This feature issue of Applied Optics presents a curated selection of papers from OIC 2025, the sixteenth Topical Meeting on Optical Interference Coatings (OIC) held May 18-23, 2025, in Tucson, Arizona, USA. The issue celebrates the 50th anniversary of OIC, reflecting the conference's depth and breadth. This triennial meeting serves as a premier international forum for the optical interference coatings community, bringing together researchers, engineers, and industry professionals to present and discuss the latest developments in the field. The meeting showcases cutting-edge advances in coating design theory, innovative materials, deposition and characterization techniques, and a wide array of applications. Topics presented span technologies for green energy, aerospace systems, gravitational wave detection, telecommunications, optical instrumentation, consumer electronics, and high-power and ultrafast laser systems, among others. Beyond the technical sessions, OIC 2025 provided valuable opportunities for collaboration, networking, and the cross-pollination of ideas, continuing its legacy as a cornerstone event in the advancement of optical coating science and technology.

The operation of a storage ring free-electron laser (FEL) is constrained by the shortest wavelength that its cavity can support. To address this limitation, significant efforts have been made in recent decades to develop high-reflective FEL mirrors supporting progressively shorter wavelengths.

In the course of continuous innovations in photonic integration optical components with ever decreasing size and increasing performance are gaining of importance in a wide range of applications. In addition, a trend towards shorter wavelengths in modern photonic systems can be observed, and frequency conversion units are commonly employed add-on modules for most laser types. Especially considering conversion to the third harmonic (TH) wavelength, present concepts are based on two second-order nonlinear processes involving two conversion crystals. Even though corresponding modules are commercially available, there is a need for simpler solutions that can be further scaled down and handled in automated assembly systems. One possibility is direct third-harmonic generation (THG) in a third-order process [1] . However compared to second-order processes, phase matching imposes a certain challenge here, because the refractive index differences between the fundamental and third harmonics are significantly larger for typical conversion materials.

For the purpose of a deeper understanding of thin film growth, in the last two decades several groups developed models for simulation on the atomistic scale. Models using molecular dynamics as their simulation method already give results comparable to experiments, however statistical analysis of the simulations themselves are lacking so far, reasoned by the limits imposed by the computational power and parallelization that can only be used in lateral dimensions. With advancements of software and hardware, an increase in simulation speed by a factor of up to 10 can be reached. This allows either larger structures and/or more throughput of the simulations. The paper analyses the significance of increasing the structure size in lateral dimensions and also the repetition of simulations to gain more insights into the statistical fluctuation contained in the simulations and how well the coincidence with the experiment is. For that, glancing angle incidence deposition (GLAD) coatings are taken as an example. The results give important insights regarding the used interaction potential, the structure size and resulting important differences for the density, surface morphology, roughness and anisotropy. While larger structures naturally can reproduce the real world in more detail, the results show which structure sizes are needed for these aspects without wasting computational resources.

We manufactured and tested ion beam sputtered antireflection coatings for 355 nm using several defect mitigation strategies to improve the damage resistance of laser optics for LIDAR satellite missions. The first approach to avoid the damage precursors in the film is based on a secondary ion source. The goal is to remove the particles that settle on the surface of the growing film. A second strategy is to avoid the damage precursors in the ultraviolet wavelength range by laser conditioning of the thin films during the coating process. A laser beam at the application wavelength scans the deposited layer on the sample during deposition still in vacuum. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in ramped raster scan tests.

The short wavelength operation of free-electron laser (FEL) oscillators is limited by the availability of high-reflectivity, thermally stable, and radiation-resistant FEL mirrors in the vacuum UV (VUV) wavelength. We report our recent work to extend the shortest lasing wavelength of the oscillator FEL to 168.6 nm using a storage ring FEL. This progress has been made possible by developing a new FEL configuration with substantially reduced undulator harmonic radiation on the FEL mirror, a thermally stable FEL optical cavity, and a new type of high-reflectivity fluoride-based multilayer coating with a protective capping layer. Using these fluoride-based mirrors, we have demonstrated storage ring FEL lasing from 168.6 to 179.7 nm with excellent beam stability. Employing this VUV FEL in Compton scattering, we have produced the first 120 MeV gamma rays at the High Intensity Gamma-ray Source (HIGS). Operating the HIGS in this new high-energy region will create many new opportunities for photonuclear physics research, in particular, the low-energy quantum chromodynamics research.

