Effect of Distribution Pattern of Molecular Dipoles on Microscopic Second-Harmonic Generation

We set up a model for dealing with the second-harmonic generation (SHG) from molecular dipoles in a line alignment pattern rather than a planar distribution under a microscope. Because of this model, it is possible to perform a flexibly quantitative investigation of SHG from collagen fibres at a molecular level. The line alignment pattern induces more significant modifications in both the angular dipole distribution structure A(θ,ψ) and the second-harmonic power structure θ_y (θ,ψ), compared to a planar distribution. Also, the line alignment angle t has strong effect on A(θ,ψ) and θ_y(θ,ψ), resulting in an irregular SHG angular power distribution. That is to say, it is unnecessary for SHG emission to present two well-defined off-axis lobes. The total SHG power shows two symmetrical peaks at angles of t=50° and t =130° while a drop at t=90°. The weakest SHG signals can be measured at t=0° and t=180°.