Optische Konstanten von Substraten im NIR/MIR‐Spektralbereich

Determination of the Optical Constants of Common Substrate Materials in the NIR/MIR‐Spectral Regions An approach is reported for determining the optical constants of substrates and single layer coatings in the near and mid‐infrared spectral regions. A combination of the algorithm from Nichelatti and a multi‐oscillator model has been used to determine the optical constants of the substrates. Thus, the approach even works when the sample transmittance is vanishing. We demonstrate the application of the approach to the evaluation of transmission and reflection spectra of common substrates (CaF₂, Q1, Sapphire) measured by a Fourier‐Transform‐Infrared spectrophotometer. The comparison of the calculated optical constants with values reported in literature confirms the validity of the used approach. We also demonstrate the IR characterisation of a tantalum oxide single layer deposited onto a Q1 substrate by means of the multi‐oscillator‐model. The results are again in good agreement with literature data.     

Design und Herstellung selektiver Dünnschichtabsorber auf Silberinselbasis. Design and fabrication of selective multilayer absorber based on silver island films

The combination of a metal island film with a dielectric multilayer represents a novel approach for preparation of spectrally selective absorbers. Metal island films show exceptional optical properties caused by the optical excitation of surface plasmon modes. The plasmon resonance frequency depends on the size and shape of the islands and is influenced by the deposition parameters. The first type of samples represents a silver island film in an ultra thin Al₂O₃ film. We analyzed these samples by means of spectrophotometry. The recorded spectra allow the calculation of the optical constants of the silver island films. These show a maximum absorptance up to 40 %. Finally, we incorporated ultrathin metal‐dielectric‐composite films on a silver/alumina basis into multilayer stacks to design tailored spectrally selective absorber coatings. The stack absorptance comes close to 100 %.     

Röntgenfluoreszenz‐Beschichtungsmessung im Vakuum‐Prozess. X‐Ray Fluorescence Coating Measurements in Vacuum Processes

The effective control of the PVD‐coating process requires to ascertain online information on the current coating thickness and/or layer composition. The energy dispersive X‐Ray fluorescence analysis (EDXRF) applies for such measuring tasks very efficiently. EDXRF allows measurements of coatings with light metals such as Al, Si and Mg as well as of their oxides up to elements such as Cu, Se, In, Ga in photovoltaic coatings or of classic precious metals like Ag and Au. The advanced design of the measuring system allows a continuous measurement of even very thin coatings from approximately 20 nm upwards. The advantages of the continuous measurement with the EDXRF‐measuring system are described by several application examples.     

Transluzente oxidfaserverstärkte Glasmatrix‐Verbundwerkstoffe

Translucent oxide fiber reinforced glasmatrix composites The aim of the work is the developement of transparent glass matrix composites. Therefore besides the mechanical properties also the optical properties of the components have to be adapted. In this study the influence of different fiber coatings (boron nitride, titanium oxide and a boron nitride/titanium oxide double coating) on the mechanical and optical properties of Nextel 440‐fiber reinforced glass was investigated. Micromechanical investigations (push‐in‐tests) and 3‐point‐bending tests have shown the best improvement of the fracture toughness for the binary boron nitride/titanium oxide coating. For single coatings of boron nitride or titanium oxide the transparency was characterized by the transmission spectra.     

A Solution‐Processed Ultrafast Optical Switch Based on a Nanostructured Epsilon‐Near‐Zero Medium

All the optical properties of materials are derived from dielectric function. In spectral region where the dielectric permittivity approaches zero, known as epsilon‐near‐zero (ENZ) region, the propagating light within the material attains a very high phase velocity, and meanwhile the material exhibits strong optical nonlinearity. The interplay between the linear and nonlinear optical response in these materials thus offers unprecedented pathways for all‐optical control and device design. Here the authors demonstrate ultrafast all‐optical modulation based on a typical ENZ material of indium tin oxide (ITO) nanocrystals (NCs), accessed by a wet‐chemistry route. In the ENZ region, the authors find that the optical response in these ITO NCs is associated with a strong nonlinear character, exhibiting sub‐picosecond response time (corresponding to frequencies over 2 THz) and modulation depth up to ≈160%. This large optical nonlinearity benefits from the highly confined geometry in addition to the ENZ enhancement effect of the ITO NCs. Based on these ENZ NCs, the authors successfully demonstrate a fiber optical switch that allows switching of continuous laser wave into femtosecond laser pulses. Combined with facile processibility and tunable optical properties, these solution‐processed ENZ NCs may offer a scalable and printable material solution for dynamic photonic and optoelectronic devices.     

