Understanding the effect of HF-based wet shallow etching on optical performance of reactive-ion-etched fused silica optics.

Laixi Sun,Wanguo Zheng,Jin Huang,Xin Ye,Weihua Li,Xinda Zhou,Ting Shao,Fenfei Li,Shufan Chen,Liming Yang,Bo Li

doi:10.1039/d1ra04174f

Abstract

The optical performance of fused silica optics used in high-power lasers is known to depend not only on their surface damage resistance, but also on their surface quality. Previous studies have shown that good fused silica damage performance and surface quality can be achieved by the use of reactive ion etching (RIE), followed by HF-based wet shallow etching (3 μm). In this study, two kinds of HF-based etchants (aqueous HF and HF/NH4F solutions) were employed to investigate the effect of HF-based etching on the optical performance of reactive-ion-etched fused silica surfaces at various HF-based shallow etching depths. The results showed that the addition of NH4F to HF solution makes it possible to produce a high-quality optical surface with a high laser-induced damage threshold, which is strongly associated with the surface roughness and fluorescence defect density. Additionally, changing the HF-based etching depth over the range from 1 μm to 3 μm can affect the surface damage resistance and absorption performance of RIE-treated fused silica. The light-scattering results indicate that the point defect density plays an important role in the determination of the HF-based etching depth. Understanding these trends can enable the advantages of the combined technique of RIE and HF-based etching during the fabrication of high-quality fused silica optics.

Highlights

In high-power fusion-class laser facilities, the service life of fused silica optics is known to be severely limited by their surfaces, as laser-induced damage (LID) initiation and damage growth lead to unacceptable obscuration or even catastrophic failure of the optics.[1,2] The LID of the fused silica surface, which typically scales with the laser uence, results from the presence of extrinsic damage precursors located on the surface or within the subsurface layer of the optics.[3]
We investigate the effect of HF-based wet shallow etching on the optical performance of reactive ion etching (RIE)-treated fused silica optics
The results show that the addition of NH4F to HF solution is highly important for improving laser damage resistance, as well as surface quality, optical transmission, and FL characteristics

Summary

Introduction

In high-power fusion-class laser facilities, the service life of fused silica optics is known to be severely limited by their surfaces, as laser-induced damage (LID) initiation and damage growth lead to unacceptable obscuration or even catastrophic failure of the optics.[1,2] The LID of the fused silica surface, which typically scales with the laser uence, results from the presence of extrinsic damage precursors located on the surface or within the subsurface layer of the optics.[3]. A possible approach to increase the laser-induced damage threshold (LIDT) of fused silica is to continually reduce the fractured subsurface defects such as scratches and microfractures, either through ne polishing or improved handling procedures.[7] in practical terms, the progress in eliminating fractures is extremely limited. Another path to further reducing the number of damage precursors on a fused silica surface is the development of a whole-optic HF or HF/NH4F chemical etching process (such as AMP described by LLNL12). This approach trades enhanced damage resistance for reduced surface quality properties, such as roughness and atness due to HF-acid isotropically exposing scratches and leaving etching traces.[14,15,16]

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Conclusion