Second harmonic generation signal in collagen fibers: role of polarization, numerical aperture, and wavelength.

The spatial distribution of second harmonic generation (SHG) signal from collagen fibers for incident elliptical polarized light has been modeled. The beam was assumed to focus on a horizontal fiber through a microscope objective. For elliptical polarized states located along a vertical meridian of the Poincare sphere, the SHG intensity has been optimized in terms of the incident wavelength and the numerical aperture (NA) of the objective. Our results show that polarization modulates the SHG signal. Elliptical polarization can generate high signals (even greater than those corresponding to linear polarization) when combined with appropriate values of both incident wavelength and NA. On the other hand, the SHG intensity might also be identically zero for particular elliptical polarization states, a condition that depends exclusively on the ratio of hyperpolarizabilities of the collagen fibers. The highest forward-to-backward SHG signal distribution occurs along the propagating direction, depends on the incident wavelength, and reduces when NA increases. Furthermore, the direction of maximum SHG emission was found to be more sensitive to changes in the NA rather than variations in the incident wavelength. These findings could help to optimize the experimental conditions of multiphoton microscopes and to increase SHG signals from biological tissues containing collagen.