Optical properties of HfO2 thin films deposited by magnetron sputtering: From the visible to the far-infrared

Hafnium oxide (HfO₂ or hafnia) holds promise as a high-index dielectric in optical devices and thermal barrier coatings, because of its transparency over a broad spectrum (from the ultraviolet to the mid-infrared) and chemical and thermal stability at high temperatures. In the present work, thin hafnia films of thicknesses from about 180 to 500 nm are deposited on Si substrates using reactive magnetron sputtering. The crystalline structure and surface topography are characterized by X-ray diffraction and atomic force microscopy, respectively. The optical and radiative properties of the film-substrate composites are measured at room temperature using spectroellipsometry and Fourier-transform infrared spectrometry. The optical constants are obtained from about 0.37 to 500 μm by fitting suitable models to the experimental results. Optical properties and dielectric function modeling are discussed with correlation to both film thickness and surface roughness. It is found that a single-oscillator dielectric-function model can describe radiative properties from about 1 to 20 μm. By combining Cauchy's formula (for the visible and near-infrared regions) with a multiple-oscillator Lorentz model (for the far-infrared region), a dielectric function is obtained for the HfO₂ films that is applicable from the visible to the far-infrared.     

Solid‐State Electrolyte‐Gated Graphene in Optical Modulators

The gate‐tunable wide‐band absorption of graphene makes it suitable for light modulation from terahertz to visible light. The realization of graphene‐based modulators, however, faces challenges connected with graphene's low absorption and the high electric fields necessary to change graphene's optical conductivity. Here, a solid‐state supercapacitor effect with the high‐k dielectric hafnium oxide is demonstrated that allows modulation from the near‐infrared to shorter wavelengths close to the visible spectrum with remarkably low voltages (≈3 V). The electroabsorption modulators are based on a Fabry–Perot‐resonator geometry that allows modulation depths over 30% for free‐space beams.     

Realization of High Performance AR‐coatings with Dust‐ and Water‐Repellent Properties

Hydrophobic coatings enable the manufacture of easy‐to‐clean surfaces having dust‐ and water‐repellent properties. In this work, a hydrophobic coating is deposited as a top layer on an antireflective (AR) multilayer system to produce low reflectance optical surfaces at a normal incident angle in the visible spectrum with dust‐ and water‐repellent properties for applications in precision optics. It is shown that the hydrophobic coating can be considered, from an optical point of view, as two adjacent thin layers having specific thicknesses and densities. In fact, the hydrophobic layer is one monolayer comprising molecular chains with anchoring groups responsible for the chemical bond with the substrate material and functional groups responsible for the water‐ and oil‐repellent properties. Their optical constants are determined and included in the final coating design. High performance AR coatings having an average reflectance of 0.14% at 7° incident angle in the 400‐680nm spectral range together with a pleasing purplered reflex color are produced. Coated lenses exhibit an excellent abrasion resistance, environmental stability, resistance to cleaning agents, homogeneity and water repellence with contact angles against water higher than 110°.     

