Optical second-harmonic observation of stimulated sacran aggregates

Abstract

Second-order nonlinear optical images of aggregates of the ampholytic megamolecular polysaccharide sacran under various stimuli were observed by optical second-harmonic generation (SHG) microscopy. SHG intensity microspots of several tens of micrometers in size are seen for sacran cotton-like lumps and fibers and they have very clear incident polarization dependence. In these microspots, the sacran molecules are oriented in concentric multilayers. We also observed SHG images of sacran prepared in a needle-ring electrode with applied voltage, where SHG was enhanced near the negatively biased needle electrode. We also observed SHG signals near the cast film edges of sacran. The appearance of the SHG image suggested a phase-separation structure in sacran aggregates. These results show that sacran molecules aggregate in several different ways.