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The determination of the effective optical constants of metal island films is an essential step towards the practical incorporation of this kind of films in optical coatings. In this work, the optical properties of aluminium island films deposited by electron beam evaporation on quartz substrates are investigated using different approaches employed by three research groups. The effective optical constants of the island films are inferred from optical measurements (spectrophotometry and spectroscopic ellipsometry) using: (i) a parameter-free dispersion model, (ii) a multiple oscillator model based on Gaussian line-shapes and (iii) theβdistributed oscillator model. All the used approaches provide similar physical insights, i.e., an increase in the effective thickness of the metal island film, a red-shift and broadening of the plasmon resonance and an enhancement of the infrared absorption as the amount of deposited material increases. However, the optimal values of the effective optical constants and thickness significantly depend on the employed model and the experimental data used for data fitting.

In plasmon-based devices, the choice of metal for the nanostructures often leads to an undesirable tradeoff between either the superb performance of Ag or the stability of Au. There is a great interest in combining Au and Ag in a way that preserves both their favourable properties, while also exploiting synergistic effects to boost surface-enhanced Raman scattering (SERS) performance beyond the capabilities of a single metal. To address this issue, a large-area nanofabrication procedure based on galvanic replacement was used to create hybrid Au–Ag nanoislands with a diverse morphology and highly tunable (through visible to NIR) plasmon resonance. Their stability was comprehensively studied and shown to be complex to achieve. However, in the right conditions, Au–Ag nanoislands with silver-like plasmonic properties were fabricated, and shown to be stable over a period of at least 5 months. Moreover, their SERS efficiency considerably outperformed even that of Ag nanoislands. Computational investigation helped explain the advantageous properties and provided insights for future sensor design. Detection of the model molecules was achieved in the nanomolar range and in mild conditions (low laser power), indicating great potential for biomedicine sensing. Beyond that, nanoislands with simultaneous stability and good plasmonic properties can find application in a wide variety of photonic devices.

A simple, versatile technique for structuring conductive thin metal films is proposed. Over the last years, glass poling and electric field assisted dissolution of metal nanoparticles and films have been the basis of new approaches for fabrication of micro and nanostructures on glass. Here we demonstrate how the sequential combination of glass poling and electric field assisted dissolution can be used to circumvent the necessity of glass as supporting substrate. In short, a patterned electrode is applied to selectively modify by glass poling an auxiliary glass substrate that is later used to induce indirect electric field assisted dissolution of a metal layer deposited on an arbitrary substrate. The proposed method is verified by microstructuring conductive thin metal layers (Ag and Cu) deposited on silicon wafers. This technique enables transferring the pattern from a primary electrode onto conductive thin metal film using inexpensive equipment and the process duration is independent on the pattern area. The primary electrode can be used many times to fabricate selectively modified auxiliary glass substrates and the only limitation on the substrate supporting the final metal microstructure is to be able to sustain moderately elevated temperature and electric field for couple of hours. This method turns out to reproduce the original pattern better than other approaches based on glass poling. Overall, the proposed method significantly expands the application range of glass poling and electric field-assisted dissolution as competitive structuring techniques.

A post-production characterization approach based on spectral photometric and ellipsometric data related to a specially developed set of samples is proposed. Reliability of the approach is demonstrated.

The process of glass poling and electric field assisted dissolution of metal films on coated glass substrates is investigated. The samples consist of soda lime glass substrates coated with thin dielectric layers of different materials (SiO2, Al2O3, TiO2) and an Ag film. The samples are subjected to the application of dc voltage at 200 °C. As a result, the metal film is dissolved and partially incorporated in the dielectric layer and glass substrate. It is shown that during the pre-heating process prior to the voltage application ions from the metal film and alkali ions from the glass substrate incorporate into the dielectric layer. The field-assisted dissolution, characterized by glass poling and drift of metal film ions into the dielectric layer and glass substrate, appears to strongly depend on the dielectric layer material and its ion incorporation during pre-heating. The process dynamics is corroborated by numerical simulations of ion drift and diffusion and compared to electrical current measurements. Overall, it turns out that the optical properties of the metal-doped dielectric layers can be varied in a complex manner by the described process, broadening the potential of glass poling and electric field assisted dissolution in photonics applications.

