Ferroeloctricity in cholesteric liquid crystals of polypeptide and its helicoidal cavity effect of SHG

在电场作用下，聚(γ-L-谷氨酸苄酯)在苯甲醇(BA)溶液中的溶致胆甾相发生了较强的二次谐波。利用倍频干涉测量技术，我们证实了通过反转电场，倍频的相位发生了反转，清楚地表明了非线性极化的转换。在三角波电压作用下，观察到开关电流峰值，表明存在自发极化现象；利用沿螺旋轴方向传播的基波光，在胆甾相PBLG固体薄膜中观察到倍频现象。当螺旋面的光距与SH光波长相同时，观察到较强的SH光，反之，产生的SH光被螺旋面抵消，SH强度可忽略不计。基于这些实验结果，我们得出结论，PBLGBA溶致体系的胆甾相实际上是一个铁电胆甾相，其中一个极平面沿螺旋轴旋转。这是液晶中第一个真正的铁电性。

Relatively strong second harmonic generation (SHG) was observed in the lyotropic cholesteric phase of the poly(γ-benzyl L-glutamate) (PBLG) in benzyl alcohol (BA) with application of an electric field. Using an SHG interferometry technique, we confirmed that the phase of SHG was reversed by reversing the electric field, clearly indicating the switching of nonlinear polarization. By applying a triangular wave voltage, a switching current peak was observed, indicating the existence of spontaneous polarization.SHG was also observed in a cholesteric solid film of PBLG using fundamental light propagating along the helicoidal axis. Relatively strong SH light was observed when the optical pitch in the helicoid was the same as the SH-light wavelength, although, otherwise, the generated SH light was canceled by the helicoid so that the SH intensity was negligibly weak.Based on these experimental results, we safely concluded that the cholesteric phase of the PBLGBA lyotropic system is in fact a ferroelectric cholesteric phase, where a polar plane rotates along the helical axis. This is the first proper ferroelectricity in liquid crystals.