Browsing by Author "Lin, PI"

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Bipolar photogenerated terahertz radiation in biased photoconductive switches
(2003-05-01)
Lin, PI
;
Luo, CW
;
Liu, HS
;
Wu, KH
;
Juang, JY
The characteristics of optically induced bipolar THz radiation in biased photoconductive switches were investigated systematically by using free-space electro-optic sampling technique. The emitted radiation shows nearly symmetrical waveform with broadband frequency spectrum spanning over 0.1-3 THz. It was observed that the bipolar nature and the emitted frequency spectrum distribution remained unchanged on varying the optical excitation fluence, strength of biased field and the emitter gap spacing. The dynamics of emitted THz transient are in agreement with the optically induced ultrafast charge transport process driven by the biased field. (C) 2003 Elsevier Science B.V. All rights reserved.
Characteristics of photogenerated bipolar terahertz radiation in biased photoconductive switches
(2002-10-15)
Lin, PI
;
Chen, SF
;
Wu, KH
;
Juang, JY
;
Uhn, TM
The characteristics of the optically induced bipolar terahertz (THz) radiation from biased semi-insulating GaAs photoconductive switches were investigated using a free-space electrooptic sampling technique, The emitted radiation shows a nearly symmetrical waveform with a broad-band frequency spectrum spanning over 0.1-3 THz, which displays essentially no dependence on the optical excitation fluence or strength of biased field. However, it does slightly depend on the file emitter gap spacing. The dynamics of the emitted THz transient is in agreement with the optically induced ultrafast charge transport process driven by the biased field.
Free-space electrooptic sampling of terahertz radiation from optically excited superconducting YBa2Cu3O7-delta thin films
(2003-03-01)
Lin, PI
;
Wu, KH
;
Juang, JY
;
Lin, JY
;
Uen, TM
The characteristics of photogenerated terahertz radiation from a current-biased superconducting YBa2Cu3O7-delta (YBCO) bow-tie antenna were investigated using a free-space electrooptic sampling technique. Picosecond electromagnetic pulses about 450 fs wide were obtained. The frequency spectrum derived by Fourier transforming the picosecond pulses spans over 0.1-4 THz. The dynamics of the quasi-particles optically induced by the ultrafast laser pulse primarily determines the performances of the transient terahertz radiation generated under different operating parameters. The results indicate a characteristic quasi-particle relaxation time of about 2.5 ps close to the critical temperature T-c, and a faster time at lower temperatures.
Metallic percolation in La0.67Ca0.33MnO3 thin films
(2003-02-24)
Chen, SF
;
Lin, PI
;
Juang, JY
;
Uen, TM
;
Wu, KH
Phase separation in La0.67Ca0.33MnO3 thin films was investigated by scanning tunneling microscopy. The correlation between the grain structure and the spatial distribution of the coexisting metallic and insulating phases was evidently established. At temperatures not far below the metal-insulator transition, the spatial variation of the coexisting metallic and insulating phases is susceptible to magnetic field in an irreversible manner. The irreversibility suggests that the metallic percolation paths can be affected randomly by magnetic field. However, the variation becomes insensitive to magnetic field at lower temperatures. (C) 2003 American Institute of Physics.
On the origin of photogenerated terahertz radiation from current-biased superconducting YBa2Cu3O7-delta thin films
(2004-06-15)
Lin, PI
;
Luo, CW
;
Liu, HS
;
Chen, SF
;
Wu, KH
The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBa2Cu3O7-delta thin films excited by femtosecond optical laser pulses is delineated. By investigating the performance of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original wave forms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed. (C) 2004 American Institute of Physics.
Superconducting NdBa2Cu3O7-delta thin films grown on bare (1(1)over-bar02) sapphire by pulsed laser deposition
(2003-05-01)
Cheng, TY
;
Lin, PI
;
Chen, SF
;
Liu, SJ
;
Juang, JY
Superconducting NdBa2Cu3O7-delta (NBCO) thin films with T-c=89 K have been. successfully grown on bare (1 (1) over bar 02) sapphire substrates by pulsed laser deposition (PLD). The X-ray diffraction results show that the as-grown NBCO films are all c-axis oriented with no observable second phase. The c-axis parameter decreases monotonically with increasing the deposition temperature (T-s), suggesting that the corresponding degradation of T-c for T-s>790degreesC might not originate from oxygen deficiency. It is conceived that the film-substrate reaction that occurring at the interface may cause Ba-deficiency and hence enhance Nd-Ba antisite substitutions.
Superconducting YBa2Cu3O7 films grown on TiO2 buffer layers derived from various processes
(2001-11-01)
Wu, KH
;
Lin, PI
;
Hsieh, CC
;
Liu, SJ
;
Juang, JY
YBa2Cu3O7 (YBCO)/TiN and YBCO/TiO2 bilayer structures have been prepared on SrTiO3 (STO)(100) substrates by in situ pulsed laser deposition (PLD). The TiN films were originally intended to serve as the lower contact electrode for studying the c-axis transport properties of YBCO thin films. It was found that, under the optimum conditions of depositing YBCO thin films, the TiN(100) layer was oxidized and transformed into rutile TiO2(110) film. On the other hand, pure anatase TiO2(001) films were prepared by PLD using a rutile TiO2(110) substrate as the target. YBCO films grown on both phases of TiO2 films show virtually the same transport properties of typical good quality singlelayer YBCO films. Comparative studies of depositing YBCO films directly onto a de-sputtered TiO2 template commonly used in the selective epitaxial growth process have, however, resulted in formation of a nonsuperconducting YBCO top layer. Experiments including interfacial X-ray absorption spectroscopy for determining the possible interactions occurring at the YBCO and TiO2 interface are reported. (C) 2001 Elsevier Science B.V. All rights reserved.