Tz. Babeva

Optical properties of photopolymerizable nanocomposites containing nanosized molecular sieves

Acrylamide-based photopolymerizable nanocomposites containing three types of nanosized crystals with controlled microporosity, Silicalite-1 (MFI-structure), AlPO-18 (AEI-structure) and Beta (BEA-structure) are studied. The influence of the porous nanoparticles on the average refractive index, optical scattering and holographic recording properties of the nanocomposite are characterized. The redistribution of nanoparticles as a result of the holographic recording in the layers is investigated by Raman spectroscopy. It is observed that in all three nanocomposites the nanoparticles are redistributed according to the illuminating light pattern. This redistribution improves the refractive index modulation only in the case of the MFI nanoparticles, while no improvement is observed in AEI and BEA doped layers. The results can be explained by the hydrophobic/hydrophilic nature of the nanoparticles and their interactions, or absence of interactions, with the host photopolymer.

Multilayer As2Se3/GeS2 quarter wave structures for photonic applications

The optical properties of single layers from As2Se3 and GeS2, double-layered stack and quarter-wave multilayer structure consisting of alternating layers from both materials are investigated. For modelling of multilayer coating the thickness dependence of the refractive index of single coatings from both materials is studied. The particularities and scope of application of different spectrophotometric methods for calculation of optical parameters of thin chalcogenide layers are discussed for film thickness, d, in the range λ/25–1.5λ (λ being the operating wavelength). Having acquired the knowledge of optical parameters (refractive index, n, and extinction coefficient, k) of the single layers, we designed and produced a one-dimensional photonic crystal with fundamental reflection band at λ = 850 nm. It was shown that the photoinduced changes of the refractive index of thin chalcogenide films can be used for enhancement of the optical contrast of both materials.

Structural and optical characterization of Ag photo-doped thin As40S60 − xSex films for non-linear applications

This paper deals with the structure and the optical properties of thin As40S60 − xSex films doped with silver. The refractive index n and the optical band gap Egopt were calculated from the transmittance and reflectance spectra. The results showed that the photo-doping leads to increase in the refractive index by about 0.25–0.27. An effect of thickness expansion was observed in the photo-doped layers. The non-linear refractive index, γ, and the two-photon absorption coefficient, β, were evaluated by applying a formula developed by Sheik-Bahae. Each of the films studied exhibits a highly non-linear refractive index at the telecommunication wavelength, 70–850 times higher than that measured for fused silica. From the Raman spectra of thin As40S30Se30 it might be concluded that under dissolution, the silver interacts with both sulfur and selenium. The surface of the thin films was investigated by using a white light interferometric profiler. It was found that the increase in the thickness of the silver layer results in roughening of the surface of the photo-doped films.

Study of the photoinduced surface relief modulation in photopolymers caused by illumination with a Gaussian beam of light

The formation of surface relief profile in photopolymerizable systems when illuminated with a focused beam of light is simulated numerically using a two-way diffusion model that takes account both for monomer and short polymer chain diffusion. The concentration and spatial distribution dynamics of monomer and short and long polymer chains are calculated. The surface profile is obtained from the calculated component concentrations considering different densities of monomer and polymer. The influence of the illumination time, intensity and spot diameter on the surface profile dynamics is discussed. A good agreement between the calculated and the experimentally measured profiles is observed, thus demonstrating the successful application of the two-way diffusion model to this system.

Photoinduced changes in the optical properties of obliquely deposited a-As2S3 thin films

Abstract The photoinduced changes in the optical properties of obliquely deposited a-As 2 S 3 thin films are followed. It is established that the increase of the refractive index and absorption coefficient as a result of illumination is highest in the sample prepared at 70° vapour incidence angle. In addition, the refractive index dispersion has been analyzed according to the Wemple–Didomenico single oscillator model and the effective As coordination number in non-illuminated and illuminated samples has been evaluated. On the basis of the results obtained the role of microstructure in the photoinduced transformations in vacuum deposited arsenic chalcogenide thin films is discussed.

Tunable Bragg stacks from sol-gel derived Ta2O5 and MEL zeolite films

In this paper we investigated sol-gel derived Ta2O5 and nanosized MEL zeolite films obtained by spin coating of Tantalum sol and colloidal zeolite solution, respectively. Refractive index and thickness of the films were determined using non-linear curve fitting of measured reflectance spectra. The influence of the post deposition annealing on the optical properties and thickness of the films was studied. Besides tunable Bragg stacks were designed and prepared by layer-by-layer deposition of Ta2O5 and MEL suspensions with quarter-wave thicknesses. The influence of water, acetone and methanol on the optical behavior of Bragg stacks was discussed.

Refractive index of corona-treated polypropylene films

The samples under investigation are 20??m thick isotactic polypropylene films corona treated at different corona polarities and grid voltages in a three-electrode system. In order to control the results of the applied treatment a combination of two refractive index (RI) measuring techniques (laser refractometry and spectrophotometry) is used to study RI dispersion in the spectral range 400?1000?nm. It is found that the surface RI was lower than the volume RI. After a corona treatment both surface and volume RI increase. The changes of the volume RI as compared to surface values are more influenced by the corona treatment conditions: the greater the grid potential, the bigger the volume RI.

One dimensional photonic crystals from As2S3 and PMMA films for photonic and sensor applications

One dimensional photonic crystal (1D-PhC) in form of 19 layered stack was prepared through layer-by-layer deposition of spin coated Poly (methyl methacrylate) (n = 1.49 at 1.55 ?m) and vacuum deposited As2S3 (n = 2.27 at 1.55 ?m) as low and high refractive index materials, respectively. The optical properties of the single layers and the multilayered stack are studied at normal and oblique incidence of linearly polarized light. The possibility of using the 1D-PhC as an omnidirectional reflector was exploited theoretically and confirmed experimentally. Besides, the prospect of using the studied structure for chemical sensing with optical read-out is demonstrated.

Nanozeolite doped photopolymerisable composites: Optical properties and holographic applications

Photopolymerisable nanocomposites containing nanozeolites have been studied. The effect of the porous nanocrystals on the average refractive index, optical scattering, holographic recording properties and shrinkage has been characterised. Results from the exposure of the holograms to analytes in a controlled gas environment demonstrate that they can be used as optical sensors.

Reflectance methods for determining the optical constants of highly absorbing films: comparative analysis of the accuracy

The accuracies of the most widely used methods for optical characterization of absorbing films with thicknesses greater than the penetration depth of light are compared quantitatively through estimation of the errors Δn and Δk in refractive index, n, and extinction coefficient, k. Among the great variety of possible combinations of reflectance, R, measured in both planes of polarization, Rp and Rs, reflectance ratios, Rp/Rs, and pseudo-Brewster angle, β, only four are selected as the most suitable methods: (R,Rp70), (Rpβ/R,β), (Rpβ,β) and (R,β). It is shown that the methods using β are more accurate than those with reflectance measurements only. Among the three methods with β, the least accurate is (Rpβ/R,β), while (R,β) and (Rpβ,β) have similar accuracies but slightly different application ranges. It is found that only samples with n, k≤3 can be characterized with methods including β if the largest incident angle, which can be measured, is 70°. Application of the error analysis predictions is illustrated through determination of the optical constants of 150 nm thick Au film.