Optical Second-Harmonic Generation in Commensurate-Incommensurate Phase Transitions

（1）光学second-harmonic wave and coherence length were measured in the ferroelectric and incommensurate phases of （NH_4） BeF_4 （AFB）、K_2ZnCl_4 （KCZ）和{N （CH_3）_4}_2ZnCl_4 （（TMA） CZ） crystals。这是在AFB的不受欢迎的第二次谐波中发现的。当KCZ和（TMA） CZ在过渡温度周围展示了弱强度。结果可能会对模型中的非线性光学常数调制与不常见的周期中的不常见的轴进行解释。它还发现了一种在KCZ观察到的第二谐波的退火效应。测量的一致性长度与X射线差法确定的不常见调制周期相比较。它被发现是因为它的相干长度比调制期的一个因素10^2-10^3。这些事实表明了quasi-phase matching i的可能性。e.，“一致性长度和调制期的一致性被排除在水晶中。”（2）Ca_2Pb （C_2H_5CO_2）_6 （DLP）的二级非线性光学（SHG）张量和米勒指标的所有独立组件都从-60^0 C到90^0 C的超电和铁电相位中确定了温度范围，其中组件d11和d14被对称性和克莱曼的条件也被遗忘。精确的测试揭示了从对称性下降中产生的现象，作为在DLP晶体中产生的强内部电场的结果。在250^0 C的情况下，对晶体的影响--降低对称性可能会被缩小。SHG张力器的温度依赖性由bond-charge理论中Pb-0和Ca-0债券假定主要为观察到的张力器。两个债券的对立信号导致SHG张力器的异常温度依赖性。

(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.