Tailoring protective coatings for all‐oxide ceramic matrix composites in high temperature‐/high heat flux environments and corrosive media

The application range of porous all‐oxide ceramic matrix composites (CMCs) can be significantly extended through deposition of protective coating systems. Typical applications include protection against erosion, wear and foreign object damage as well as a reduced permeability. Environmental barrier coatings (EBC) are mandatory in order to guarantee sufficient lifetime of the CMC components under high temperature‐, high heat flux conditions and corrosive attack (combustor liners, thermal protection systems for atmospheric reentry). Limited thermal stability of today’s oxide fibers requires additional thermal barrier functionality for EBCs in order to keep the effective CMC bulk temperatures below 1200 °C. Depending on the specific application DLR’s coating concept for all‐oxide CMCs is based on either a single reaction‐bonded aluminium oxide (RBAO) coating or a hybrid coating system consisting of a RBAO bond coat followed by an EB‐PVD YSZ/FSZ top coat and is highlighted for three case studies. Deposition techniques (magnetron sputtering, MOCVD) alternative to EB‐PVD as well as the suitability of fibrous and cellular materials for thick EBC/TBC layers are explored.     

A comparative study of the UV optical and structural properties of SiO2, Al2O3, and HfO2 single layers deposited by reactive evaporation, ion-assisted deposition and plasma ion-assisted deposition

Growing requirements for the optical and environmental stability, as well as the radiation resistance against high-power laser radiation, especially for optical interference coatings used in the ultraviolet spectral range, have to be met by new, optimised, thin-film deposition technologies. For applications in the UV spectral range, the number of useful oxide thin film materials is very limited due to the higher absorption at wavelengths near to the electronic bandgap of the materials. Applying ion-assisted processes offers the ability to grow dense and stable films, but in each case careful optimisation of the deposition process (evaporation rate, substrate temperature, bombarding gas, ion energy and ion current density) has to achieve a balance between densification of the layers and the absorption. High-quality coatings and multilayer interference systems with SiO₂ as the low-index material can be deposited by various physical vapour deposition technologies, including reactive e-beam evaporation, ion-assisted deposition and plasma ion-assisted deposition. In order to improve the degradation stability of dielectric mirrors for use in UV free-electron laser optical cavities, a comparative study of the properties of SiO₂, Al₂O₃ and HfO₂ single layers was performed, and was addressed to grow very dense films with minimum absorption in the spectral range from 200 to 300 nm. The films were deposited by low-loss reactive electron-beam evaporation, by ion-assisted deposition using a ‘Mark II’ ion source, and by plasma ion-assisted deposition using the advanced plasma source. Optical and structural properties of the samples were studied by spectral photometry, infrared spectroscopy, X-ray diffraction and reflectometry, as well as by investigation of the surface morphology. The interaction of UV radiation with photon energy values close to the bandgap was studied. For HfO₂ single layers, laser-induced damage thresholds at 248 nm were determined in the 1-on-1 and 1000-on-1 test modes as a function of the deposition technology and film thickness.     

Hybrid Copper‐Nanowire–Reduced‐Graphene‐Oxide Coatings: A “Green Solution” Toward Highly Transparent,Highly Conductive,and Flexible Electrodes for (Opto)Electronics

This study reports a novel green chemistry approach to assemble copper‐nanowires/reduced‐graphene‐oxide hybrid coatings onto inorganic and organic supports. Such films are robust and combine sheet resistances (<30 Ω sq^?1) and transparencies in the visible region (transmittance > 70%) that are rivalling those of indium–tin oxide. These electrodes are suitable for flexible electronic applications as they show a sheet resistance change of <4% after 10 000 bending cycles at a bending radius of 1.0 cm, when supported on polyethylene terephthalate foils. Significantly, the wet‐chemistry method involves the preparation of dispersions in environmentally friendly solvents and avoids the use of harmful reagents. Such inks are processed at room temperature on a wide variety of surfaces by spray coating. As a proof‐of‐concept, this study demonstrates the successful use of such coatings as electrodes in high‐performance electrochromic devices. The robustness of the electrodes is demonstrated by performing several tens of thousands of cycles of device operation. These unique conducting coatings hold potential for being exploited as transparent electrodes in numerous optoelectronic applications such as solar cells, light‐emitting diodes, and displays.     

