Antoni C. Mitus

Size Selection During Crystallization of Oppositely Charged Nanoparticles

Opposites attract (selectively): Oppositely charged nanoparticles characterized by different size distributions form 3D supracrystals (see figure) only if the distributions overlap. Crystal quality decreases rapidly with decreasing degree of overlap, and, irrespective of the ratio of particle diameters/charges, no crystals are observed for non-overlapping distributions.

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...

Local structures in a computer-generated liquid. Two-dimensional Lennard-Jones liquid

We study the local structures in a 2D Lennard-Jones liquid of 2500 atoms near the melting line (ϱ∗ = 0.76, T∗ = 0.47) with the help of methods of mathematical statistics based on an earlier proposed probabilistic approach. We analyze the local structures in individual configurations and study the time evolution of patterns of matter which is close to the solid hexagonal structure. We conclude that the model liquid displays two types of local structure: hexagonal and “chaotic”. The first one corresponds to the fluctuations of the hexagon with root-mean-square fluctuations ξ of atoms equal to ξ = 0.14-0.16 while the second one can be represented by a strongly fluctuating (ξ = 0.25-0.30) “defect” pattern. We discuss the consequences of the physical picture of the liquid as a locally ordered two-structure system for the methodology of computer simulations and for theories of the 2D liquid phase.

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.

Kinetic Monte Carlo study of diffraction grating recording/erasure in DNA-based azo-dye systems

Recently, we have proposed a simplified kinetic Monte Carlo model which mimics the inscription/erasure of diffraction gratings in DNA matrix with azodyes. Preliminary results correctly reproduce the observations made for the photochromic system DR1:DNA-CTMA: very short operational times (inscription/erasure), optical stability and reversibility. In this paper we analyze semi-intercalation model of paper analytically and discuss its predictions. Next, we modify this model by taking the DNA model chain directly into account. Local free volume determines the transition probabilities of trans-cis photoisomerization reactin. Using the model we address the open questions: (i), short operational time; (ii), small diffraction efficiency; and (iii), one exponential inscription.

Towards modelling of stochastic kinetics for process related to photochromic dye semi-intercalation in DNA-based polymer matrix

The semi-intercalation of an azo-dye Disperse Red 1 (DR1) molecule into a biopolymeric material made of deoxyribonucleic acid (DNA) complexed with the cationic surfactant hexadecyltrimethyl-ammonium chloride (CTMA) formulated recently1-3 has successfully explained the main experimental results4 of laser dynamic inscription of diffraction gratings: short response time, low diffraction efficiency, single-exponential kinetics and flat wavelength dependence.5 In this paper we generalize the analytic model of Ref.2 to account for a more realistic dynamics of DNA-CTMA matrix. To this end we extend the model of paper5 by including into it probabilistic features of local free volume in DNA matrix which characterize, in a simple way, the spatial distribution of local voids which, in turn play the central role for the kinetics of photoinduced trans-cis-trans cycles of DR1 dye under the polarized laser light illumination. We discuss a stochastic master equation which generalizes the simple model of Ref.2 and address briefly the topic of non-exponential grating inscription in modelling and in recent experiments.

Complex Dynamics of Photo-Switchable Guest Molecules in All-Optical Poling Close to the Glass Transition: Kinetic Monte Carlo Modeling

We study theoretically the kinetics of noninteracting photoswitchable guest molecules (model azo-dye) dispersed at low concentration in host (model polymer matrix) in the all-optical poling process close to the glass transition temperature Tg. We modify kinetic Monte Carlo model used in our previous studies of nonlinear optical processes in host-guest systems. The polymer matrix is simulated using the bond-fluctuation model. The kinetics of multiple trans-cis-trans cycles is formulated in terms of transition probabilities which depend on local free volume in the matrix and its dynamics. Close to Tg, the buildup of polar order, monitored in terms of angular probability density functions, follows a power-law in time while the evolution of the nonlinear susceptibilities related to second harmonic generation effect follows the stretched-exponential law. This complex dynamics of guest molecules implies the presence of dynamic heterogeneities of the matrix in space and time which spread the complexity from the matrix to the otherwise simple dynamics of noninteracting guest molecules. A qualitative physical picture of mosaic-like states-intertwined areas of free- and hindered angular motion of guest molecules-is proposed and the role of related short and longer scales in space for the promotion of complex dynamics of guest molecules is discussed. A brief comparison of the theory to available experimental data is given.

Random lasing in dye doped bio-organic based systems: recent experiments and stochastic approach

We review the results of recent experimental studies on random lasing phenomenon in biopolymeric matrices: DNA-CTMA and starch, loaded with different luminescent dyes (DCNP and Rh6G). New experimental results for DNA-CTMA:DCNP system are presented. The random lasing originates due to the light scattering induced by formation of microcrystals or clusters in the bulk of biosystem. We propose a simple model for light transport in the scattering medium accounting for the inhomogeneities in polymer matrices simulated using Monte Carlo method and present some preliminary results related to ray scattering.

Holographic grating inscription in DR1: DNA-CTMA thin films: the puzzle of time scales

We study experimentally the dynamics of holographic inscription of gratings in DR1:DNA-CTMA thin films using a degenerate two-wave mixing (DTWM) setup in its initial phase (30 ms) and in a longer time interval (30 s). The temporal pattern of evolution of diffraction efficiency is complex, simple fitting procedures fail to reproduce the data. We point out that the complex dynamics can originate a large span of temporal scales, closely related to the microscale inhomogeneity of local free volume. Some of its hallmarks are found through Monte Carlo simulations.

Grating inscription in DR1:DNA-CTMA thin films: theory and experiment

Recent experimental results indicate that the inscription of gratings in DR1:DNA-CTMA thin films displays some features of non-exponential grating amplitude growth with time.1 The origin of this behavior is hypothetically assigned to a complex distribution of local voids in a polymeric matrix1, 2 which strongly influences the dynamics of grating inscription modelled using the semi-intercalation hypothesis.3–7 We discuss critically those topics, review the theoretical methods used for modelling of the grating inscription and point-out a hypothetical relation to complex systems. New experimental results of holographic DTWM recording of the gratings in DNA-CTMA:DR1 and PS:DR1 are presented. The two observed types of dynamics are hypothetically assigned to various distributions of local voids in corresponding polymeric matrices.