Metal nanoparticles are used in optical coatings and sensors due to their absorption in optical part of spectrum and its sensitivity to the environment induced by localized surface plasmon resonance. Glass is the most common substrate used for optical coatings. However, its surface does not have optimal properties for coating with metal nanoparticles grown in situ by reduction of metal salt. Glass surface optimization methods may involve environmentally hostile chemicals or processes that have time limited or atmosphere sensitive effects. In this study it is demonstrated and discussed effectiveness, mechanisms and advantages of glass poling as pre-treatment method for improving glass surface properties for maximization of coatings plasmonic performance. Pre-treatment of glass surfaces by poling is highly efficient for the purpose. Glass poling quenches ion exchange between metal ions from the solution and alkali ions from glass, favouring nanoparticles formation. Surface prepared in such way is not affected by ageing in normal atmosphere and is effective even after coating with ultrathin dielectric or Cr layers., This work was supported by the Croatian Science Foundation [Grant no. IP-2016-06-2168 and DOK-2018-01-3956]. M.C.S. and J.A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327. ICN2 is supported by the Severo Ochoa program from Spanish MINECO [Grant no. SEV-2017-0706] and is funded by the CERCA Programme/Generalitat de Catalunya. M.C.S. has received funding from the post doctoral fellowship Juan de la Cierva Incorporation from MICINN (JCI-2019) and the Severo Ochoa programme.

Different approaches for tailoring the effective optical constants of complex island films in the near-infrared range are presented. Nearly-percolated films with finely tuned optical properties may be used as highly-lossy materials in novel optical coatings.

Hollow nanoparticles show enhanced plasmonic response with respect to their solid counterparts. Formation of hollow nanostructures is usually carried out in nanoparticle solutions via galvanic replacement and Kirkendall effect driven by the redox potential difference between metals of a redox couple. In this work, the formation of hollow nanoparticle layers using metal island films fabricated by physical vapor deposition as templates is demonstrated. Ag metal island films on glass and silicon substrates are titrated with a solution containing tetrachloroauric acid as oxidizing agent. The islands morphology and their plasmonic properties gradually change with the increase of titration solution concentration due to the effects of galvanic replacement. In comparison to untreated samples, a four-fold enhancement of the refractive index sensitivity factor (360 nm/RIU vs 92 nm/RIU) and two and a half times increase of the figure of merit (1.37 vs 0.56) are observed for hollow island films obtained with large titration concentration. The presented approach merges the advantages of large area coverage and high particle density of island films with the plasmonic properties of hollow nanoparticles in a straightforward way that is suitable for cost-effective fabrication of plasmonic sensors.

The incorporation of metal ions in dielectric layers by means of electric field assisted film dissolution is investigated. The samples consist of alkali-containing glass substrates coated first with SiO 2 and then Ag thin films. The application of moderately elevated temperatures and DC voltages induces thermal poling in the glass matrix and metal film dissolution, resulting in the incorporation of metal ions in both dielectric layer and glass matrix. First, the process dynamics are simulated by modelling the migration of metal film ions and alkali species under an applied electric field. Numerical solution of the model indicates that metal ions progressively dope the dielectric layer until they reach the glass matrix. Then the dopant distribution in the layer becomes steady-state and further injection of ions contributes to increase the dopant concentration in glass. The influence on the process of alkali and dopant ion mobilities and alkali ion concentration is analysed. Additionally, Ag doping of SiO 2 layers deposited on soda-lime and borosilicate glasses is experimentally carried out and characterized using spectroscopic ellipsometry. The evolution of refractive index profiles through both, SiO 2 layer and glass substrate, is correlated with ion migration and confirms the model trends. Overall, this study shows that glass poling and film dissolution can be used to control metal doping of dielectric layers, with potential application in optical and photonic devices.

