Paweł Karasiński

Sol–gel-derived silica–titania (SiOx–TiOy) films were investigated as functional oxide thin films for surface microstructuring and photonic components. High-refractive-index SiOx–TiOy layers (n ≈ 1.8) were deposited by dip-coating on glass and silicon substrates and patterned by photolithography followed by inductively coupled plasma reactive ion etching (ICP-RIE). The study focused on pattern transfer at the photoresist/oxide interface and on the effect of lithographic and etching parameters on surface morphology, sidewall profile, and optical response. Exposure dose, development time, and post-lithography thermal treatment were evaluated for AZ 15nXT masks formed on SiOx–TiOy layers. Controlled ICP-RIE processing enabled the fabrication of rib structures with widths from 1.2 to 24 μm and heights from 14 to 170 nm. SEM/FIB analysis showed smooth rib surfaces and step-like sidewall profiles, confirming effective transfer of the resist pattern into the oxide layer. The microstructured films were then optically validated in the visible spectral range using straight and curved rib waveguides. Single-mode guiding was obtained for 1.5 μm-wide ribs with a height of 20 nm, and a Mach–Zehnder interferometer was fabricated using these parameters. Optical losses as low as 1.8 dB/cm at 532 nm were measured. The results show that sol–gel-derived SiOx–TiOy films can be patterned by scalable lithography and dry etching into oxide microstructures with controlled geometry and functional optical performance.

This study reports the results of an investigation of a niobium-doped titania dioxide thin film fabricated via a sol-gel method and dip-coating. Niobium-doped and undoped titania films were deposited on silicon wafers and soda-lime glass slides. The niobium-doped and undoped titanium dioxide films exhibit a high refractive index of ∼ 2.1 and thicknesses exceeding 110 nm. The study addresses the technological procedures, characterisation methods, and the influence of niobium doping on the optical properties of TiO₂ films.

The article discusses the design, fabrication, and experimental evaluation of a large-area vertical grating coupler (VGC) enabling simultaneous coupling of multiple input optical beams. The presented VCG was fabricated by direct nanoimprinting of a grating pattern in a non-hardened SiO_X:TiO_Y waveguide (WG) film. The WG film was deposited on a glass substrate using a combination of the sol–gel method and the dip-coating technique. The fabrication process allowed precise control of the waveguide film thickness and refractive index, as well as the VGC geometry. The relevance of the process was proved by a demonstration of optical coupling of multiple quasi-parallel input beams via the VGC to the WG layer. To make this possible, a dedicated optical coupling system was designed, including a polymer microlens array and optical fiber array positioned in a V-groove. This opens promising perspectives on using the proposed structure for the fabrication of low-cost multichannel optical sensor chips, as highlighted in the article’s final section.

The work presents TiOx:SiOy nanocomposite layers doped with Eu3+ions. Layers were fabricated using the sol-gel method and dip-coating technique. It was shown that the viscosity of sols and the size of optical band gaps increase with increasing concentration of europium ions. Considering the quantum size effect, it was found that the presence of europium ions causes a weakening of the crystallization of titanium oxide. This means that the diameters of scattering centers are smaller and the layer’s surfaces have lower roughness. In this way, the presence of europium ions in TiOx:SiOy waveguide layers contributes to reducing optical losses in the latter. Full Text: PDF References B. Zheng et.al. "Rare-Earth Doping in Nanostructured Inorganic Materials", Chemical Reviews , 122(6), 5519 (2022). CrossRef R.G Manju et.al. "pectroscopic analysis of Eu3+ doped silica–titania–polydimethylsiloxane hybrid ORMOSILs", RSC Adv., 10, 20057 (2020). CrossRef V. P. Prakashan et.al., "Perceiving impressive optical properties of ternary SiO2-TiO2-ZrO2:Eu3+ sol-gel glasses with high reluctance for concentration quenching: An experimental approach", J. Non-Cryst. Solids, 482, 116 (2018). CrossRef X. Chen et.al., "Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications", Chem. Rev., 107, 2891 (2007) CrossRef L.H Slooff et.al., "Effects of heat treatment and concentration on the luminescence properties of erbium-doped silica sol–gel films", J. Non Cryst. Solids, 296(3), 158 (2001). CrossRef M. Zięba et.al. "High Refractive Index Silica-Titania Films Fabricated via the Sol–Gel Method and Dip-Coating Technique—Physical and Chemical Characterization", Materials, 14, 7125 (2021). CrossRef P. Karasiński et.al. "Optical rib waveguides based on sol-gel derived silica–titania films", Thin Solid Films, 519, 5544 (2011). CrossRef J. Puetz et.al., Dip Coating Technique. In: Aegerter M.A., Mennig M. (eds) Sol-Gel Technologies for Glass Producers and Users. 2004 Springer, Boston, MA. CrossRef C.J. Brinker, G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Wiley & Sons Library: Hoboken, NJ, USA, (2013). CrossRef P. Karasiński, M. Zięba, E. Gondek, J. Nizioł, S. Gorantla, K. Rola, A. Bachmatiuk, C. Tyszkiewicz, "Sol-Gel Derived Silica-Titania Waveguide Films for Applications in Evanescent Wave Sensors—Comprehensive Study", Materials, 15, 7641 (2022). CrossRef P. Karasiński, E. Gondek, S. Drewniak, I.V. Kityk, "Nano-sized blue spectral shift in sol–gel derived mesoporous titania films", J. Sol-Gel Sci. Technol., 61, 355 (2012). CrossRef

