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Structural ordering transition and repulsion of the giant LO-TO splitting in polycrystalline BaSr1-xTiO3
(2001-12-01)
Kuo, SY
;
Liao, WY
;
Hsieh, WF
;
光電工程研究所
;
Institute of EO Enginerring
We report Raman and x-ray diffraction studies of polycrystalline BaxSr1-xTiO3 (BST) prepared by the sol-gel technique in the range of x = 0 to 1.0. Other than a tetragonal to cubic phase transition at composition x approximate to0.75 of the BST powders detected by x-ray diffraction and Raman spectroscopy, a new structural ordering transition around x approximate to0.4-0.5 is observed due to the atomic arrangement. By taking the anharmonic coupling between the three A(1) (TO) modes into account, the observed repulsion of the giant LO-TO splitting is a result of decreasing cell dimension and strengthening the electron-phonon coupling. The results help us to explain the weakening of the ferroelectricity in the BST tetragonal phase as x decreases. The anomalous inconsistency in our experimental results and theoretical prediction are attributed to the lack of detailed information of dynamical effective charges in the BaxSr1-xTiO3 compounds.
Lattice dynamics of perovskite PbxCa1-xTiO3
(2004-05-01)
Kuo, SY
;
Li, CT
;
Hsieh, WF
;
光電工程研究所
;
Institute of EO Enginerring
Investigation of the structural and vibrational properties of the PbxCa1-xTiO3 (PCT) system exhibits two phase transitions in the range of x=0-1 by using x-ray powder diffraction and Raman spectroscopy. The first transition occurring at xsimilar to0.65 corresponds to the tetragonal-to-cubic phase and the second one occurs near x=0.35 is attributed to the orthorhombic-to-cubic phase. The absence of the intermediate tetragonal phase between orthorhombic and cubic phases (0
Lasing spectral blue shifts of fluorescent saturable absorbing dye in microdroplets
(1998-06-01)
Yu, IJ
;
Hsieh, WF
;
光電工程學系
;
Department of Photonics
We report blue shifts of emission spectra of microdroplet lasers containing fluorescent saturable absorbing dye as compared with those of conventional dye lasers. The results cannot be simply explained by matching the lasing peaks to the minima of threshold condition without taking into account cavity enhancement. We find that the microdroplet lasers have quality factors of 10(4) with a 3X cavity enhancement and their lasing peaks at fixed concentration are independent of the incident pump power.
Fabrication and raman analysis of ZnSe quantum dots in glass matrix thin films by pulsed laser evaporation
(1998-07-01)
Yin, SB
;
Chen, L
;
Hsieh, WF
;
光電工程學系
;
Department of Photonics
We have successfully fabricated ZnSe-microcrystal-doped silica-glass thin films by pulsed laser evaporation from ZnSe powder embedded in sol-gel glass targets and demonstrated that the spatial cell-elation model call be used to analyze the LO Raman mode of ZnSe semiconductor doped glass thin films. The obtained correlation lengths differ from the real sizes of the particulates observed by scanning electron microscopy which may bz due to the fact that the particulates art polycrystalline, with different orientations separated by grain boundaries, and the correlation length just provides a measure of the separated average grain dimension. The photoluminescence spectrum indicates that the carriers may tunnel through the grain walls in the polycrystalline particulates. Low temperature annealing only causes aggregate of particulates without merging the grain boundaries.
The preferable resonators for Kerr-lens mode-locking determined by stability factors of their iterative maps
(1998-10-15)
Wei, MD
;
Hsieh, WF
;
Sung, CC
;
光電工程學系
;
Department of Photonics
The generalized stability factor of a general resonator was obtained from its iterative map based on the linear stability analysis. Since a physical system tends to stay with high stability, the preferred resonator for Kerr-lens mode-locking (KLM) is determined by the relative stability between KLM and CW operations. With this criterion the preferable KLM regions agree with the previous experimental self-starting regions when the Ken medium is located around the center of the resonator. (C) 1998 Elsevier Science B.V. All rights reserved.
