Sum frequency generation microscopy study of cellulose fibers

Area of Science:

• Materials Science
• Biophysics
• Spectroscopy

Background:

• Cellulose is a key structural component in plants, and understanding its microfibril organization is crucial for material properties.
• Sum frequency generation (SFG) microscopy is a powerful nonlinear optical technique for probing molecular structure and orientation at interfaces.

Purpose of the Study:

• To investigate the microstructural organization of cotton cellulose fibers using SFG microscopy.
• To determine the relationship between cellulose microfibril orientation and nonlinear optical responses.
• To elucidate the contribution of structural chirality to the observed SFG signals.

Main Methods:

• Sum frequency generation (SFG) microscopy was employed to image cotton cellulose fibers.
• Observations were made at an infrared wavenumber of ~2945 cm⁻¹, targeting the asymmetric CH(2) stretching mode.
• Spatial resolution of 2 μm was achieved, and the influence of electric field orientation was analyzed.

Main Results:

• Distinct domains of cellulose microfibril bunches exhibited varying second-order nonlinear responses.
• The intensity of the SFG peak at 2945 cm⁻¹ was highly dependent on the orientation of incident visible and infrared light electric fields relative to the fiber axis.
• The second-order nonlinear susceptibility was dominated by the chirality inherent in the cellulose structure.
• SFG spectra from the cellulose fiber cross-section were weaker and spectrally distinct from those obtained from the fiber's side.

Conclusions:

• SFG microscopy effectively visualizes microfibril organization and orientation within cotton cellulose fibers.
• The chirality of the cellulose structure plays a dominant role in its nonlinear optical properties.
• Fiber orientation significantly impacts SFG signal intensity, providing insights into structural anisotropy.