Barriereausrüstung von Kunststoffen mittels PECVD

Effect Mechanisms in PECVD Barrier Coating of Plastics Especially optical transparency as well as very high barrier functionality, preventing the transmission of gas and aroma, qualifies silicon‐oxide as material of choice for thin PECVD‐coatings on plastics. Whereas permeation through plastics can be described by the Arrhenius‐model, there is no universal model for transport mechanisms through single or multilayer PECVD‐coatings. Models that do exist describe diffusion through defects. These models presume that no permeation besides transmission through these defects occurs. Topic of this research is to determine which model fits the experimental data of oxygen‐ and water vapour‐transmission of experimental data. In order to investigate the applicability of different models, single‐ and both‐sided are compared using different layer architectures.     

In‐situ Messung dünner Schichten auf Architekturglas

For the deposition of modern coatings on architectural glass (energy‐saving, solar control, anti‐reflective), reactive magnetron sputtering plays an outstanding role. The production of these coatings by reactive sputtering requires a manufactoring equipment, that ensures high quality as well as efficient deposition of the coatings. Thin film measurement by in‐situ ellipsometry can very impressively used for monitoring and controlling the film properties, in particular in the case of more complex coatings. With regard to long‐term stability the in‐situ film measurement is of special importance with respect to the use of the novel mid‐frequency magnetron sputter technique. This technique allows the deposition rate to be increased up to 500 %, however, a dynamical plasma stabilization in the so called »transition mode« is necessary. Within the framework of a project supported by the BMBF, the spectroscopic ellipsometry was implemented on a large scale glass deposition plant (Semco Glasbeschichtung, Neubrandenburg) and was tested during the production. The investigations show that ellipsometry is outstandingly suitable for an accurate determination of the optical layer properties of coatings on architecture glass, even for complex layer systems. Therefore, the basis for an improvement of the efficiency of the plant is given.     

UV‐verstärkte Silberschichten

UV‐enhanced Ag‐coatings with optimized environmental stability For the construction of optical instruments, mirrors with silver (Ag) coatings are of great importance, because Ag has the highest reflectivity of all metals from the visible (VIS) throughout the infrared (IR) spectral range. Investigations performed at IOF show, that the deposition of a closed and dense protective layer is a necessary but not a sufficient condition for the effective protection of silver: hygroscopic particles can harm even silver coatings protected by a previously defect‐free protective layer. In order to improve the protection, a nanolaminate approach for Ag‐protection has been developed. By this approach, both the environmental stability and the optical performance can be optimized.     

Second‐Harmonic Generation from a Single Core/Shell Quantum Dot

Nanoparticles emitting two‐photon luminescence are broadly used as photostable emitters for nonlinear microscopy. Second‐harmonic generation (SHG) as another two‐photon mechanism offers complementary optical properties but the reported sizes of nanoparticles are still large, of a few tens of nanometers. Herein, coherent SHG from single core/shell CdTe/CdS nanocrystals with a diameter of 10 to 15 nm is reported. The nanocrystal excitation spectrum reveals resonances in the nonlinear efficiency with an overall maximum at about 970 nm. Polarization analysis of the second‐harmonic emission confirms the expected zinc blende symmetry, and allows extraction of the three‐dimensional nanocrystal orientation. The small size of these nonlinearly active quantum dots, together with the intrinsic coherence and orientation sensitivity of the SHG process, are well adapted for ultrafast probing of optical near‐fields with high resolution as well as for orientation tracking for bioimaging applications.     

Three‐Layered Hollow Nanospheres Based Coatings with Ultrahigh‐Performance of Energy‐Saving,Antireflection, and Self‐Cleaning for Smart Windows