The plasmonic response of Cu-Ag metal islands films is investigated. Films are obtained by subsequent electron beam deposition of Ag and Cu using different fabrication conditions: deposited mass thickness, substrate temperature and post-deposition annealing in vacuum. Optical properties of films are investigated by spectroscopic ellipsometry and correlated with the structural characterization results obtained by electron microscopy. It is observed that Ag enhances island growth and increases the percolation threshold of Cu films. The localized surface plasmon resonance of isolated particles shows signatures of both Cu and Ag. Moderate thermal annealing enhances island growth and favours Janus-like morphology, increasing the Ag contribution to the surface plasmon resonance. In case of percolated films, annealing-induced dewetting can lead to the appearance of large and irregular particles with a remarkable absorption peak in the near-infrared range. Composition and optical properties of the films can be further modified by Ag partial evaporation upon annealing at high temperatures. The variation of optical properties with aging is related to Cu oxidization and follows different trends depending on the sample morphology. Overall, it is shown that Cu-Ag island films are compelling systems for plasmonic applications, as their optical response can be widely and easily tuned by adjusting fabrication conditions., The authors thank the financial support of the Croatian Science Foundation through the Grant No. IP-2019-04-5424. M. C. Spadaro and J. Arbiol acknowledge funding from Generalitat de Catalunya 2017 SGR 327. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme/ Generalitat de Catalunya. M.C.S. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 754510 (PROBIST) and the Severo Ochoa programme. Authors acknowledge F.J. Belarre for the TEM sample preparation.

The optical properties of Cu island films, in particular their plasmonic behaviour, are investigated. The films are fabricated by electron beam evaporation using different deposition parameters (deposited mass thickness and substrate temperature). The optical response can be tuned from a metal-like behaviour to well-defined localized surface plasmon resonances and is well correlated with the structural and morphological properties of the samples. Significant changes of optical properties take place with sample aging, including quenching and non-monotonic frequency shift of the plasmon resonance. These changes are interpreted in terms of Cu oxidization process, that appears to strongly depend on the initial morphology of samples. Theoretical calculations in the framework of effective medium theories qualitatively explain the experimental observations. Overall, the results give a detailed insight on the morphology dependence and time evolution of the optical response of Cu island films that may provide useful guidelines for applying these nanostructures in plasmonic applications.

Metal–dielectric phase-shifting multilayer optical elements have been developed, providing broadband, virtually dispersion-free polarization manipulation down to the few-cycle level. These optical elements are A g / A l 2 O 3 mirrors that operate in the spectral range from 500 to 100 nm, exhibiting reflectance higher than 95%, and a differential phase shift between the s- and p-polarization of about 90° distributed over four bounces. The mirrors have been designed, produced, and reliably characterized based on spectral photometric and ellipsometric data using a non-parametric approach as well as a multi-oscillator model. The optical elements were implemented into a few-cycle laser system, where they transformed linearly polarized few-cycle light pulses to circular polarization.

Application of electric field and moderately elevated temperature depletes the side facing anode from alkali present in glasses. The change of composition of the treated glass results in variation of refractive index depth profile within the treated glass. Spectroscopic ellipsometry is employed for characterization of optical properties of glass treated in different conditions. The results of optical characterization are verified by secondary ion mass spectroscopy. It is found that the refractive index profile obtained from ellipsometry has a maximum value higher than the one of untreated glass. The obtained refractive index profiles are in very good agreement with concentration profiles.

The silver (Ag) films were deposited on the monodispersed polystyrene spheres that were drop-coated on hydrophilic glass substrates in order to form a self-assembled 2D monolayer. Thus prepared Ag films over polystyrene nanospheres (AgFONs) were used to record the surface-enhanced Raman scattering (SERS) spectra of rhodamine 6G (R6G) and pyridine (λex = 514.5 nm). AgFONs were prepared by depositing 120, 180 and 240 nm thick Ag layer on the 1000 nm polystyrene spheres and 80, 120, 160 and 200 nm thick Ag layer on the 350 nm spheres as well as on their mixture (350 + 1000 nm). The silver was deposited by electron beam evaporation technique. The best enhancement of the Raman signal for both test molecules was obtained using 180 nm Ag film deposited on the 1000 nm spheres and using 80 nm Ag film deposited on the 350 nm polystyrene spheres. The lowest detectable concentrations of R6G and pyridine were 10−9 mol L−1 and 1.2 × 10−3 mol L−1, respectively. This study has shown that AgFONs could be regarded as good and reproducible SERS substrate for analytical detection of various organic molecules.

