# Second-order Nonlinear Optical Microscopy of Spider Silk

## TL;DR

This study demonstrates second harmonic generation in spider silk due to asymmetric protein structures, revealing the relationship between protein orientation and nonlinear optical response using femtosecond laser microscopy.

## Contribution

It provides the first experimental observation of SHG in spider silk and models the orientation of protein $eta$-sheets responsible for the nonlinear response.

## Key findings

- SHG signal correlates with $eta$-sheet presence in spider silk
- Polarization dependence reveals $eta$-sheet orientation
- Different silks show varying SHG responses

## Abstract

Asymmetric $\rm \beta$-sheet protein structures in spider silk should induce nonlinear optical interaction such as second harmonic generation (SHG) which is experimentally observed for a radial line and dragline spider silk by using an imaging femtosecond laser SHG microscope. By comparing different spider silks, we found that the SHG signal correlates with the existence of the protein $\rm \beta$-sheets. Measurements of the polarization dependence of SHG from the dragline indicated that the $\rm \beta$-sheet has a nonlinear response depending on the direction of the incident electric field. We propose a model of what orientation the $\rm \beta$-sheet takes in spider silk.