Grzegorz Pawlik

Infrared cylindrical cloak in nanosphere dispersed liquid crystal metamaterial.

We present a design of an infrared cylindrical cloak using nanosphere dispersed nematic liquid crystal (NLC) metamaterial following the approach of Smiths group [Science 314, 977 (2006)]. Cloaking conditions require spatial distribution of liquid crystal birefringence with constant extraordinary index of refraction and radially dependent ordinary index of refraction. An approximate analytical formula for the latter is derived. Finite element (FE) simulations confirm the cloaking effect. Owing to the tunable birefringence of the liquid crystal component, such cloaking material offers the interesting possibilities of real-time control of invisibility. The possibility of experimental realization is briefly discussed.

Liquid crystal hyperbolic metamaterial for wide-angle negative–positive refraction and reflection

We show that nanosphere dispersed liquid crystal (NDLC) metamaterial can be characterized in near IR spectral region as an indefinite medium whose real parts of effective ordinary and extraordinary permittivities are opposite in signs. Based on this fact we designed an electro-optic effect: an external electric-field-driven switch between normal refraction, negative refraction, and reflection of TM incident electromagnetic wave from the boundary vacuum/NDLC. A detailed analysis of its functionality is given based on effective medium theory combined with a study of negative refraction in anisotropic metamaterials and finite elements simulations.

Kinetics of diffraction gratings formation in a polymer matrix containing azobenzene chromophores: Experiments and Monte Carlo simulations

The physical processes accompanying an illumination of azobenzene chromophores embedded in a polymer matrix with spatially modulated and linearly polarized light are studied experimentally and simulated using the Monte Carlo (MC) method. The MC simulations base on a simple three-channel kinetic model of trans-cis type photoisomerization cycles in azobenezene molecules. The method focus on studying the kinetics of diffraction grating recording and erasing under various conditions of illumination, light polarization and recording in the presence or absence of a background light. By modeling the “macroscopic” light diffraction efficiency one gets insight into the role played by microscopic processes (angular hole burning, angular redistribution, rotational diffusion) up to the high-temperature limit, characterized by a high mobility of polymer chains. The obtained qualitative agreement between experimental data and the modeling confirms that the main processes underlying grating recording and erasing are wel...

Second–harmonic generation in poled polymers: pre–poling history paradigm

Experimental studies of second harmonic generation (SHG) from electric-field poled PMMA - DR1 system show occurrence of a maximum in diagonal and off diagonal tensor components χ(2)(-2ω;ω,ω) at 15 mol % concentration and a rapid decrease above, with a stabilization. The origin of the observed concentration dependence is studied using the Monte Carlo (MC) modeling. We find that presence of maximum is conditioned by the pre-poling history of the sample, when entanglement of linear dipolar structures takes place. Length of the pre-poling interval is an important kinetic parameter which differentiates between various nonexponential kinetics of build-up of polar phase responsible for strong/weak SHG susceptibility.

Electro-optic phenomena in nematic liquid crystals studied experimentally and by Monte-Carlo simulations

In this article we compare results of experiments on light self-diffraction in nematic liquid crystal panels with corresponding results of the Monte-Carlo simulations of a two-dimensional nematic liquid crystal model in the presence of a spatially modulated electric field. In the simulations molecular interactions were described by the Lebwohl–Lasher Hamiltonian. The results obtained on the diffraction efficiency and spatial and temporal behavior of refractive index changes in nematic liquid crystal are satisfactorily reproduced by Monte-Carlo simulations. We discuss the complementarity of both methods in studying and designing systems for optical information processing using liquid crystals.

Concentration Variation of Quadratic NLO Susceptibility in PMMA-DR1 Side Chain Polymer

Chemical synthesis and concentration dependence of two second order NLO tensor components: and was studied for a side chain polymer PMMA-DR1 in thin films by optical second harmonic generation techniques. The noncentrosymmetry was created by the corona poling technique. The results show that for small chromophore concentrations the susceptibility follows linearly the density number of active chromophores, reaches a maximum, than decreases and stabilizes.

Monte–Carlo simulations of refractive index changes in nematic liquid crystal upon spatially nonuniform illumination

Abstract We study the influence of a spatially modulated light intensity on the refractive index of a nematic liquid crystal, using Monte–Carlo simulations with a generalized version of the Lebwohl–Lasher hamiltonian. We have considered a model of 2D liquid crystal cell in which one of the electrodes is covered by a photoconducting polymeric layer. The index profiles along the cell thickness are calculated and compared with experimental data.

Generic stochastic Monte Carlo model of the photoinduced mass transport in azo-polymers and fine structure of Surface Relief Gratings

We study theoretically the possible origin of a double-peak fine structure of Surface Relief Gratings in azo-functionalized poly(etherimide) reported recently in experiments. To improve the statistics of experimental data additional measurements were done. For the theoretical analysis we develop a stochastic Monte Carlo model for photoinduced mass transport in azobenzene-functionalized polymer matrix. The long sought-after transport of polymer chains from bright to dark places of the illumination pattern is demonstrated and characterized, various scenarios for the intertwined processes of build-up of density and SRG gratings are examined. Model predicts that for some azo-functionalized materials double-peak SRG maxima can develop in the permanent, quasi-permanent or transient regimes. Available experimental data are interpreted in terms of models predictions.

MONTE CARLO SIMULATIONS OF TEMPERATURE DEPENDENCE OF THE KINETICS OF DIFFRACTION GRATINGS FORMATION IN A POLYMER MATRIX CONTAINING AZOBENZENE CHROMOPHORES

A simple kinetic model of temperature-dependent processes accompanying the writing and decay of diffraction gratings in a polymer matrix doped with azo-dye chromophores under illumination with spatially modulated and linearly polarized light is introduced and studied using Monte Carlo simulations. The model generalizes a recently proposed athermal kinetic model based on three-channel photoizomerization cycles of trans–cis type of azobenzene molecules and orientational redistribution and diffusion. These processes are influenced by a polymer matrix in a thermal equilibrium. The Monte Carlo simulations show the important role of temperature on the kinetic processes during building and erasure of diffraction gratings. We discuss some interesting applications for photonic devices, related to all-optical processing and storage of information using holographic methods.

Kinetics of grating inscription in DR1:DNA-CTMA thin film: experiment and semi-intercalation approach

The semi-intercalation hypothesis1–5 which states that an azo-dye Disperse Red 1 (DR1) molecule intercalates in a specific way into a biopolymeric material made of DNA complexed with the cationic surfactant CTMA, has successfully explained the main experimental results6 of laser dynamic inscription of diffraction gratings: short response time, low diffraction efficiency, single-exponential kinetics and flat wavelength dependence.4 Recent experiments indicate that the inscription of the grating displays some features of non-exponential behavior. To understand this complex dynamics we characterize local environment of polymeric chains in Monte Carlo modelling by analyzing some features of local free-volume (void) distribution.