In this study, a well‐controlled interfacial engineering method for the synthesis of SiO₂/TiO₂/VO₂ three‐layered hollow nanospheres (TLHNs) and TLHNs‐based multifunctional coatings is reported. The as‐prepared coatings allow for an outstanding integration of thermochromism from the outer VO₂(M) layer, photocatalytic self‐cleaning capability from the middle TiO₂(A) layer, and antireflective property from internal SiO₂ HNs. The TLHNs coatings exhibit excellent optical performance with ultrahigh luminous transmittance (T_lum‐l = 74%) and an improved solar modulation ability (ΔT_sol = 12%). To the best knowledge, this integrated optical performance is the highest ever reported for TiO₂/VO₂‐based thermochromic coatings. An ingenious computation model is proposed, which allows the n_eff of nanostructured coatings to be rapidly obtained. The experimental and calculated results reveal that the unique three‐layered structure significantly reduces the refractive index (from 2.25 to 1.33 at 600 nm) and reflectance (Rave, from 22.3 to 5.3%) in the visible region as compared with dense coatings. Infrared thermal imaging characterization and self‐cleaning tests provide valid evidence of SiO₂/TiO₂/VO₂ TLHNs coatings' potential for energy‐saving and self‐cleaning smart windows. The exciting inexpensive and universal fabrication process for well‐defined structures may inspire various developments in processable and multifunctional devices.     

Real time <Emphasis Type="Italic">in situ</Emphasis> spectroscopic ellipsometry of the growth and plasmonic properties of au nanoparticles on SiO<Subscript>2</Subscript>

The evolution of the film thickness and plasmonic properties for sputtered deposited Au nanoparticles on SiO₂ layers have been monitored in real time using in situ spectroscopic ellipsometry in the photon energy range 0.75–4.1 eV. The spectroscopic ellipsometry data were analyzed with an optical model in which the optical constants for the Au nanoparticles were parameterized by B-splines which simultaneously provide an accurate determination of an effective thickness and an effective dielectric function. The effective thickness is interpreted with support of transmission and scanning electron microscopy and Rutherford backscattering measurements. Further parameterization of the optical constants by physical oscillators in the isolated spherical particle region allows the microstructural parameters such as size and Au fraction to be extracted. Real time in situ monitoring allows the growth of nanoparticles from the nucleation phase to near percolation to be followed, and there is a red-shift of the plasmon resonance absorption peak as the nanoparticles increase in size and their interaction becomes stronger.      

Magnetron sputter‐deposition on atom layer scale

For many applications there is an increasing request to control the deposition process on an atom layer scale. This offers a lot of advantages like in accuracy, layer homogeneity and tailoring of layer properties. On the other hand the speed and throughput of the process should not suffer from the control on an atom layer scale as it is the case for classical atom layer deposition (ALD). For optical applications especially high‐end interference filter coatings we developed a plasma assisted reactive magnetron sputtering process in combination with a high speed drive for the substrates. This combination allows controlling the layer thicknesses and layer properties on an atom layer scale while maintaining a high deposition rate. The advantages of this process are demonstrated on single layer results of SiO2, HfO2, ZrO2, Ta2O5 and mixed oxides of SiO2‐Nb2O5. Morphology, surface roughness, film stress, refractive index and losses are controlled by the oxygen partial pressure, the substrate temperature, the energy input by the sputtering ‐and assist process and by cosputtering. The outstanding performance of high‐end interference filter coatings like a multi notch filter for fluorescence microscopy is achieved by the very stable and reproducible deposition process in combination with an advanced thickness control strategy based on in‐situ optical thickness control and time control.     

Micrometer‐Thick Graphene Oxide–Layered Double Hydroxide Nacre‐Inspired Coatings and Their Properties

Robust, functional, and flame retardant coatings are attractive in various fields such as building construction, food packaging, electronics encapsulation, and so on. Here, strong, colorful, and fire‐retardant micrometer‐thick hybrid coatings are reported, which can be constructed via an enhanced layer‐by‐layer assembly of graphene oxide (GO) nanosheets and layered double hydroxide (LDH) nanoplatelets. The fabricated GO–LDH hybrid coatings show uniform nacre‐like layered structures that endow them good mechanic properties with Young's modulus of ≈18 GPa and hardness of ≈0.68 GPa. In addition, the GO–LDH hybrid coatings exhibit nacre‐like iridescence and attractive flame retardancy as well due to their well‐defined 2D microstructures. This kind of nacre‐inspired GO–LDH hybrid thick coatings will be applied in various fields in future due to their high strength and multifunctionalities.