ABSTRACT Collagen is a key protein in mammals that maintains structural integrity within tissues. A failure in fibrillar collagen reorganization can induce cancer or fibrosis formation, such as in spinal cord injury (SCI), where the healing process after the initial trauma leads to the formation of scar tissue, which includes fibrosis. As there is no current treatment targeting the fibrotic process directly, a better understanding of collagen properties can thus help to apprehend malignant states. Characterization of collagen fibers has been widely explored on second‐harmonic generation (SHG) images, due to the label‐free nature of the SHG imaging technique. It has been performed with various fibers extraction methods such as curvelet transform (CT) implemented in the open‐source software CurveAlign. However, when it comes to investigating undergrowth collagen fibers (collagen fibers that are still under reorganization) as observed in SCI, the CT method becomes complex to tune for nonadvanced users in order to properly segment the fibers. To improve collagen detection in the case of undergrowth fibers, we propose a methodology based on the fingerprint enhancement (FP‐E) algorithm that requires fewer user input parameters and is less time‐consuming. Our method was extensively tested on SHG data from injured spinal cord samples. We obtained metrics that depicted changes in collagen organization over time, particularly a significant increase in fiber density, demonstrating the FP‐E algorithm was properly adapted to address the evolution of collagen properties after SCI. Besides the simpler tuning of the method compared to commonly used software, the combination with further characterization of the extracted fibers could lead to consider fibrillar collagen as a biomarker in diseases where fibers are under development. The FP‐E algorithm is provided in the article.

In this work, the first hyperpolarizability of Au x @Ag 100-x core-shell nanoparticles with x the gold molar fraction in percent and the gold core of which is 12 nm in diameter, is determined and compared to that of their corresponding laser annealed nanoparticles using hyper Rayleigh scattering experiments. Laser annealing transforms the initially composite Au x @Ag 100-x core-shell nanoparticles into alloyed Au x Ag 100-x homogeneous nanoparticles, providing a reference for comparison. It is observed that the evolution with the relative molar ratio between gold and silver of the first hyperpolarizability magnitude of both the Au x @Ag 100-x core-shell and the alloyed Au x Ag 100-x nanoparticles is driven by the SPR resonance enhancement occurring at the harmonic wavelength due to red shifting of the SPR band away from the harmonic wavelength. Furthermore, the first hyperpolarizability magnitude of the Au x @Ag 100-x core-shell nanoparticles is found to be about three orders of magnitude larger than that of the annealed alloyed Au x Ag 100-x nanoparticles. This feature may be attributed to the existence of the two nonlinearities, namely the surface nonlinearity due to the surrounding mediumsilver layer interface and the silvergold metal -metal interface constructively contributing due to their close localization. The core-shell morphology is thus highly beneficial in view of applications as compared to the alloyed one.

Liquid water still remains an ubiquitous liquid the molecular organization of which requires careful investigation. In this work, we present a study of the second harmonic scattering (SHS) intensity for two different scattering angles, namely the forward and the right angle geometries.This method performed at optical wavelengths is indeed selective towards long correlation lengths. A polarization analysis demonstrates that it is then possible to clearly evidence a coherent component to the total SHS intensity using a fast Fourier transform analysis of the SHS intensity as a function of the fundamental angle of polarization despite its weak relative contribution.A model is then derived based on the rotational invariants to fully unravel the different contributions to the SHS intensity for the two right angle and forward angles of scattering as well as the dependence with the fundamental beam polarization angle. This model fully supports the experimental results further confirming an azimuthal geometry of the water-water orientational correlation function in neat water.