Special Issue: Coatings Optical Coatings: Current Problems, Achievements and Prospects

Optical coatings represent a ubiquitous technology with significant applications across optics, optoelectronics, and photovoltaics. The current Special Issue is dedicated to the integration of thin films into advanced optical and optoelectronic devices. The emphasis is placed on examining the relevant structures, material properties, and fabrication techniques employed in film development.

The past few decades have seen the emergence of various optical thin film strategies that are crucial for advancing integrated optics [1]. The realization of optical waveguide thin films utilizes diverse approaches, notably those involving chemical, physical, and refractive index modification [2]. While these established methods are effective, their high fabrication costs and complex equipment requirements necessitate simpler, cost-effective alternatives suitable for mass production, and, consequently, the successful commercialization of waveguide technology. Thin film technology has matured over several decades to offer processes, including material modification techniques, for developing integrated photonic devices like structured optical fibers [3]. Although the functional versatility of microstructured fibers significantly benefits quantum technologies, the realization of their full potential is hindered by key persistent challenges [4].

Transparent electrodes are necessary for displays and solar cells, demanding a combination of high transparency and low electrical resistance. Research is focusing on new materials—such as graphene, carbon nanotubes, and conductive polymers—that can offer an effective balance of optical and electrical properties alongside the mechanical flexibility required for future flexible and wearable electronic devices [5].

One-dimensional photonic crystals are extensively utilized in optical coatings, primarily in planar multilayer interference structures, for their AR and HR functionality. Significant progress has been made, including the formation of 2D photonic crystals achieved through physical vapor deposition on pre-patterned substrates. This Special Issue highlights novel technological approaches, specifically Direct Laser Interference Patterning (DLIP) [6], for realizing submicron non-absorbing gratings. These methods are designed to advance laser optics by establishing a scalable and efficient technology for fabricating precise metal-oxide photonic structures.

Aerospace materials research is currently characterized by advances in various compounds, including high-strength Carbon-Fiber-Reinforced Polymers (CFRP). A substantial challenge remains in the selection of specially engineered coatings to mitigate laser attacks. Due to the increasing threat posed by directed-energy weapons, enhancing resistance to laser ablation is a major priority in the aerospace sector. The recent progress made in coatings designed for laser ablation resistance and improved thermal shock performance is comprehensively detailed in [7].

The recent breakthroughs in Additive Manufacturing (AM) enable the creation of high-precision optical components, including microlenses and photonic devices, with micro- and nanoscale features [8]. The range of printable materials now includes metals, polymers, and ceramics. These advances in AM, combined with the rapid growth of nanophotonics (e.g., metasurfaces and photonic crystals), are leading to the development of new ultrathin optical analog computing devices. A popular trend in this field is the use of lithography-free ultrathin film optical coatings (UFOCs) for longitudinal nanophotonic structures [9].

Developing flexible optoelectronic devices for practical use is difficult because mechanically strained light-emitting components perform poorly compared to solid materials [10]. Improving performance is crucial for commercialization, but initial efforts to create new stretchable and human-friendly electronic systems are vital. Separately, the creation of high-quality optics is limited by polishing methods and insufficient instrumentation sensitivity. This gap is being addressed by the introduction of specialized differential coatings [11]. Finally, attaining High Reflectance (HR) from the visible to near-infrared ranges is an essential prerequisite for all future space programs, both commercial and research-based [12].

The development and implementation of coatings that exhibit emissive properties across various wavelength ranges is critically important. This category primarily includes different types of films with luminescent characteristics. A key method that is used to modify the desired emission parameters is the structuring (or nanostructuring) of these films, particularly their surfaces. Furthermore, these properties are also influenced by other coating material optical parameters, including their chemical composition, crystal structure, and geometry.

The publication by Liu et al. [13] details the synthesis of multimode fluorescent materials for practical security enhancement. Using the sol–gel method, they created core–shell SrAl₂O₄:Eu phosphors. Upon exposure to UV radiation at 254 nm, the materials demonstrated a dual optical response: immediate red emission and subsequent intense green persistent luminescence (afterglow). The observed multimodal luminescence, which is dependent on excitation parameters like wavelength and duration, offers a robust foundation for developing advanced anti-counterfeiting technologies.

Li et al. [14] introduced a luminescent coating technology intended for multifunctional road markings. They employed long-persistent CaAl₂O₄ phosphor, which they incorporated into the coating via a mixing method. Notably, this phosphor offers an impressive afterglow duration exceeding 8 h after being excited by daylight. The study also revealed that adding components like SiO₂ and CaCO₃ significantly improves the homogeneity and density of the final coating. The study successfully identified the optimal material composition and processes for the application of this system on expressways.