Modern laser systems have paved the way for spaceborne laser applications such as Earth's surface and atmosphere monitoring. Well known technologies like Nd:YAG lasers are often employed; however, they do not always comply with all the different requirements for space missions. High optical efficiencies and tunable wavelength, which are desirable for many applications, can be reconciled with a simple laser design employing Alexandrite crystals. Horizon 2020 project presented here discuss the results on the development of alexandrite laser crystal treatment prior to coating deposition, as well as future plans on crystal interference coating deposition for LIDT improvement.

Recent developments of nanostructured coatings have reached a point where extensive investigations within multi-layer systems are necessary for further implementation in novel photonic systems. Although sculptured thin films are explored for decades, no optical and structural measurements have been performed for anisotropic nanostructured multi-layer coatings with different deposition conditions of the dense layer. In this paper, we present extensive morphological analysis on silica nanostructured anisotropic films. Changing the deposition angle from 66 degrees to 84 degrees, indicate the changes in surface filling from 84% to 57%, respectively, while phase retardance has a maximal value of 0.032 degrees/nm at 70 degrees and 72 degrees angles. We also present the investigation of covering such structures with the dense layer at different conditions. As a result, the technology for maintaining initial anisotropic properties is developed for extending spectral difference 1.6 times and phase retardation by 5% in anisotropic multi-layer coatings. Furthermore, we present simulations of growing silica layer using experimental conditions in the Virtual Coater framework resulting in virtual anisotropic films for comparison with measurements. The minimal impact on the anisotropy of porous layer is reached with the deposition of dense layer at 30 degrees angle during constant substrate rotation.

By analyzing the third harmonic generation from gradient layers of the amorphous dielectric ternary mixture material Hf x Al y O z we are able to derive the third order nonlinear susceptibility of the material.

Hybrid integrated photonics open up new application perspectives due to compact size and the shift to cost-efficient components. Therefore, integration of optical and electro-optical functionalities into photonic chips has recently attracted great interest. Research has been directed towards miniaturization of demanding spectral transfer properties for individual applications. However, it remains challenging to implement highly complex transmission and reflection characteristics with few additional process steps. In this contribution, we report on our advancement in the field of optical thin-film coating fabrication, which enables a manufacturing process comparable to die assembly in electronics. We have combined a sacrificial-substrate approach with the production of miniaturized optical thin-film coatings by ion-beam sputtering. The concept is applicable to high precision coatings with more than 130 individual layers and adding up to over 26 µm total film thickness. Segmentation down to sizes of 25 μm x 25 μm pieces is realized by laser cutting of the coating. By completely removing the substrate afterwards, we achieve a freestanding thin-film and thus minimized thickness. Our measurements indicate no general performance loss compared to coatings on glass substrates. Additionally, the substrates refractive index and absorption do not have to be considered in the multilayer-coating design. Therefore, the design can be optimized and matched to the refractive index of specific waveguides on the chip. Furthermore, we demonstrate the compatibility to releasable transfer tape. With this, we aim for enabling a high-volume feed of miniaturized thin-film filters to an automated assembly process.

Third harmonic generation (THG) from multiple thin layer structures has proven its capability to reach conversion efficiencies up to the percentage level [1] , becoming an interesting alternative to the cascaded third harmonic generation. However, such multi-layer structures need to be carefully optimized by computer algorithms. Here, we present a study of a single amorphous Hf x Al y O z layer with varying thickness. The advantage of ternary mixture materials such as Hf x Al y O z is the tunability of the bandgap, which directly influences the nonlinear susceptibility χ (3) . In comparison to previous works, where only layers of specific thickness or theoretical models were studied [2] , [3] , we present both, continuous experimental measurements and theoretical studies of the same sample.

Dielectric cavity mirrors for storage-ring free-electron lasers (FELs) in the VUV range have to overcome several challenges including resistance to synchrotron radiation and long-term stability in a contaminated vacuum. While oxide materials are not suitable below 190 nm, pure fluoride materials are not stable in this challenging environment. It has not been possible until now to generate lasing below 176 nm in a storage ring FEL. Duke University/TUNL and Laser Zentrum Hannover have successfully developed new strategies and produced mirrors with a high reflectivity of 95 %, which enabled FEL lasing between 168,6 and 176,7 nm.