The evolution of electric field assisted dissolution (EFAD) of nanoparticles process is studied by means of spectroscopic ellipsometry. Multilayer structures with embedded Ag islands films are prepared by electron beam evaporation and subjected to EFAD process using different voltages and treatment times. Modelling of ellipsometric data evidences the dissolution process by the progressive quenching of the localized surface plasmon resonance of nanoparticles. Further quantitative information is revealed by the analysis of the effective dielectric function of Ag islands films in terms of the spectral density function. The amount of dissolved metal appears to follow a non-linear dependence with the applied voltage. Ag islands become elongated in the film plane as dissolution takes place. In the case of multiple Ag islands films, the process occurs in a sequential way, starting from the film closest to the glass substrate. Accumulation of Ag in a near-surface glass region is revealed by the presence of a refractive index gradient. Overall, the study demonstrates the usefulness of ellipsometry for providing novel insights into the EFAD process.

The strong dispersion in the effective optical constants of plasmonic nanocomposite films is used to generate interference effects that are highly sensitive to changes in the dielectric environment of particles. Numerical simulations show that reflectance measurements on metal nanoparticle systems close to a metallic substrate have larger sensitivities to changes in the surrounding refractive index than the standard transmittance measurements of the same system when placed on a transparent substrate. The scheme is particularly advantageous in case of disordered nanoparticle systems, where the difference between reflectance and transmittance‐based sensing approaches is enhanced due to local‐field fluctuations that modify the effective optical constants dispersion. Exceptional to most plasmonic applications, in the present case, a disordered system provides larger sensitivity and figure of merit than its ordered counterpart. The key concepts suggested from numerical calculations are verified by the fabrication of metal island films coated with different dielectric layers, that confirm the superiority of the reflectance‐based sensing scheme. Overall, the present approach profits from the disorder to improve refractive index sensitivity in metal nanoparticle systems that are produced with industrially appealing techniques. Thus, the proposed scheme may be valuable for the broad implementation of low‐cost and highly efficient plasmonic sensors.

Electric field assisted dissolution is a process that enables micro- and nano- structuring of metal nanocomposite glasses, metal island films and compact metal layers on glass substrates, using simple equipment and technical setup. During the process, metal ions drift into a glass matrix under the influence of moderately elevated temperature and the applied electric field. The surface quality of nanostructured metal layers has great importance in many applications as it influences the device performance. In this work, a study of the surface morphology evolution during the dissolution of a compact metal layer is presented. It is found that in the course of gradual change from a compact to a completely dissolved layer, drift of metal ion s from glass/layer interface is followed by the simultaneous deterioration of the surface quality already from the very beginning of the process. The surface morphology evolution could be described by the occurrence of surface roughness of the metal layer and continuous thinning of the layer. Understanding how this process takes place is important for improvement of electric field assisted dissolution application in structuring metal layers for optoelectronic devices.

Glass poling changes composition of the treated glass depleting the side facing anode from alkali species naturally present in glasses. The resulting change of the treated glass refractive index profiles was studied using ellipsometry.

Application of DC voltage and moderate temperature induces dissolution of metal nanoparticles in nano-composite materials. In this work spectroscopic ellipsometry is used to monitor changes in multilayered nano-composite coatings as the dissolution process takes place.

A technique for patterning compact thin metal layers is presented. The technique is based on a successive application of well known, cost-effective, and simple methods of glass poling and electric field assisted dissolution. A microstructure from the patterned anode was copied onto a thin silver layer on a glass substrate. It is demonstrated that electric field assisted dissolution can be used for selective dissolution of compact metal layers. Nanostructuring of compact metal layers is interesting as it can be used in the production of metamaterials, metasurfaces, and optical circuits. The proposed two-step technique does not involve expensive equipment, and the duration of the structuring process is independent of the size of the required structured surface.

