Örs Sepsi

Use of the optical admittance function and its WKB approximation to simulate and evaluate transmittance spectra of graded-index colloidal films

A method is presented for the simulation of transmittance spectra of graded-index dielectric layers deposited on a transparent substrate of finite thickness, and the simulation process is applied to films of silica and zinc oxide nanoparticles. The method is based on the optical admittance by solving its differential equation for an appropriate refractive index profile. When the refractive index changes with depth z according to the function N(z) = N(0)/(1+z/ζ)2, an exact solution exists which is equivalent to the WKB (geometric optical) approximate solution derived for the electromagnetic field in weakly inhomogeneous media. Using this solution allows us to characterize transparent graded-index films by two optical parameters—the mean refractive index and the grade of inhomogeneity, in addition to the film thickness through a fast curve fitting process.

Complex Langmuir−Blodgett Films of SiO2 and ZnO Nanoparticles with Advantageous Optical and Photocatalytical Properties

Multifunctional Langmuir-Blodgett (LB) films were fabricated on the surface of glass substrates using sol-gel derived ZnO and SiO2 particles. ZnO particles of 6 and 110 nm diameter were synthesized according to the methods of Meulenkamp and Seelig et al. (Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566; Seelig, E. W.; Tang, B.; Yamilov, A.; Cao, H.; Chang, R. P. H. Mater. Chem. Phys. 2003, 80, 257). Silica particles of 37 and 96 nm were prepared by the Stober method (Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62). Alternate deposition of monoparticulate Langmuir films of SiO2 and ZnO nanoparticles provided complex (six- and nine-layered) LB films with both antireflective and photocatalytic properties. The LB films were investigated with scanning electron microscopy (morphology and structure) and UV-vis spectroscopy (optical properties and stability). The photocatalytic activity was measured by immersing the UV-irradiated films into an aqueous solution of Methyl Orange and following the photodegradation of the dye by optical spectroscopy. Adding ZnO particles to the silica films slightly lowered the antireflection property but ensured strong photocatalytic activity. Both the photocatalytic activity and antireflection properties were proved to be sensitive to the sequence of the silica and ZnO layers, with optimum properties in the case of nine-layered films with a repeated (SiO2-ZnO-ZnO) structure.

Investigation of polarized light emitting diodes with integrated wire grid polarizer

The polarization properties of light emitting diodes with integrated wire grid polarizers are investigated. Rigorous coupled wave analysis and Monte-Carlo ray tracing are used for modeling the gratings and the entire LED structure respectively. We show that it is more advantageous to place the polarizer onto the LED encapsulation rather than onto the die. With the proposed arrangement the average extinction ratio is 2.37 in the uncollimated case and 76.86 in the collimated case, while the light extraction efficiency is significantly higher than that of the LED + external polarizer combination. The achieved results compare favorably to other polarized LED solutions proposed in the literature.

Polarized light emitting diodes using silver nanoellipsoids

We investigate the polarizing properties of periodic array of silver nanoellipsoids placed on top of a planar LED structure. The response of the particles is calculated with the periodic layered Greens tensor in the electrostatic limit with dynamic depolarization and radiation damping corrections. We investigate the degree of polarization and the total extracted power spectra depending on parameters like lattice period, axial ratio and particle size. The proposed model is applicable over a wide range of parameters and appropriate to optimize the given structure. The optimization procedure shows that particles in the size range of 100 nm are optimal to reach 50% degree of polarization and less than 15% absorbance for an uncollimated and unpolarized dipole source.

Preparation and characterization of two-dimensional metallic nanoparticle and void films derived from a colloidal template layer.

A novel and simple bottom-up fabrication method for the realization of metallic nanovoid and metallic film on nanoparticle (dome) array is presented and their optical performance assessed based on experimental and theoretical investigations. The structures are created by a simple, annealing induced replica formation of a template monolayer, which is composed of submicron particles deposited on top of a thin polymer film. Angle and wavelength dependent reflection measurements indicate the possibility to excite Bragg plasmons at the prepared structures. We found an excellent agreement between the measured and simulated reflection curves, but only when the simulated reflection was averaged over several possible azimuthal lattice orientations of the hexagonal unit cell with respect to the plane of incidence.

Modeling the optical properties of nanocomposite media using effective transfer matrices

In this study we suggest an effective matrix method (EMM) for the optical modeling of nanocomposite media. We show that an effective transfer matrix of a nanocomposite medium, comprising an assumed periodic arrangement of nanoparticles embedded in a surrounding matrix, can be extracted from a rigorous finite element simulation of the structure. The effective matrix of the nanocomposite can then be used in a standard transfer matrix calculation to forward-calculate the optical spectra of arbitrary stratified structures that contain the nanocomposite. The computational complexity of this approach is significantly less than a rigorous electromagnetic simulation of such arbitrary stratified structures, while its accuracy is practically the same. We compare this EMM to various effective medium approximations based on analytical formulas and numerical retrieval techniques. We show that the proposed EMM can be successfully applied to certain nanocomposites that cannot be described with an effective refractive index.