The study focuses on reliable reverse engineering of electron-beam deposited TiO2/SiO2 coatings. It is known that optical constants of evaporated TiO2 films are dependent on deposition conditions and may vary from layer to layer. Also, the nominal optical constants, used during the theoretical designing, may differ from the actual optical constants of coating layers, determined based on characterization of thicker single layers. Typically, post-production characterization of e-beam evaporated coatings is based on spectral photometric or/and ellipsometric data measured ex-situ. The study reports a new reliable algorithm that allows reliable estimation of layer thicknesses and optical constants based on ex-situ measurements. The reliability of the results is verified using a specially produced unique set of samples including single layers identical to the ones included in the multilayer sample. The obtained results, based on the photometric and ellipsometric data, are in correspondence with each other. The algorithm delivers practical results and avoids overfitting.
Thin films of gold (Au) have found use in various optoelectronic applications due to their unique optical properties. Depending on the film morphology, the optical response can display localized surface plasmon resonance related to isolated metal clusters and a Drude-like response emerging from a connected metal network. Therefore, Au films, especially those with nearly percolated morphology display a very broad optical response that can be drastically varied by control of the fabrication conditions or post-deposition treatments. In this study, we investigate the optical and morphological changes observed in thin Au films subjected to thermal annealing as potential building units for optical-based thermal sensors. Three different film morphologies (island film, nearly percolated film, and compact film) are obtained by controlling the amount of deposited metal. The evolution of morphological properties of these three types of films upon thermal annealing follows different mechanisms, resulting in enhanced optical changes in different spectral regions. In addition, we show that the incorporation of nearly percolated films in multilayer interference coatings can significantly boost their potential as irreversible temperature sensors. Overall, we show that the unique morphological changes induced by annealing combined with interference effects hold great promise for thermal sensing.
Aziz Genç, Javier Patarroyo, Jordi Sancho-Parramon, Martial Duchamp, Edgar Gonzalez, Neus Bastus, Lothar Houben, Rafal Dunin-Borkowski, Victor Puntes, Jordi Arbiol
Complex metal nanoparticles offer a great playground for plasmonic nanoengineering, where it is possible to cover plasmon resonances from ultraviolet to near infrared by modifying the morphologies from solid nanocubes to nanoframes, multiwalled hollow nanoboxes or even nanotubes with hybrid (alternating solid and hollow) structures. We experimentally show that structural modifications, i.e. void size and final morphology, are the dominant determinants for the final plasmonic properties, while compositional variations allow us to get a fine tuning. EELS mappings of localized surface plasmon resonances (LSPRs) reveal an enhanced plasmon field inside the voids of hollow AuAg nanostructures along with a more homogeneous distributions of the plasmon fields around the nanostructures. With the present methodology and the appropriate samples we are able to compare the effects of hybridization at the nanoscale in hollow nanostructures.
Boundary element method (BEM) simulations also reveal the effects of structural nanoengineering on plasmonic properties of hollow metal nanostructures. Possibility of tuning the LSPR properties of hollow metal nanostructures in a wide range of energy by modifying the void size/shell thickness is shown by BEM simulations, which reveals that void size is the dominant factor for tuning the LSPRs. As a proof of concept for enhanced plasmonic properties, we show effective label free sensing of bovine serum albumin (BSA) with some of our hollow nanostructures. In addition, the different plasmonic modes observed have also been studied and mapped in 3D.
In the present study, we theoretically investigate how shaping of the light distribution that illuminates a large numerical aperture lens can be used to efficiently couple light to a particular element of optical nano-circuits located at the focal region of the lens. In addition to standard plane waves, we consider illumination schemes that provide peculiar light distributions at focus, such as cylindrical vector beams. These results show that light excitation may be included as an additional degree of freedom in the optimization of optical and metatronic nano-circuitry and their coupling to conventional optical systems.
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.
Dark plasmon modes in metal nanoparticle systems are usually excited by non-optical means. We show that strongly focused illumination can lead to excitation of dark modes. We first use rigorous vectorial diffraction theory to compute the distribution of light at the focus and then numerically calculate the response of single particles and particle dimers. Controlling the distribution of light arriving at the focusing lens by pupil filters enables enhancing the excitation of dark modes. Overall, these results present guidelines for the excitation of dark plasmon modes using standard optical instrumentation.
The force exerted by optical tweezers can be measured by tracking the momentum changes of the trapping beam, a method which is more general and powerful than traditional calibration techniques as it is based on first principles, but which has not been brought to its full potential yet, probably due to practical difficulties when combined with high-NA optical traps, such as the necessity to capture a large fraction of the scattered light. We show that it is possible to measure forces on arbitrary biological objects inside cells without an in situ calibration, using this approach. The instrument can be calibrated by measuring three scaling parameters that are exclusively determined by the design of the system, thus obtaining a conversion factor from volts to piconewtons that is theoretically independent of the physical properties of the sample and its environment. We prove that this factor keeps valid inside cells as it shows good agreement with other calibration methods developed in recent years for viscoelastic media. Finally, we apply the method to measuring the stall forces of kinesin and dynein in living A549 cells.
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 of Ag-SiO2 composites obtained by electron beam evaporation was studied. Optical characterization of the composites was performed by means of spectroscopic ellipsometry measurements. The effective dielectric function of the composites, modeled using a multiple oscillator approach, could be widely tuned by controlling the deposition temperature. The spectral dependence of the composite absorption appeared to be better described with a Gaussian line shape than with the classical Lorentz oscillator model. The description of the effective dielectric function using standard effective medium theories failed and the experimental results could be explained only in the general framework of the Bergman spectral density theory.
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.
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