Thin-films of vertically aligned titanium dioxide nanotubes are key constituents of charge transfer layers in 3rd generation photovoltaics. The beneficial charge transport primarily occurs due to the favourable microstructural features, i.e. the large effective surface-to-volume ratio. Here measuring methods are selected which are sensitive to microstructural features: Simultaneous Grazing Incidence Small and Wide Angle X-Ray Scattering (GISWAXS) and Scanning Electron Microscopy (SEM). Further, a model is developed for the simulation and explanation of the GISWAXS experimental data. Titanium nanotube arrays were investigated, which were previously successfully produced by electrochemical anodization of titanium thin film evaporated onto a zinc-oxide coated glass substrate. The developed model was shown to be appropriate to describe the obtained samples, which consist of arrays of single nanotubes with a diameter of 40- 80 nm, and a titanium dioxide layer with a porosity of 30 – 50%. Voltage applied during the anodization can be used to tailor the average nanotube diameter. Within this work, we presented simultaneous GISWAXS as a favourable tool for the fast and successful study of the average nanotubes’ diameters in nanotubes arrays.

Alternative materials that can potentially replace Au and Ag in plasmonics and broaden its application potential have been actively investigated over the last decade. Cu and Al have been usually overlooked as plasmonic material candidates because they are prone to oxidisation. In this work the plasmonic performance of Cu and Al is investigated using numerical simulations of different nanostructures (spheres, cubes, rods and particle dimers) and taking into account the presence of oxidisation. It is shown that geometry can play a dominant role over material properties and the performance of Cu and Al becomes comparable to that of Ag and Au for systems of non-spherical particles and strong electromagnetic coupling among particles. This observation is experimentally confirmed by the fabrication and characterisation of Cu and Al metal island films. Optical characterisation of the samples reveals a comparable performance of these metals to that obtained for Ag and Au and suggests that Cu and Al metal island films can offer an efficient low-cost platform for solar energy harvesting, as shown in water vapour generation proof of concept experiments.

Bimetallic islands films consisting of composite Au-Ag nanoparticles are deposited on glass substrates by electron beam evaporation. Broad tuning of the surface-plasmon resonance (SPR) characteristics can be achieved by controlling film composition, deposition temperature and post-deposition thermal annealing. Optical and structural characterization of the samples enable one to establish the link between the SPR and the morphological and compositional characteristics of nanoparticles.

The formation of three dimensional (3D) photonic microstructures by the locally selective dissolution of metal clusters embedded in dielectric multilayer stacks is presented. Dissolution of clusters is performed by the simultaneous application of electric field and temperature. The produced photonic structures show a highly tailorable optical behavior that combines the interferential effects of multilayer stacks and the surface plasmon resonance of non-dissolved metal clusters. Due to its feasibility and the possibility to widely modify the optical properties of the resulting structures, the current approach represents a promising method for the production of novel components based on 3D-metallodielectric photonic structures.

The coatings having refractive index changing with the thickness present interesting optical performance, improved mechanical properties and smaller light scattering in comparison with classical multilayer stacks. Lot of theoretical work and experimental advances have been done for designing and production of mixture layers with such particular performances. The effective refractive index of the mixture coatings can be calculated by the use of effective medium theories. The refractive index profile characterization of inhomogeneous films that are mixtures of SiO 2 and Nb 2 O 5 is presented. The composition is linearly changed through the thickness of the layers. Ex-situ spectrophotometric measurements, i.e. reflectance and transmittance at different incidence angles, are used for the precise characterization of the refractive index profiles. Linear, Maxwell-Garnet, Bruggeman and Lorentz-Lorenz effective medium theories are applied and quality and differences of the results are studied and analyzed. It is shown that the Lorentz-Lorenz model is the most appropriate for the given mixture, suggesting components are well mixed and there are no separated phases.

In this work is studied the influence of annealing on metal island and compact Au films formed on the substrates with and without Ge seed layer. Samples with different thicknesses of Au were annealed at different temperatures. Optical characterization of the samples was performed based on ellipsometric measurements. The obtained results demonstrate that, even in the conditions of moderately elevated temperature, Ge seed layer promotes the percolation of metal islands.

