Sum frequency generation (SFG) microscopy images of cotton cellulose fibers were observed at the infrared wavenumber of ∼ 2945 cm<sup>–1</sup> and with a spatial resolution of 2 μm. Domains of different cellulose microfibril bunches were observed and they showed different second-order nonlinear responses. The intensity of the peak of the asymmetric CH<sub>2</sub> stretching mode at 2945 cm<sup>–1</sup> depended strongly on the orientation of the electric fields of the incident visible and infrared light with respect to the cellulose fiber axis. The second-order nonlinear susceptibility arising from the chirality in the cellulose structure was found to be dominant. The SFG of the cross section of the cellulose fiber was relatively weak and showed a different spectrum from that measured from the side of the fiber axis.
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