Titanium dioxide (TiO₂) thin films prepared by the sol–gel technique have been shown to be optically isotropic and, unlike the films obtained by competitive methods, do not exhibit measurable birefringence. A series of submicrometer-thin titanium dioxide films were prepared using the sol–gel technique and then thermally annealed at different temperatures. The samples were analyzed by spectroscopic ellipsometry using the Mueller matrix formalism, X-ray diffractometry and scanning electron microscopy. The conversion of amorphous titanium dioxide to polycrystalline anatase occurred at 400 °C or higher. Crystallites of a few percent of the film thickness were observed. Nevertheless, the crystallization process did not trigger the appearance of birefringence. These observations demonstrate that high-quality planar optical waveguides can be successfully fabricated on flexible substrates, in particular those composed of efficient polymers that can withstand the aforementioned temperatures.

Titania dioxide (TiO2) layers were synthesized via the acid-catalysed sol-gel route using titania (IV) ethoxide, and then annealed at temperatures varying in the range of 150–700 °C. The research concerned the effect of annealing temperature on the structure of TiO2 layers, their surface morphology, and their optical properties. Further, X-ray diffractometry, and Raman spectroscopy were used to determine the structure of TiO2 layers. Scanning electron and atomic force microscopy were used to study the surface morphology of TiO2 layers. Transmittance, reflectance, absorption edge, and optical homogeneity were investigated by UV-VIS spectrophotometry, while the refractive index and thicknesses of TiO2 layers were measured using a monochromatic ellipsometer. Chromatic dispersion characteristics of the complex refractive index were determined using spectroscopic ellipsometry. Structural studies have shown that the TiO2 layers annealed at temperatures up to 300 °C are amorphous, while those annealed at temperatures exceeding 300 °C are polycrystalline containing only anatase nanocrystals with sizes increasing from 6 to 20 nm with the increase of the annealing temperature. Investigations on the surface morphology of TiO2 layers have shown that the surface roughness increases with the increase in annealing temperature. Spectrophotometric investigations have shown that TiO2 layers are homogeneous and the width of the indirect optical band gap varies with annealing temperature from 3.53 eV to 3.73 eV.

Zinc oxide layers on soda-lime glass substrates were fabricated using the sol-gel method and the dip-coating technique. Zinc acetate dihydrate was applied as the precursor, while diethanolamine as the stabilizing agent. This study aimed to determine what effect has the duration of the sol aging process on the properties of fabricated ZnO films. Investigations were carried out with the sol that was aged during the period from 2 to 64 days. The sol was studied using the dynamic light scattering method to determine its distribution of molecule size. The properties of ZnO layers were studied using the following methods: scanning electron microscopy, atomic force microscopy, transmission and reflection spectroscopy in the UV-Vis range, and the goniometric method for determination of the water contact angle. Furthermore, photocatalytic properties of ZnO layers were studied by the observation and quantification of the methylene blue dye degradation in an aqueous solution under UV illumination. Our studies showed that ZnO layers have grain structure, and their physical–chemical properties depend on the duration of aging. The strongest photocatalytic activity was observed for layers produced from the sol that was aged over 30 days. These layers have also the greatest porosity (37.1%) and the largest water contact angle (68.53°). Our studies have also shown that there are two absorption bands in studied ZnO layers, and values of optical energy band gaps determined from positions of maxima in reflectance characteristics are equal to those determined using the Tauc method. Optical energy band gaps of the ZnO layer fabricated from the sol aged over 30 days are E_g^I = 4.485 eV and E_g^II = 3.300 eV for the first and second bands, respectively. This layer also showed the highest photocatalytic activity, causing the pollution to degrade 79.5% after 120 min of UV irradiation. We believe that ZnO layers presented here, thanks to their attractive photocatalytic properties, may find application in environmental protection for the degradation of organic pollutants.

