Peter Sharlandjiev

Anisotropic hybrid organic/inorganic (azopolymer/SiO2 NP) materials with enhanced photoinduced birefringence.

Hybrid materials based on combination of polymers and inorganic nanoparticles (NP) attracted considerable attention in the last decade due to their advantageous electrical, optical, or mechanical properties. Recently, we reported a significant improvement of the photoresponse by doping azopolymers with ZnO NP. To study the influence of the composition of the dopant, in our present work we have synthesized anisotropic organic/inorganic nanocomposite materials by incorporating 5-15 nm sized SiO2 NP in a side-chain azopolymer. As a result we observe an enhancement of the photoinduced birefringence in these composite materials with about 20% compared to the nondoped sample. Additionally, we discuss possible mechanisms leading to this enhancement related with the scattering caused by the NP and the increased mobility of the azochromophores in the vicinity of NP.

Resonant optical transmission from a one-dimensional relief metalized subwavelength grating

We have studied one-dimensional (1D) relief metalized subwavelength gratings, which support resonant optical transmission. We have used pregrooved DVD stampers, metalized with a thin Al layer. The sensitivity of resonant transmission of the gratings to the cladding environment was investigated by the help of matching fluids. We have shown that the shift of the spectral position of the resonance peak can be used for sensor applications, e.g., for determination of very low concentrations of nanosized dielectric particles in distilled water.

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.

Photoinduced anisotropy : new photopolarimetric setup for real-time measurements

We describe an experimental set up that measures material optical anisotropy and its eventual evolution in real time. The setup is based on a new photopolarimeter that measures the four Stokes parameters of quasi-monochromatic light quickly and simultaneously. The photoinduced anisotropy in the material optical constants is determined from the measured Stokes parameters. The features of the experimental setup are demonstrated by simultaneous measurement of the photoinduced changes in dichroism and birefringence of layers of rigid solutions of azodyes in polymethyl methacrylate, for which it is known that linearly polarized light orients the dye molecules.

Electron donors and heat treatment: their effect on the diffraction efficiency of dichromated gelatin reflection holograms.

Holographic parameters of dichromated gelatin layers are improved by introducing electron donors such as N-N′ dimethylformamide into the layers. Simple reflection holograms are recorded. Their diffraction efficiency is greater compared with that of standard dichromated gelatin holograms. Recording at 6 mJ/cm2 (λ = 488 nm) gives a diffraction efficiency of ∼40%. The presence of electron donors does not deteriorate the hologram SNR. Heat treatment of recorded but undeveloped holograms leads to higher holographic sensitivity. EPR experiments show that the speed of photoreduction of chromium ions is changed by electron donors and by baking. This is confirmed by refractive-index and gelatin-melting-point temperature measurements. Baking affects the position of the diffraction efficiency spectral maximum as well.

Determination of optical parameters of very thin ( λ /50) films

A straightforward approach for estimation of thickness (d), real (e1) and imaginary parts (e2) of the complex permittivity of very thin films from spectrophotometric measurements is presented. The uncertainties in e1, e2, and d due to methodical error and the uncertainties in the measured quantities are investigated. It is shown that the influence of these factors is considerable when e1, e2, and d are obtained simultaneously for each wavelength. The accuracy of e1, e2, and d is significantly increased if the value of d is evaluated first, its value is kept constant over the whole spectral region, and then e1 and e2 are calculated for each wavelength.

Optical Characterization of Very Thin Films for Nanotechnological Basis of Sensors

Thin films and photonic crystals are used as elements of a nanotechnological basis for sensors due to their well-defined optical properties such as transmittance, reflectance, or diffraction. Key issue is the optical characterization of structures obtained by nanotechnological methods, i.e. a determination of the physical thickness of each individual layer, its dielectric permittivity, magnetic permeability as well as the evaluation of its homogeneity and corrugation. Herein, we focus on a specific problem of great importance to fundamental and applied science: we analyze the case of very thin films, meaning that the ratio ‘layer thickness to wavelength’ is much less than 1/50. We also demonstrate the potential of a new direct approach developed by us to inverse optical problems, based on a position estimation of the probability density function of partial solutions.

Control of the spectral position of dichromated gelatin reflection holograms

Dichromated gelatin (DCG) is one of the best light-sensitive materials for holographic recording. However, strict technological control should be observed during deposition, gelation, and drying of DCG plates and the chemical processing of the recorded structures. Usually, for reflection holograms the gelatin layer should be sealed in order to protect it from ambient humidity, which affects the value and position of the reflection maximum. Herein we give three different recipes for tuning the reflection maximum from 430 to 700 nm by swelling the hologram thickness with a filler material that is commercially available. The mechanism of the hologram swelling, which changes the period of the recorded holographic structure, is also briefly discussed.

Simulation of the diffraction by CD-R: thickness determination of the dye recording layer

In this paper an approach for evaluating the diffraction of a plane light wave by a compact discs-recordable (CD-R) surface, considered as a grating with a certain period and structure, is demonstrated. Its comparison with the rigorous treatment of the problem shows a very good agreement between the results. The main feature of the proposed approach is that in the calculations boundary effects are included. It is shown that the optical behaviour of the unrecorded discs can be well described by the realized approximate model. Our approach for the diffraction allows the solution of the inverse optical problems, one of which is the determination of the dye thicknesses in the groove and in the land. The possibility of studying the dye layer thicknesses in the land and in the groove as dependent on the spin-coating conditions is illustrated. The results obtained show the feasibility of two CD-R versions - with a thicker and a thinner dye layer. The model can also be used for CD-R optimization.

Polarimeter - Photometer for Quasi-Monochromatic Light

An instrumental device for complete, accurate, fast and simultaneous measurement of all parameters of quasi-mono-chromatic light: wavelenth, intensity, degree and state of polarization is presented. The performance of the device is based on a new principle which makes use of a polarization holographic grating. The measured light is split into four beams of intensities I , 12 , 13 and 14 . To determine the four Stokes parameters, these intensities are measured simultaneously by four photocietectors and the electric signals are processed by a microcomputer. At the output of the device all the four Stokes parameters are given as well as the ratio of the polarization ellipse axes and its azimouth. The use of linear ray detector for measurement of 13 and 14 intensities makes it possible the evaluation of light wavelenth through analysis of light beams angle of diffraction.