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Item type:Publication, Access status: with files , On the applicability of the Maxwell Garnett effective medium model to media with a high density of cylindrical pores(Optica Publishing Group, 2024-04-01); ; ; ; The optical properties of dielectric materials with subwavelength cylindrical pores are commonly described by effective medium models. We compare the Maxwell Garnett and the Bruggeman effective medium models for porous silicon with simulations and experiments for the case of polarization orthogonal to the pore axis. The Maxwell Garnett model matches the results of the simulations even up to very high porosities. An experimental study of the effective permittivity of macroporous and mesoporous silicon is conducted by analyzing the Fabry-Pérot oscillations in the long-wavelength limit. These experimental results are also consistent with the Maxwell Garnett model. We advocate using this model for media with cylindrical pores in the future.Publicationtype: Journal Article36 57 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Machine learning models for photonic crystals band diagram prediction and gap optimisation(2022-12); ; ; ; Data-driven methods of machine learning (ML) have attracted a lot of interest in various fields of physics. Inverse design and optimisation of structured optical metamaterials such as photonic crystals, metasurfaces, and other nanostructured components seem to benefit a lot from this approach in the nearest future. Here we develop several approaches to use ML methods to predict and optimise properties of photonic crystals (e.g. size of bandgaps) effectively. We use a dataset of 2D photonic crystals produced recently in [T.Christinsen et al., Nanophotonics 9, 4183 (2020)]. For improving performance of predictive models, we apply symmetry-aware augmentations and hybrid ML-solver approaches. As a result, considerable improvement in prediction accuracy could be achieved as compared to baseline models. For generative models, we apply variational autoencoders (VAEs) combined with predictor architecture, inspired by related works in chemical design realm. By using latent space optimisation, we achieve good results in the task of increasing bandgaps of photonic structures. The approach seems to be very promising and can be extended to 3D geometries.Publicationtype: Journal Article84 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Direct Plasmonic Excitation of the Hybridized Surface States in Metal Nanoparticles(2021-07-21); ; ; Plasmon-driven chemical reactions are a subject that is currently capturing the attention of the research community and generates a fair amount of arguments about their origin. Taking into account that the lifetime of excited hot carriers in metals is very short, some mechanism is required to store carriers long enough and in sites that allow chemical reactions with the environment. One established mechanism is the injection of charges into either the valence or conduction band of a semiconductor, followed by a chemical reaction at the semiconductor surface. Here, we consider a somewhat less-explored pathway by which plasmon decay can cause a chemical reaction: the direct excitation of hybridized surface states by plasmons. Using a simple model, we evaluate theoretically the rate of direct excitation and find that it can be comparable and often exceed the rate of indirect excitation of surface states. Our findings correspond to prior experimental results. We also identify the conditions under which one can enhance the direct excitation efficiency and, thus, bring plasmon-driven photochemistry closer to practical applications.Publicationtype: Journal Article90 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Tailoring disorder and quality of photonic glass templates for structural coloration by particle charge interactions(American Chemical Society (ACS), 2021-04-20); ; ; ; To obtain high-quality homogeneous photonic glass-based structural color films over large areas, it is essential to precisely control the degree of disorder of the spherical particles used and reduce the crack density within the films as much as possible. To tailor the disorder and quality of photonic glasses, a heteroaggregation-based process was developed by employing two oppositely charged equal-sized polystyrene (PS) particle types. The influence of the particle size ratio on the extent of heteroaggregation in the suspension mixes is investigated and correlated with both the morphology and the resultant optical properties of the films. The results show that the oppositely charged particle size ratio within the mix greatly influences the assembled structure in the films, affecting their roughness, crack density, and the coffee-ring formation. To better differentiate the morphology of the films, scanning electron microscopy images of the microstructures were classified by a supervised training of a deep convolutional neural network model to find distinctions that are inaccessible by conventional image analysis methods. Selected compositions were then infiltrated with TiO2 via atomic layer deposition, and after removal of the PS spheres, surfacelated inverse photonic glasses were obtained. Different color impressions and optical properties were obtained depending on the heteroaggregation level and thus the quality of the resultant films. The best results regarding the stability of the films and suppression of coffee-ring formation are obtained with a 35 wt % positively charged over negatively charged particle mix, which yielded enhanced structural coloration associated with improved film quality, tailored by the heteroaggregation fabrication process.Publicationtype: Journal ArticleScopus© Citations 7 127 575 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Influence of alumina addition on the optical properties and the thermal stability of titania thin films and inverse opals produced by atomic layer deposition(Multidisciplinary Digital Publishing Institute (MDPI), 2021-04); ; ; ; TiO2 thin films deposited by atomic layer deposition (ALD) at low temperatures (lower than 100 °C) are, in general, amorphous and exhibit a smaller refractive index in comparison to their crystalline counterparts. Nonetheless, low-temperature ALD is needed when the substrates or templates are based on polymeric materials, as the deposition has to be performed below their glass transition or melting temperatures. This is the case for photonic crystals generated via ALD infiltration of self-assembled polystyrene templates. When heated up, crystal phase transformations take place in the thin films or photonic structures, and the accompanying volume reduction as well as the burn-out of residual impurities can lead to mechanical instability. The introduction of cation doping (e.g., Al or Nb) in bulk TiO2 parts is known to alter phase transitions and to stabilize crystalline phases. In this work, we have developed low-temperature ALD super-cycles to introduce Al2O3 into TiO2 thin films and photonic crystals. The aluminum oxide content was adjusted by varying the TiO2:Al2O3 internal loop ratio within the ALD super-cycle. Both thin films and inverse opal photonic crystal structures were subjected to thermal treatments ranging from 200 to 1200 °C and were characterized by in- and ex-situ X-ray diffraction, spectroscopic ellipsometry, and spectroscopic reflectance measurements. The results show that the introduction of alumina affects the crystallization and phase transition temperatures of titania as well as the optical properties of the inverse opal photonic crystals (iPhC). The thermal stability of the titania iPhCs was increased by the alumina introduction, maintaining their photonic bandgap even after heat treatment at 900 °C and outperforming the pure titania, with the best results being achieved with the super-cycles corresponding to an estimated alumina content of 26 wt.