  We have detected a second-order nonlinear optical response from aggregates of
the ampholytic megamolecular polysaccharide sacran extracted from
cyanobacterial biomaterials, by using optical second harmonic generation (SHG)
microscopy. The SHG images of sacran cotton-like lump, fibers, and cast films
showed SHG intensity microspots of several tens of micrometers in size. The
dependence of the SHG spot intensity on an excitation light polarization angle
was observed to illustrate sacran molecular orientation in these microdomains.
We also observed SHG signals around a special region of the cast film edges of
sacran. These results show that sacran megamolecules aggregate in several
different ways.
