Fig. 3.
SHG amplification and observation of the HN* structure. (A) The temperature dependencies of SH signal for RM734/S1 = 95/5, 70/30, 50/50, 40/60, and 30/70 mixtures. The SH signal is amplified in RM734/S1 = 40/60 (also refer to B). The horizontal axis is the reduced temperature. TN*-HN* is the transition temperature between the N* and HN* states. The SHG efficiency, defined as the SH signal intensity ratio of the HN* materials to that of a Y-cut quartz plate. The Inset on the right top shows the SH signal in a log scale in the HN* state. The polar plot clearly shows the strongest SHG occurs perpendicular to the helical axis of the helielectric structure, suggesting the helical structure is helicoidal-like. (B) The saturated SHG efficiency in the HN* state and helical pitch of the HN* state as a function of the weight percentage of S1. (C) Optical setup for the simultaneous observation of SHG and SHG-I images. In the SHG-I imaging area, the quartz, and sample SH signals overlap, causing the interference of SHG. The SH signal of the polar domains interferes with that of the quartz plate constructively or destructively depending on the rotation angle of the fused silica plate. In the imaging, we choose the proper angle for best contrast of the SHG interferometry. (D) PLM (Upper) and traditional (Lower) microscopy observations of the “fingerprint” helical structure (scale bar, 5 μm). The latter was made without polarizers during the SHG measurement. (E) Simultaneous observation of SHG and SHG-I microscopies. The SHG-I area, where a Gaussian reference SH beam from a Y-cut quartz plate is overlapped with the sample SH signal, is indicated by a white dotted circle. The incident polarization is indicated by a white arrow. The width of the image is 5.6 μm. (F) The SHG and SHG-I signal plots along a-a′ and b-b′ lines. The blue chain and green dotted line are the fitting curves to the spatial variation of the SH and SHG-I signals. The corresponding polarity field is depicted in E. (G) The schematic of the HN* structure under an IPE field application. (H) The SH signal as a function of the applied DC voltage. Over 9 VDC, electrohydrodynamic convection occurs.