In order to enhance the reliability and performance of space-based Lidar systems, it is desirable to increase the damage resistance of ultraviolet antireflective coatings. For laser pulses with nanosecond pulse duration, laser-induced damage is known to be triggered by nano-sized defects embedded in the optical coating. In this work, we demonstrate the mitigation of damage precursors during the manufacturing of ion-beam sputtered (IBS) coatings using two approaches: ion bombardment with a secondary ion source and laser irradiation with a nanosecond-pulsed laser. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in large-area raster scans.

Spaceborne Earth observation based on laser instruments provides new technologies to monitor the atmosphere or our planet's surface. Space-qualified Alexandrite laser crystals show convincing properties as a laser-active medium in high power laser systems for space-based missions, e.g. the wavelength tunability and the excellent material properties, such as high thermal conductivity and a good breaking strength. Therefore, the Horizon 2020 project GALACTIC was initiated to realize space-qualified, high-quality coated Alexandrite crystals relying on a purely European-based supply chain. The project consortium will push the development of Alexandrite crystals and coatings within the EU from the current Technology Readiness Level (TRL) 4 up to TRL 6. The Horizon 2020 project GALACTIC, which will be presented, has been initiated to realize space qualified high quality, high performance coated Alexandrite crystals rest on a purely European-based supply chain. The project consortium, consisting of Optomaterials S.r.l., Altechna Coatings and the Laser Zentrum Hannover e.V., provides the necessary expertise in the fields of laser crystal growth, optical coatings and laser systems to push the development of high quality, high damage threshold Alexandrite coatings within the EU up to TRL 6. In order to reach the GALACTIC goals, different technology areas will be addressed. On the one hand, Optomaterials S.r.l. as an experienced optical crystal manufacturer will continuously refine the crystal growth process and improve the raw crystals as well as the cutting and polishing quality step-by-step. As an experienced company in optical coatings production for space applications, Altechna Coatings will secondly develop high quality, low loss, high damage threshold coatings specifically tailored for application onto Alexandrite laser crystals. The superior performance of the coated crystals will then be proven in demonstrator laser systems. The Laser Zentrum Hannover e.V. has the required knowledge to set up laser demonstrator prototypes, whose specifications mimic typical earth observation laser source requirements. Finally, the GALACTIC consortium will work closely together to characterize and qualify the developed coated laser crystals. By collaborating with the European space industry and deriving the detailed requirements from typical earth observation space missions, a detailed TRL 6 components test plan will prove the TRL 6 qualification of the developed coated laser crystals. This will conclude the development process and will enhance the European non-dependence status of the Alexandrite laser crystal market. At the conference, we will present the project and its consortium as well as detailed strategies to enable a non-dependence of Europe on coated Alexandrite laser crystal technology and enabling long term European availability of this technology. This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No 870427.

We demonstrate the mitigation of nano-sized damage precursors during the manufacturing of IBS-coatings using bombardment with a secondary ion source and laser conditioning with a nanosecond-pulsed laser. Optical coatings show a significantly increased damage threshold.

The development of advanced laser systems has paved the way for spaceborne laser applications like Earth's atmosphere and surface monitoring. Prominent technologies like Nd:YAG are often exploited, however they have proven a challenge to combine the different requirements for space missions. High optical efficiencies and tunable wavelength, which are desirable for many applications, can be reconciled with a simple laser design employing Alexandrite crystals. Horizon 2020 project presented here focuses on the development of high quality and high performance coated Alexandrite crystals along with the construction of a laboratory prototype Alexandrite laser system with specifications suitable for Earth observation missions.

Manufacturing of optical coatings is about the precise deposition of material and its control on the nanometer scale. This has to be achieved for a stack of multiple layers with a total thickness that can reach several micrometer. In addition, this has to be maintained for the complete functional area. In this contribution we demonstrate a process capable of delivering these accuracies not only to flat but also to curved surfaces.

Spaceborne LIDAR systems with high power laser optics enable advanced understanding of the climate change. However, nano-scaled particles originating from film deposition processes reduce the laser-induced damage threshold. A secondary ion source is used to remove the particles that settle on the surface of the growing film. A second option is to remove precursors by laser irradiation before they are covered with additional layers. To verify the impact on the defect mitigation, laser-induced damage threshold tests and raster scan tests were conducted. This work has been carried under the ESA contract AO 1-8683/16/NL/BJ activity with name “Particle mitigation in high power laser optics”.