;
JUANG, JY
;
LEE, CL
;
LAI, TC
;
UEN, TM
The optimum deposition conditions for growing Y1Ba2Cu3O7-delta superconducting thin films in situ by using xenon, Nd:YAG, and KrF excimer lasers with CO2 laser-heated substrates were studied. The dependences of the T(c0) and microstructure of the films on the deposition parameters such as substrate temperature, oxygen partial pressure, laser energy density and repetition rates have been investigated systematically. It was found that both the surface mobility on the substrate and the reaction temperatures for the particulates arriving at the substrates are the key factors for obtaining high-quality films. High-quality Y1Ba2Cu3O7-delta thin films with mirror-like surface morphology, T(c0) almost-equal-to 90 K, and nearly perfect c-axis oriented normal to the film surface were routinely grown in situ on SrTiO3 substrates with the following deposition conditions: substrate temperature at 670-degrees-C, oxygen partial pressure of 0.1 torr, laser energy density of 4 J/cm2 per pulse, and repetition rate of 10 Hz. More significantly, it took only about 3 min to grow a 0.3-mu-m thick film and less than 50 s to cool down the film to room temperature after deposition by the present process. Subsequent processes, such as slow cooling in oxygen atmosphere or any further heat treatments, were unnecessary for the present process. The unique advantage of the present process further offered us good opportunities to delineate the effects of each deposition parameter on the detailed film growth mechanisms without having to deal with the complications introduced by prolonged post-deposition treatments. The details of all the interesting features observed will be presented and their physical implications will be discussed.
POWER-LAW DISSIPATION, COLLECTIVE PINNING, AND GRANULARITY IN TLBACACUO THIN-FILMS
(1994-10-01)
GOU, YS
;
UEN, TM
;
WU, KH
;
JUANG, JY
;
交大名義發表
The influences of the applied magnetic field and temperature on flux dynamics have been investigated in dc-sputtered high-T-c Tl-Ba-Ca-Cu-O thin films. It was found that the current-voltage characteristics can be fitted by an empirical power-law equation over a wide range of dissipation. It then allowed us to easily derive an exponent showing a strong dependence on the magnetic field at lower field regime (< 400 Oe). The exponent, on the other hand, showed a relatively weaker dependence os temperature. The physical implications of the temperature and field dependiencies of the power-law exponent were discussed within the frameworks of collective pinning model. Evidences for collective pinning effects induced by the intergranular weak-link networks were further demonstrated to lend strong support to the present conjecture on the flux dynamics in these films.
PULSE-FORMING DYNAMICS OF A CW PASSIVELY MODE-LOCKED TI-SAPPHIRE DDI LASER
(1992-03-01)
KUO, JC
;
SHIEH, JM
;
HWANG, CD
;
CHANG, CS
;
PAN, CL
The dynamics of pulse formation in a cw passively mode-locked Ti:sapphire/DDI laser has been investigated. It is found that the time required for the circulating energy in the cavity to build up and bleach the dye absorber is approximately 140-mu-s. A further 230-mu-s is required for the buildup of steady-state picosecond pulses. Concurrently the laser output spectrum initially narrows in the cw regime, begins to broaden as the laser becomes modulated, and finally establishes the steady-state spectral distribution.