We demonstrate a sum frequency generation (SFG) holographic imaging method by integrating the capabilities of holography and SFG spectroscopy. SFG can probe the molecular vibrational resonance in non-centrosymmetric media. Holographic recording can capture both the amplitude and the phase of the SFG signal, thus leading to label-free spectroscopic three-dimensional imaging.
The noncentrosymmetry requirement of sum frequency generation (SFG) spectroscopy allows selective detection of crystalline cellulose in plant cell walls and lignocellulose biomass without spectral interferences from hemicelluloses and lignin. In addition, the phase synchronization requirement of the SFG process allows noninvasive investigation of spatial arrangement of crystalline cellulose microfibrils in the sample. This paper reviews how these principles are applied to reveal structural information of crystalline cellulose in plant cell walls and biomass.