Browsing by Author "Tang, LC"

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Ab initio study of the structural and optical properties of orthorhombic ternary nitride crystals
(2001-11-19)
Huang, JY
;
Tang, LC
;
Lee, MH
;
光電工程學系
;
Department of Photonics
The structural, optical and mechanical properties of orthorhombic ternary nitride crystals have been analysed theoretically with first-principles calculation. Our results indicate that these nitrides possess fairly large optical bandgap (4-6 eV), second-order nonlinear optical susceptibility (13-18 pm V-1), and bulk modulus 170-370 GPa. Therefore these materials could be useful for optical and protective coating applications. Our analysis with the band-by-band and atomic species projection techniques not only yields useful information about material properties, but also provides deep insight into the fundamental understanding of the mechanical and optical properties of orthorhombic ternary nitrides.
Cation substitution effects on structural, electronic and optical properties of nonlinear optical AgGa(SxSe1-x)(2) crystals
(2003-09-10)
Tang, LC
;
Lee, MH
;
Yang, CH
;
Huang, JY
;
Chang, CS
The structural, electronic and optical properties of tetragonal nonlinear optical (NLO) crystals, AgGa(SxSe1-x)(2) (x = 0.0, 0.25, 0.5, 0.75, and 1.0), were investigated theoretically and experimentally. The results obtained indicated that the electronic bandgaps, optical properties and bulk moduli of these compounds were linearly dependent on the substitution concentration of cations. From partial density of state analysis, it was found that the electronic states near the band edges of AgGa(SxSe1-x)(2) were a simple proportional mixture of the atomic orbitals of sulfur and selenium. A cell-volume effect was proposed as the major cause of the linear dependence of material properties on the substitution concentration. It was calculated that the second-order NLO susceptibilities were scaled with the cubic power of bandgap, although a minor deviation existed. This deviation arose from the optical transition moment products.
Electronic structure and optical properties of rhombohedral CsGeI3 crystal
(2000-10-30)
Tang, LC
;
Chang, CS
;
Huang, JY
;
光電工程學系
;
Department of Photonics
We analyse the electronic structure of the perovskite crystal CsGeI3 by means of first-principles calculations and compare our findings to experimental results. Our calculation indicates that CsGeI3 has a direct-transition gap of 0.74 eV at (k) over right arrow = (pi /a)(111). The top of the valence bands was found to mainly comprise the 5p orbitals of iodine, while the bottom of the conduction bands is dominated by the 4p orbital of germanium. Photoluminescence (PL) measurements on a single crystal of CsGeI3 indicate two peaks, one at 0.82 mum (1.51 eV) and the other at 1.15 mum (1.08 eV). The shorter-wavelength PL peak is assigned as arising from an interband transition at (k) over right arrow = (pi /a)(111) and the longer-wavelength PL is presumably ascribable as originating from a transition involving an energy level within the fundamental band gap. Fourier-transformed infrared spectroscopy reveals that the transparent range of CsGeI3 could extend from similar to2 mum to >12 mum The short-wavelength cut-off is mainly limited by the energy band gap, while the long-wavelength limit possibly originates from lattice phonon absorption. Raman spectra of the crystal exhibit two major peaks at 105 cm(-1) and 151 cm(-1) and the corresponding overtones at 220 cm(-1) and 293 cm(-1).
Elimination of dynamic instabilities in the plus c-face incident photorefractive BaTiO3 mutually pumped phase conjugator
(1999-05-15)
Chang, CC
;
Chen, TC
;
Tang, LC
;
Yau, HF
;
光電工程學系
Experimental observations of the dynamic instabilities in the +c-face incident mutually pumped phase conjugator (MPPC) of a BaTiO3 crystal are reported for the first time. Experiments show that it is possible to stabilize the phase-conjugate output by choosing a proper geometry formed from the crystal and two incident beams. By choosing the proper geometry the essential configuration of the MPPC attained is a "kite" rather than a "fish head". Data also indicate that the phase-conjugate output can reach a very steady state with a high value (similar to 32%) and is insensitive to angular and positional variations.
High-resolution double self-pumped phase conjugations with plus c-face incident configuration in a BaTiO3 crystal
(2000-11-01)
Chen, TC
;
Chang, CC
;
Tang, LC
;
Yau, HF
;
光電工程學系
The geometry of creating two stable self-pumped phase-conjugate outputs in one photorefractive BaTiO3 crystal is demonstrated. This novel geometry provides two phase-conjugate images with a resolution as high as 4 mum (or 114 1p/mm). The reflectivity versus the angle of the incident beam is also investigated.
NBTI effects of pMOSFETs with different nitrogen dose imlantation
(2004)
Lee, YJ
;
Tang, LC
;
Wu, MH
;
Chao, TS
;
Ho, PT
NBTI effects with different nitrogen dose implantation and regions were investigated. High nitrogen dose implantation in the channel or source/drain extension results in serious NBTI degradation. Both the dynamic NBTI effects and substrate hot holes effects were also discussed. DNBTI and I-CP were measured simultaneously. Reduction of DeltaV(TH) and I-CP after positive gate bias stressing is related with the recovery of interface states.
New infrared nonlinear optical crystal CsGeBr3: synthesis, structure and powder second-harmonic generation properties
(2005-11-23)
Tang, LC
;
Huang, JY
;
Chang, CS
;
Lee, MH
;
Liu, LQ
An innovative infrared nonlinear optical crystal CsGeBr3 was synthesized. Ab initio calculations on CsGeBr3 were also carried out in order to analyse the second-order nonlinear susceptibilities. From its powder x-ray diffraction pattern, this crystal was characterized as a rhombohedral structure with an (R3m, No 160) space group symmetry. The reflection powder second-harmonic generation (PSHG) measurement of CGBr showed that its nonlinear optical efficiency is 1.62 times larger than that of rhombohedral CsGeCl3 and is 9.63 times larger than that of KH2PO4 (KDP), and most important of all that CsGeBr3 is phase-matchable. The rescaled d(eff)((2)) of CGBr was about 2.45 times larger than that of rhombohedral CsGeCl3, and this trend was coincident with the ab initio calculation results. The infrared transparent spectrum of rhombohedral CsGeBr3 was extended to more than 22.5 mu m. The rhombohedral CsGeBr3 shows the potential in the realm of nonlinear optics and can be applied to the infrared region.