Sensitive optical coating defect detection via second harmonic generation

TL;DR

This paper presents second harmonic generation (SHG) as a highly sensitive optical technique for detecting minute defects and damage in thin film coatings, outperforming traditional diagnostic methods.

Contribution

It introduces SHG as a novel, sensitive diagnostic tool for early detection of laser-induced damage in optical coatings, with demonstrated advantages over existing techniques.

Findings

SHG can detect subtle defects undetectable by standard methods

SHG mapping reveals modifications in material composition and stress

SHG's sensitivity is enhanced by adjusting incidence angles and polarization

Abstract

Optical coating, an integral part of many optical systems, is prone to damage from environmental exposure and laser irradiation. This underscores the need for reliable and sensitive coating diagnostics. We introduce second harmonic generation (SHG) as a method for the sensitive detection of defects and inhomogeneities within optical thin films. We demonstrate the use of SHG on Si3N4 layers tested for their laser-induced damage threshold. The SHG mapping was compared with commonly used diagnostic techniques, including Nomarski microscopy, white light interferometry, and electron microscopy. Owing to its sensitivity to variations in local symmetry, SHG is able to discern minute alterations in material composition, mechanical stress, and interface structures. Therefore, SHG identified modifications in the tested layers, highly extending the damage recognized by standard methods. Furthermore, we demonstrate SHG's ability to enhance specific features by modulation of incidence angles and polarization modes. Based on these findings, we propose SHG as an ideal diagnostic tool for the early identification of laser-induced modifications in centrosymmetric thin films.