Grineviciute et al. [15] introduced periodically modulated optical coatings designed for spatial light filtering. Their fabrication method involved depositing conformal films onto modulated substrates, and they systematically compared various thin film deposition techniques, including electron beam evaporation, atomic layer deposition (ALD), and ion beam sputtering, specifically for use on these periodically modulated surfaces. A key part of the study involved conducting a series of experiments to evaluate the method’s effectiveness in replicating modulated submicron surfaces.

Nowak and colleagues [16] revealed that short-pulse laser irradiation induces the formation and local reorganization of nanostructures within aluminum thin films. This phenomenon fundamentally alters the film’s optical properties, leading to a spectrally selective change in its response. The study successfully demonstrated the generation of plasmonic colors via this precise femtosecond laser patterning. This technique has significant implications for both security applications (anti-counterfeiting) and advanced color printing. Additionally, the authors suggest that the diffuse reflection characteristics of these plasmonic multilayer systems could be beneficial for sensory technologies.

Additional functional coatings for energy devices are also relevant, namely photovoltaics, solar batteries, energy storage and accumulation devices.

In their paper, Jeon and colleagues [17] presented a high-efficiency heterojunction solar cell technology with simple fabrication processes. They used copper iodide (CuI) thin films (work function > 5.0 eV), which remained relatively stable under atmospheric exposure compared to TMO thin films. The work function of the charge carriers, the high hole concentration in the hole-selective contacts (HSC layer), and the use of an ideal passivation material at the interface between the HSC layer and n-Si layer all contribute to the development of high-efficiency carrier-selective contact (CSC) Si solar cells.

Modern energy storage research increasingly emphasizes the development of sustainable, affordable, and ecologically sound supercapacitors. In response to this, Bronusiene and colleagues [18] focused their work on the synthesis and characterization of tin sulfide (SnS_x) films. The collected data allowed them to effectively assess SnS_x viability as an electrode material for supercapacitors, thereby confirming its potential role in sustainable energy systems. Crucially, the optimal energy performance was achieved after the films underwent a specific thermal annealing process.

Candelas-Urrea et al. [19] presented research on A₂B₆ semiconductor film structures. CdS, CdTe, HgCdTe, and CdTe films were electrodeposited to create short- and mid-wave infrared photodetectors. These photodetectors achieved high infrared sensitivity, boasting a detectivity (D∗) of 2.86 × 10¹² cm·Hz^1/2·W⁻¹, and were also self-powered. This effective multilayer electrodeposition strategy is scalable for developing various sensor applications.

Fiber optic technology constitutes a cornerstone of modern high-speed communication networks, facilitating inter-connectivity with enhanced data transmission bandwidth, speed, and fidelity [20]. The rapid emergence and evolution of optical fiber technology over recent decades have yielded substantial global technological progress. Furthermore, customized fibers are employed in specialized applications, including fiber optic sensors and fiber lasers. Technological efforts are directed towards advancing laser optics through the implementation of scalable and efficient methodologies for creating precise metal-oxide-based photonic structures [21]. An integral step in the development of fiber-optic photonic sensors involves the engineering of functional layers [22], with research heavily focused on the design and performance characteristics of transducers and techniques for sensitivity enhancement [23].

The work by Qu et al. [24] underscores the crucial role of coatings in boosting the performance of fiber-optic sensors, focusing on their sensitivity and stability. The authors provided a comprehensive review of the sensor operating principles and coating applications, discussing how different designs enhance reliability. The study is particularly relevant as it highlights the increasing use of these sensors in the medical sector—for instance, in measuring glucose levels, protein concentrations, and blood properties. The inherent advantages of fiber-optic sensors, including their small size, resistance to interference, high sensitivity, affordability, and rapid response, are shown to be significantly beneficial in these demanding applications.

The global market provides optical fiber manufacturers with a diverse selection of coating options [25]. Currently, the coatings most frequently employed are acrylate, polyimide, and silicone-based compounds. Furthermore, metallic coatings (e.g., gold, aluminum, copper) constitute a significant category. Driven by the rapid progress in laser and optoelectronic technologies, coupled with the expanding scope of fiber applications, there is an increasing demand for highly durable optical fibers capable of performing reliably in challenging environmental conditions.

This Special Issue, “Optical Coatings: From Materials to Applications”, collects relevant research articles addressing the latest developments in this research topic. The expert integration of optical fiber sensor technology and coating technology significantly boosts sensors’ adaptability across diverse real-world applications. This powerful combination provides researchers in related disciplines with new tools and methodologies, actively accelerating progress and development in their fields. The relevant academic research findings are comprehensively covered in this Special Issue.