Figure 1.

Discontinuity THGM imaging of bead-string FINEs

(A) Both bead-string FINEs and regular fibers oriented horizontally relative to the direction of light illumination can be visualized with a tightly focused Gaussian beam through THG. The THG signal generated by the 800-nm diameter bead is 50 times stronger than that generated by the 200-nm diameter fiber.

(B) The discontinuous varicosities of FINEs disrupt the structural axial symmetry and create the optical inhomogeneity necessary for THG generation. However, the Guoy phase shift effect can cause coherent cancellation of the THG signal due to axial symmetry if a regular fiber with a uniform diameter is stretched in the direction of light propagation. For this vertical geometry, the THG signal generated by the 800-nm diameter bead is 1 million times stronger than that generated by the 200-nm diameter fiber.

(C) The digital reconstruction of the discontinuous THG bulges within a distance of 10.4 μm can reveal FINEs.