Metal island films (MIF) are good candidates for sensors due to the strong sensitivity of the localised surface plasmon resonance to the environment refractive index. The strong near field enhancement in the vicinity of the island surface can be even higher if a metal layer (ML) is placed close to a MIF. Structures containing MIF with and without ML are prepared and sensitivities of spectrophotometric and ellipsometric features of the measurements are compared. It is shown that simple MIF is preferable for ellipsometry-based sensing and the one including ML in the case of spectrophotometric measurements.

We report on the structure and arrangement of particles created in the Fe 2 O 3 /TiO 2 + SiO 2 multilayers. X-ray diffraction and extended X-ray absorption fine structure spectroscopy reveal the presence of crystalline rutile-TiO 2 while the iron oxide remains either amorphous or forms very small clusters of Fe 2 O 3 . The Fe 3 + oxidation state of iron atoms has been confirmed by Mossbauer and X-ray spectroscopy. The degree of the particle ordering has been studied by grazing-incidence small-angle X-ray scattering. It was demonstrated that with increasing temperature partially ordered nanoparticles are created and grow up to a critical temperature when the ordering is destroyed. Both particle sizes and inter-particle distances depend strongly on the thickness of the Ti/Fe containing layer.

We explain reasons of oscillations frequently observed in total losses spectra (1 − R − T) calculated on the basis of measurement spectral photometric data of thin film samples. The first reason of oscillations is related to difference in angles of incidence at which spectral transmittance and reflectance are measured. The second reason is an absorption in a thin film. The third reason is a slight thickness non-uniformity of the film. We observe a good agreement between theoretical models and corresponding measurements, which proves above statements on the origins of oscillations in total losses.

Optical properties of metal island films (MIFs) can be combined with interference of dielectric coatings. A set of multilayer designs containing metal clusters reflecting different colours from front and back side of the coating was obtained by numerical optimization. The chosen designs presenting the range of feasible colours were deposited by electron beam evaporation. Spectrophotometric and ellipsometric measurements verified that the produced coatings present an excellent agreement with the optical performance calculated from the designs. Numerical optimization was verified as a useful method in designing of coatings containing MIFs. This approach can ease the implementation of metal clusters into multilayer designs and broaden the applications of MIFs.

Interface traps at the Si–SiO2 interface have been and will be an important performance limit in many (future) semiconductor devices. In this paper, we present a study of fast neutron radiation induced changes in the density of Si–SiO2 interface-related defects. Interface related defects (Pb centers) are detected before and upon the irradiation. The density of interface-related defects is increasing with the fast neutron fluence.

The potential of random metal-dielectric nanocomposites as constituent elements of metamaterial structures is explored. Classical effective medium theories indicate that these composites can provide a tunable negative dielectric function with small absorption losses. However, the tuning potential of real random composites is significantly lower than the one predicted by classical theories, due to the underestimation of the spectral range where topological resonances take place. This result suggests that a random mixture consisting of a metal matrix with embedded isolated dielectric inclusions is a promising design guideline for the fabrication of tunable composites for metamaterial purposes.

Thin metal island films exhibit unique optical properties and possess a high potential in design and fabrication of multilayer coatings with sophisticated spectral performances over wide wavelength and angular ranges. Optical properties of these films are dependent on film thickness. In the present study we consider and solve a problem of designing multilayers which reflect different colors from their front and back sides and have specified average transmittance values. Additionally, in many cases the reflected colors are stable to variations of the incidence angle. In the design process we use optical constants of Ag metal island films, that were carefully determined based on recently proposed characterization approach.