Bifurcation of fundamental Gaussian modes in Kerr-lens mode-locked lasers
(1999-09-01)
Wei, MD
;
Hsieh, WF
;
光電工程學系
;
Department of Photonics
We propose that more than one spatial fundamental Gaussian mode may occur in concentric and confocal resonators under persisting nonlinear effect. Extensively studying the influence of the resonator's parameters in a Kerr-lens mode-locked resonator, pitchfork bifurcation results in more symmetrical configurations, and saddle-node bifurcation appears as the symmetry being broken. From the properties of bifurcation, we suggest that the equal-arm and near-confocal resonator is suitable for the emergence of bistability in Kerr-lens mode-locked lasers. (C) 1999 Elsevier Science B.V. All rights reserved.
Nanomachining of (110)-oriented silicon by scanning probe lithography and anisotropic wet etching
(1999-10-18)
Chien, FSS
;
Wu, CL
;
Chou, YC
;
Chen, TT
;
Gwo, S
We have demonstrated that silicon nanostructures with high aspect ratios, having similar to 400 nm structural height and similar to 55 nm lateral dimension, may be fabricated by scanning probe lithography and aqueous KOH orientation-dependent etching on the H-passivated (110) Si wafer. The high spatial resolution of fabricated features is achieved by using the atomic force microscope based nano-oxidation process in ambient. Due to the large (110)/(111) anisotropic ratio of etch rate and the large Si/SiO2 etch selectivity at a relatively low etching temperature and an optimal KOH concentration, high-aspect-ratio gratings with (111)-oriented structural sidewalls as well as hexagonal etch pit structures determined by the terminal etch geometry can be obtained. (C) 1999 American Institute of Physics. [S0003-6951(99)03242-8].
Nanometer-scale conversion of Si3N4 to SiOx
(2000-01-17)
Chien, FSS
;
Chang, JW
;
Lin, SW
;
Chou, YC
;
Chen, TT
It has been found that atomic force microscope (AFM) induced local oxidation is an effective way for converting thin (< 5 nm) Si3N4 films to SiOx. The threshold voltage for the 4.2 nm film is as low as 5 V and the initial growth rate is on the order of 10(3) nm/s at 10 V. Micro-Auger analysis of the selectively oxidized region revealed the formation of SiOx. Due to the large chemical selectivity in various etchants and great thermal oxidation rate difference between Si3N4, SiO2, and Si, AFM patterning of Si3N4 films can be a promising method for fabricating nanoscale structures. (C) 2000 American Institute of Physics. [S0003-6951(00)03003-5].
Study of linear and nonlinear optical properties of distorted Ti-O-6 perovskite structure in BaxSr1-xTiO3
(2000-06-01)
Chen, WK
;
Cheng, CM
;
Huang, JY
;
Hsieh, WF
;
Tseng, TY
We report the linear and nonlinear optical properties of sol-gel prepared BaxSr1-xTiO3 (BST) crystalline powders. The results from powder X-ray diffraction, birefringence, and second-order optical nonlinearity measurements indicate that the paraelectric-ferroelectric phase transition of Ba0.75Sr0.25TiO3 occurs near room temperature. The second-order optical nonlinearity of the ferroelectric BST is about 7.9 pm/V. The index of refraction, n = 2.2, and birefringence, Delta n(B) similar to 0.05, are also appropriate for practical nonlinear optical applications. (C) 2000 Elsevier Science Ltd. All rights reserved.
Cavity-configuration-dependent nonlinear dynamics in Kerr-lens mode-locked lasers
(2000-08-01)
Wei, MD
;
Hsieh, WF
;
光電工程學系
;
Department of Photonics
We determined theoretically that the nonlinear dynamics of a Gaussian beam is configuration dependent in a general cavity. This prediction was confirmed by numerical simulation in a Kerr-lens mode-locked cavity for which the self-focusing effect is considered the nonlinear source in both the spatial and the temporal domains. Period doubling, tripling, and quadrupling can occur in these configurations with the products of generalized cavity G parameters equal to 1/2, 1/4 (or 3/4), and (2 +/- root 2)/4, respectively. The dynamic behavior of the cavity beam will become irregular if the nonlinear effect is further increased. (C) 2000 Optical Society of America [S0740-3224(00)00708-6] OCIS codes: 140.3410, 140.4050, 260.5950, 140.1540.

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