Theoretical simulation study of linearly polarized light on microscopic second-harmonic generation in collagen type I.

A theoretical model is established for dealing with second-harmonic generation (SHG) in type I collagen excited by linearly polarized light focused by a microscope. With this model, the effects of the polarization angle alpha, numerical aperture (NA), as well as the ratio of hyperpolarizability rho=beta(xxx)beta(xyy) on SHG emission have been investigated. Simulation results reveal that SHG emission power changes periodically as alpha. The use of lower NA leads to weaker SHG emission but is more concentrated in two closer lobes, whereas more distributed emission in two detached lobes appear at higher NA. As the introduction of polarization direction, which is not along with the fiber axis (alpha not equal0 deg), one more element beta(xyy) is valid in our case than beta(xxx) alone, while their ratio rho plays a very important role for collagen features characterization. SHG emission with rho shows complicated modality that SHG emission is different at different alpha and not symmetric at +/-rho except at alpha=0 deg, suggesting the important impact of polarization working on rho for SHG emission. Our theoretical simulation results provide useful clues for experimental study of microscopic SHG emission in collagen excited by linearly polarized beam.