The influence of the substrate temperature on the effective optical behavior ofAg-SiO 2 composites obtained by electron beam evaporation was studied. Optical characteriza-tion of the composites was performed by means of spectroscopic ellipsometry measurements.Theeffectivedielectricfunctionofthecomposites,modeledusingamultipleoscillatorapproach,could be widely tuned by controlling the deposition temperature. The spectral dependence ofthe composite absorption appeared to be better described with a Gaussian line shape than withthe classical Lorentz oscillator model. The description of the effective dielectric function usingstandard effective medium theories failed and the experimental results could be explained onlyin the general framework of the Bergman spectral density theory. C 2011 Society of Photo-OpticalInstrumentation Engineers (SPIE) . [DOI: 10.1117/1.3590238] Keywords: metal-dielectric composites; metal island films; surface plasmon resonance; opticalconstants; effective medium theory; spectroscopic ellipsometry.Paper 11015SSR received Jan. 28, 2011; revised manuscript received Apr. 15, 2011; acceptedfor publication Apr. 15, 2011; published online May 13, 2011.

A three-dimensional (3D) photonic microstructure consisting of metal clusters embedded in dielectric matrix is coated with a dielectric mirror. The produced photonic structure shows optical behaviour that combines the interferential effects of the multilayer stack and the surface plasmon resonance of metal clusters. Due to its feasibility and the possibility to widely modify the optical properties of the resulting interferential-plasmonic structure, this approach represents a promising method for the production of novel optical components.

Optical constants of thin metal films are strongly dependent on deposition conditions, growth mode, and thickness. We propose a universal characterization approach that allows reliable determination of thin metal film optical constants as functions of wavelength and thickness. We apply this approach to determination of refractive index dispersion of silver island films embedded between silica layers.

Metal island films show a characteristic absorption peak related to the surface plasmon resonance of free electrons. This kind of film can be used in absorbing coatings, together with dielectric layers. Such absorbing multilayer coatings, with and without the gradient of the silver mass thickness in metal island films throughout the coating, have been deposited by electron beam evaporation. It is shown experimentally that coatings with a gradient in the mass thickness of silver nanoparticles have higher absorption than equivalent nongradient coatings with the same total mass thickness of silver nanoparticles.

A two-step procedure for the formation of silver nanoparticles embedded in a glass matrix is studied. The procedure consists of: (i) the inclusion of silver ions in the glass matrix by electric field assisted dissolution of Ag film deposited on the glass and (ii) the aggregation of silver by thermal annealing. The optical properties of the sample, dominated by the surface plasmon resonance of metal nanoparticles in the visible spectral range, are studied by optical spectroscopy. The structural characterization is carried out by grazing-incidence small-angle X-ray scattering measurements performed at the Synchrotron Elettra (Italy) and silver depth profiles are determined using Rutherford backscattering. The results suggest that the depth profile of Ag might be tailored by modification of the parameters of metal film dissolution (electric field and temperature). Variation of thermal annealing parameters (temperature and time) allows control of the nanoparticles size. Thus, the surface plasmon absorption intensity and line shape are changed, enabling tuning of the optical properties of the sample.

We describe the optical properties of gold island films embedded between SiO2and TiO2layers. Plasmonic properties of gold films have been characterized for various combinations of embedded media and used in design of reflectors.

Metal island films show unique optical properties owing to the local surface plasmon resonance of islands free electrons. In the present study, the electric field assisted dissolution of clusters in metal island films is reported. Island films of Au, Ag, and Cu are deposited under different conditions by thermal evaporation and coated with thin dielectric layers. The samples are treated with the simultaneous application of an intense electric field and temperature, leading to the sample partial or total bleaching due to the dissolution of metal clusters in the films. Owing the facility of production of metal island films and the inexpensive technical requirements of the dissolution process, this approach suggests a novel path for the production of low-cost photonic structures.

A class of problem dealing with narrow-band reflectors under oblique light incidence has been selected, and a group of design procedures leading to satisfactory results is analyzed. Initially performance criteria and design restrictions are set. Various designs are then created and analyzed. Conclusions can then be drawn on the relative merits of the designs. Two different types of target function have been chosen. A global search for 50 or more layers with varied optical thicknesses between 0 and lambda/2 and a needle design method have been applied to the synthesis. Gradient and variable metrics have been used for further refinement. Results are compared, and it is shown that all the design methods used yield similar results for this problem. The issue of how to determine the best method is addressed.

