The forward and backward second-harmonic generation from crystallized collagen fibre with tightly focused linearly polarized beams

In this paper, a theoretical model to reveal the second-harmonic generation (SHG) from the crystallized collagen fibre, that is, the collagen is highly organized by a bundle of closely packed thin fibrils excited by tightly focused linearly polarized beams, has been established. With this model, we first theoretically demonstrate the phenomenon of the coexisting of forward (F) and backward (B) second-harmonic emissions in a specific SHG medium. The ratio of forward-to-backward (F/B) SHG is dependent on the constituted SHG medium by a particular collagen fibril diameter d1 as well as the structural order (m,l) of collagen fibre.