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

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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.