The dissolution of metal clusters in metal island films by the simultaneous application of electric field and temperature is reported. The consequent fading of surface plasmon resonance greatly modifies the optical properties of the samples. The dissolution process is verified in island films of different metals, obtained under different conditions and covered by different dielectric materials, as well as on multilayer dielectric stacks showing interferential properties. The tailoring possibilities of the optical behavior of metal island films combined with the inexpensive technical requirements of this approach open up the possibility to produce low-cost photonic heterostructures.

The absorption coefficient of composite films consisting of niobia (Nb2O5) and silica (SiO2) mixtures is studied for photon energies around the band gap. The films were deposited by co-evaporation and their composition was varied by changing the ratio of deposition rates of the two materials. Both, as-deposited and thermally annealed films were characterized by different techniques: the absorption coefficient was determined by spectrophotometric measurements and the structural properties were investigated using infrared spectroscopy, transmission electron microscopy and X-ray diffraction. The correlation between the variations of absorption properties and film composition and structure is established. The absorption coefficients determined experimentally are compared with the results derived from effective medium theories in order to evaluate the suitability of these theories for the studied composites.

A hybrid antireflective coating combining homogeneous layers and linear gradient refractive index layers has been deposited using different techniques. The samples were analysed by optical characterisation based on spectrophotometric and spectroscopic ellipsometric measurements under different angles of incidence in order to precisely characterise the coatings. The Lorentz-Lorenz model has been used for calculation of refractive index of material mixtures in gradient index layers as well as some homogenous parts of the coating. The obtained refractive index profiles have been compared with the targeted ones to detect errors in processes of deposition.

Gradient index coatings and optical filters are a challenge for fabrication. In a round-robin experiment, basically the same hybrid antireflection coating for the visible spectral region, combining homogeneous refractive index layers of pure materials and linear gradient refractive index layers of material mixtures, has been deposited. The experiment involved three different deposition techniques: electron-beam evaporation, ion-beam sputtering, and radio frequency magnetron sputtering. The material combinations used by these techniques were Nb(2)O(5)/SiO(2), TiO(2)/SiO(2), and Ta(2)O(5)/SiO(2), respectively. The spectral performances of samples coated on one side and on both sides have been compared to the corresponding theoretical spectra of the designed profile. Also, the reproducibility of results for each process is verified. Finally, it is shown that ion-beam sputtering gave the best results in terms of deviation from the theoretical performance and reproducibility.

The deposition of graded-index layers and rugate structures was performed by coevaporation of silicon dioxide as the low index material and niobium pentoxide as the high index material. To obtain information about the composition depth profile of the films, we used cross-sectional transmission electron microscopy to supplement deposition rate data recorded by two indipendent crystal quartz monitors during film preparation. The concentration depth profile was transformed to a refractive index profile using the effective medium approximation. The thus obtained refractive index profiles turned out to represent efficient initial approximations for re-engineering purposes.

Rugate structures, as well as gradient refractive index films in general, attract a lot of interest. The gradient index systems may provide advantages in both, optical performance and mechanical properties of the optical coatings. Rugates have shown to be especially interesting for design of notch filters. A lot of theoretical work on design of rugate filters has been done in the last decades. However, only few of the designs could be deposited, which is often caused by practical problems, e.g. preparing materials with the desired refractive index values. In this paper two different gradient refractive index designs are compared to a classical high-low stack. One gradient design is synthesized by an apodized sinusoidal structure that is approximated by homogeneous sublayers. The other one is based on an apodized sinusoidal structure as well, but it is approximated by a hybrid structure, i.e. a combination of linear gradient index ramps between the lowest and the highest refractive index applicable and homogeneous layers of high index values. The two gradient designs take into account the constraints posed by limitations of the real deposition systems. Both designs are compared to a classical high-low stack and the advantages and drawbacks of each approach are commented.

We present a new design for an omnidirectional antireflection coating for the visible spectral range. In contrast to classical designs, it combines homogeneous layers and linear gradient index layers into one hybrid design with a full thickness of approximately 500 nm. The coating may be practically produced based on silicon dioxide as low index material and niobium pentoxide as high index material, while intermediate indices may be obtained from corresponding mixtures.