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Optical Second-Harmonic Generation in Commensurate-Incommensurate Phase Transitions

相变-非相变中的光学二次谐波产生

基本信息

批准号：
01540283
负责人：
UESU Yoshiaki
金额：
$ 1.02万
依托单位：
Waseda University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1989
资助国家：
日本
起止时间：
1989 至 1990
项目状态：
已结题

项目摘要

(1) Intensities of optical second-harmonic wave and coherence lengths were measured in the ferroelectric and incommensurate phases of (NH_4) BeF_4 (AFB) , K_2ZnCl_4 (KCZ) , and {N (CH_3)_4}_2ZnCl_4 ((TMA) CZ) crystals. It was found that no appreciable second-harmonics was observed in the incommensurate phase of AFB. While KCZ and (TMA) CZ exhibited weak intensities around their lock-in transition temperatures. The results could be interpreted on the model where nonlinear optical constant modulates with the incommensurate period along the incommensurate axis. It was also found that an annealing effect on the second harmonics was observed in KCZ. Measured coherence lengths were compared with the incommensurate modulation period determined by the Xーray diffraction method. It was found that the coherence lengths were larger than the modulation period by a factor of 10^2-10^3. These facts indicate that the possibility of quasi-phase matching i. e., the coincidence of the coherence length and the modulation period, is excluded in the crystal.(2) All independent components of the second-order nonlinear optical (SHG) tensors and Miller's indices of Ca_2Pb (C_2H_5CO_2)_6 (DLP) were determined in the temperature range from -60^0C to 90^0C Both in paraelectric and ferroelectric phases, the components d11 and d14 which are forbidden by the symmetry and Kleimann's condition were also observed. Precise examination revealed that the phenomenon originated from the symmetry-lowering as a result of strong internal electric field produced in the DLP crystal. By annealing the crystal at 250^0C, the effect of symmetry-lowering could be diminished. Temperature dependences of the SHG tensors were analyzed by the bond-charge theory where Pb-0 and Ca-0 bonds were assumed to contribute mainly to the observed tensors.Opposite sign of the two bond-contributions causes the anomalous temperature dependence of the SHG tensors.

(1)测量了（NH_4）BeF_4（AFB）、K_2ZnCl_4（KCZ）和{N（CH_3）_4}_2ZnCl_4（（TMA）CZ）晶体铁电相和无公度相中的光学二次谐波强度和相干长度。结果表明，在AFB的无公度相中没有观察到明显的二次谐波。而KCZ和（TMA）CZ在其锁定转变温度附近表现出较弱的强度。结果可以用非线性光学常数随无公度轴沿着方向的无公度周期调制的模型来解释。还发现在KCZ中观察到对二次谐波的退火效应。测量的相干长度进行了比较，通过X射线衍射方法确定的无公度调制周期。发现相干长度比调制周期大10^2-10^3倍。这些事实表明，准相位匹配的可能性。例如，在晶体中排除了相干长度和调制周期的重合。(2)在-60 ~90 ℃温度范围内测定了Ca_2Pb（C_2H_5CO_2）_6（DLP）的二阶非线性光学（SHG）张量和米勒指数的所有独立分量。在顺电相和铁电相中，都观察到对称性和Kleimann条件所禁止的分量d_（11）和d_（14）。通过对这种现象的详细分析，认为这种现象是由于DLP晶体内部产生的强内电场引起的晶体质量下降所致。通过在250 ℃下退火，可以减小晶体的低折射率效应。用键荷理论分析了SHG张量的温度依赖性，其中Pb-0和Ca-0键对SHG张量的贡献是主要的，这两种键的贡献符号相反导致了SHG张量的反常温度依赖性。