Composite silica-titania waveguide films of refractive index ca. 1.8 are fabricated on glass substrates using a sol-gel method and dip-coating technique. Tetraethyl orthosilicate and tetraethyl orthotitanate with molar ratio 1:1 are precursors. Fabricated waveguides are annealed at 500 °C for 60 min. Their optical properties are studied using ellipsometry and UV-Vis spectrophotometry. Optical losses are determined using the streak method. The material structure and chemical composition, of the silica-titania films are analyzed using transmission electron microscopy (TEM) and electron dispersive spectroscopy (EDS), respectively. The surface morphology was investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM) methods. The results presented in this work show that the waveguide films are amorphous, and their parameters are stable for over a 13 years. The optical losses depend on their thickness and light polarization. Their lowest values are less than 0.06 dB cm⁻¹. The paper presents the results of theoretical analysis of scattering losses on nanocrystals and pores in the bulk and interfaces of the waveguide film. These results combined with experimental data clearly indicate that light scattering at the interface to a glass substrate is the main source of optical losses. Presented waveguide films are suitable for application in evanescent wave sensors.

The solar module output power is the power generated by all individual cells in their specific electrical circuit configuration, multiplied by the cell-to-module power ratio. The cell-to-module power ratio thus reflects the sum of the losses and gains produced by the structure of the module. The biggest process change in module design during the last few years was the introduction of half cells. Another important trend is the use of bifacial cells to build bifacial modules. These two trends increase parts of the module that correspond to the intercell gaps, and the light does not meet the cell in its path. This part of the radiation is therefore not used efficiently. Scientific efforts focus on the texturing surface of covering glass and cells, and the introduction of narrower ribbons and encapsulation materials with improved UV performance, etc. The concept of a diffusor that actively redirects light from the intercell space into the cell was proposed in the past, in the form of a micro-structured prismatic film, but this is not applicable for bifacial modules. The conclusion is that losses caused by the incidence of light on the areas of the photovoltaic panel not covered with solar cells yet are to be explored further. A sawtooth-shaped reflecting diffusor placed between cells is proposed. This article addresses the issue in a novel way, primarily because the theoretical range of the optimum sawtooth profile is defined. In the experimental part of the study, the possibility of producing such a profile directly on glass using a CO₂ laser is demonstrated. The theoretical model enables discrimination between advantageous and disadvantageous sawtooth profiles. As a proof of concept, minimodules based on the optimum parameters were built and tested for their electrical performance. The result confirms that the proposed sawtooth-shaped reflecting diffusor placed between cells creates cell-to-module power gain. The proposed laser technology can be incorporated into existing production lines, and can increase the output of any photovoltaic technology, including and beyond silicon.

In the past few decades, several methods concerning optical thin films have been established to facilitate the development of integrated optics. This paper provides a brief depiction of different techniques for implementing optical waveguide thin films that involve chemical, physical, and refractive index modification methods. Recent advances in these fabrication methods are also been presented. Most of the methods developed for the realization of the thin-films are quite efficient, but they are expensive and require sophisticated equipment. The major interest of the scientists is to develop simple and cost-effective methods for mass production of optical thin films resulting in the effective commercialization of the waveguide technology. Our research group is focused on developing a silica-titania optical waveguide platform via the sol-gel dip-coating method and implementing active and passive optical elements via the wet etching method. We are also exploring the possibility of using nanoimprint lithography (NIL) for patterning these films so that the fabrication process is efficient and economical. The recent developments of this platform are discussed. We believe that silica-titania waveguide technology developed via the sol-gel dip-coating method is highly attractive and economical, such that it can be commercialized for applications such as sensing and optical interconnects.