JEOS RP ISSN01 | Page 240

J. Eur. Opt. Society-Rapid Publ. 2025, 21, 24 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025021 Available online at: https:// jeos. edpsciences. org

EOSAM 2024 Guest editors: Luca De Stefano and Raffaele Velotta

Journal of the European Optical Society-Rapid Publications

RESEARCH ARTICLE UV coatings using Ta 2 O 5-SiO 2 quantized nanolaminates

Manuel Bärtschi 1,*

, Stephan Waldner 2, Silvia Schwyn Thöny 2, Xavier Maeder 3, Fabian Steger 1, Thomas Frei 2, and Vivek Devulapalli 3

Received 31 January 2025 / Accepted 15 April 2025

Abstract. The use of Quantized Nanolaminates( QNL) in optical interference coatings has gained significant attention in recent years. By using a magnetron sputtering tool from Evatec with rotating substrate table and multiple sputtering sources operating simultaneously, we are able to deposit these metamaterials consistently at exceptionally high rates, comparable to and sometimes even exceeding magnetron sputter rates for standard materials. This combination of high deposition rate and precision enables us to treat QNL as a stand-alone material, effectively replacing high-refractive index materials in complex interference filter designs. In our experiments, we used a combination of Ta 2 O 5-SiO 2 QNL to produce filters like anti-reflective and shortpass coatings within the UV range of 266 – 350 nm, which would be not possible with Ta 2 O 5 as bulk material in similar designs.

Keywords: Optical interference coatings, Quantized nanolaminates, Magnetron sputtering, Ta 2 O 5, SiO 2.

1 Introduction

Quantized Nanolaminates( QNL) represent an innovative material system for optical coatings which was first developed by the Laser Zentrum Hannover( LZH) in 2016 [ 1 ]. This system involves the alternate deposition of two optical materials with different refractive indices as extremely thin layers, typically 0.1 – 2.0 nm thick. When numerous such layers are sequentially deposited on top of each other, they form a system which, as a stand-alone material has new optical properties compared to the standard bulk oxide materials. The refractive index of the resulting material is determined by the thickness ratio of the two constituent layers. However, the extreme thinness of the high index material layers in the stack, restricts the valence electron mobility, causing a shift in the absorption edge towards shorter wavelengths. This phenomenon has been demonstrated multiple times, primarily using Ion Beam Sputtering( IBS) or Atomic Layer Deposition( ALD) systems [ 1 – 3 ]. Unfortunately, the process of switching between the deposition of the two different coat materials, is time consuming, limiting the practical application of QNL in complex layer designs due to significant effort and costs. We have developed a magnetron sputtering process that allows the production of QNL systems from Ta 2 O 5 and SiO 2 as a continuous process [ 4 ]. This process achieves sputtering

* Corresponding author: manuel. baertschi @ rhysearch. ch rates of up to 0.8 nm / s and enables the deposition of up to 2400 laminate pairs per hour without interruption during the process. For the first time, this advancement enables the economical use of QNL as an independent high refractive index material for complex optical filter systems. In this paper we demonstrate first our approach to develop QNL single layers. Subsequently, we use two different QNL variants as high refractive index materials to create two complex multilayer systems. The first is a shortpass filter below 320 nm with a reflective region after 340 nm. We compare it with a similar shortpass made with standard Ta 2 O 5 layers as high refractive index material. Next, we present a double-side coated anti-reflective filter on quartz glass, which shows no significant absorption at 266 nm. This highlights the great potential of Ta 2 O 5-SiO 2 QNL compared to standard Ta 2 O 5 based coatings when it comes to manufacturing filters for the UV range.

2 Material and methods

2.1 Quantized nanolaminates

At first glance the idea behind QNL appears to be a relatively simple system. They consist of the periodic and alternating stacking of materials with different band gap energies. The band gap energy describes the amount of energy that a photon must have to be absorbed by the

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