%.Publicationtype: Journal ArticleScopus© Citations 11 188 176 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Improved thermal stability of zirconia macroporous structures via homogeneous aluminum oxide doping and nanostructuring using atomic layer deposition(Elsevier, 2021-02-11); ; ; ; Dopants are regularly used in sol-gel and powder metallurgy routes, however, the controlled insertion of such is quite challenging, especially in the case of nanostructures. Here we investigate the use of atomic layer deposition (ALD) as a potential technique to precisely introduce aluminum oxide as dopant or second phase into zirconia 3D macroporous nanostructures. The results show that the introduction of high Al O contents into the zirconia nanostructures successfully inhibited sintering when in comparison to undoped zirconia. Moreover, for the multi-nanolaminated and full-mix structures, the tetragonal phase was stabilized up to 1200 °C. Furthermore, the structures presented a photonic band gap even after heat treatment at 1200 °C for 2 h, enabling its application as inverse opal photonic crystals in high-temperature environments. The enhancement of thermal stability and high-temperature tetragonal phase stabilization is enabled jointly by the nanostructuring and homogeneous distribution of aluminum oxide provided by ALD super-cycles. 2 3Publicationtype: Journal ArticleScopus© Citations 8 193 377 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Conductive and radiative heat transfer inhibition in YSZ photonic glass(2020-08-01); ; ; ; A high temperature stable ceramic photonic structure is demonstrated with low thermal conductivity and suppressed external radiative heat transfer. The structure is based on a disordered arrangement of yttria-stabilized zirconia (YSZ) microparticles, called photonic glass (PhG). The prepared YSZ-PhG film exhibits low thermal conductivity of 0.03 Wm−1K−1 comparable to that of the air. The small point contacts of the adjacent YSZ particles are the main cause of such low thermal conductivity. After annealing at 1400 °C for 5 h, the solid thermal conductivity increased to 0.3 Wm−1K−1 at room temperature due to the thermally induced neck formation, associated with an increased contact area between adjacent particles. This thermal conductivity is still much lower than that of conventional YSZ thermal barrier coatings (TBCs) with approximately 1 Wm−1K−1. At the same time, the PhG structure is an efficient scatterer for thermal radiation in the wavelength range between 1 and 6 μm. In an only 100 μm thick structure an average reflection of 84% was obtained. At 1400 °C, the effective thermal conductivity is 0.2 Wm−1K−1. The presented structure is applicable to other oxides with even lower bulk thermal conductivity and can be considered for future TBCs.Publicationtype: Journal Article262 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Photonic glass based structural colorStructural coloration, which is based on spectrally selective scattering from optical structures, has recently attracted wide attention as a replacement of pigment colors based on the selective light absorption in chemical structures. Structural colors can be produced from transparent non-toxic materials and provide high stability under solar radiation. To provide angle independent non-iridescent colors, the structure should combine spectral selectivity with an isotropic response. Photonic glass (PhG), a disordered arrangement of monodisperse spheres, is a versatile structure to achieve that, which provides isotropic spectral selectivity via short-range order and Mie resonances. However, conventional PhGs show low color purity that hinders their future application. The interplay of single-particle scattering, short-range order, broadband absorption, and Fresnel reflection is a route to improve the color. In this perspective, we review the field of PhG based structural colors and discuss the physical mechanism behind the color generation by several established theories. We point out the current challenges in the theory and possible directions to improve color purity.Publicationtype: Journal ArticleScopus© Citations 36 234 316 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , High-Contrast Structural Color Based on Photonic Glass from Core-Shell Particles(2020-05); ; ; Photonic glass provides angle-independent, spectrally selective scattering that can be used as structural colors. We show how the core-shell geometry of the particles can be used to improve the saturation of blue color.Publicationtype: Conference Paper81 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Surface templated inverse photonic glass for saturated blue structural color(2020-03-16); ; ; ; To substitute conventional pigments, which often are toxic or suffer from fading in ultraviolet light, non-iridescent structural colors should demonstrate high spectral selectivity, while being also mechanically stable. However, conventional photonic glass (PhG) shows low color saturation due to the gradual transition in the reflection spectrum and low mechanical stability due to weak interparticle attachment. Here, a PhG with sharp spectral transition in comparison with the conventional full sphere PhG is designed by a conformal coating via atomic layer deposition (ALD) onto an organic PhG template. The ALD deposition allows to control the film thickness precisely for the highly saturated color. This structure can be described by hollow particle motifs with the effective size larger than the interparticle distance. Such unusual PhG is motivated by the achievable features in the spatial Fourier transform of a disordered assembly of such motifs. The surface-templated inverse PhG shows much higher color saturation than the direct PhG from full spheres. Moreover, the dense and solid connected shell will be beneficial for mechanical stability. These results pave the way for highly saturated structural colors. The demonstrated sharp spectral selection feature can be also considered for many related applications such as sunscreens, photovoltaics and radiative cooling by adjusting the reflection transition to the required wavelength. This can be achieved by proportionally scaling the motif and lattice dimensions as well as the film thickness.Publicationtype: Journal Article163