Surface Relief Gratings (SRG) were demonstrated experimentally more than 20 years ago. Despite many years of research efforts the underlying physical mechanisms remain unclear. In this paper we present a short overview of the main concepts related to SRG - photofluidization and its counterpart, the orientational approach - based on a seminal paper by Saphiannikova et al. Next, we summarize the derivation of the cos2 θ potential, following the lines of recent paper of this group. Those results validate the generic Monte Carlo model for the photoinduced build-up of the density and SRG gratings in a model polymer matrix functionalized with azo-dyes, presented in another part of the paper. The characterization of the photoinduced motion of polymer chains, based on our recent paper, is briefly discussed in the last part of the paper. This discussion offers a sound insight into the mechanisms responsible for inscription of SRG as well as for single functionalized nanoparticle studies.
Our recent research results showed that DNA chains undergo a compactisation when DNA solutions are doped with different chromophores. In this paper we will present our attempt to model this behaviour in order to predict the DNA solutions characterisation.
KEYWORDS: Polymers, Diffusion, Motion models, Modeling, Monte Carlo methods, Nanoparticles, Photochromic materials, Curium, 3D modeling, Chromophores, Temperature metrology, Physics
Recently we have formulated a generic Monte Carlo model for the photoinduced build up of the density grating and surface relief grating (SRG) in a model polymer matrix functionalized with azo–dyes.1 Mass transport from illuminated to dark places was demonstrated and ascribed to a hypothetical complex dynamics of polymer chains in bond–fluctuation model. This paper constitutes a first step towards verification of this hypothesis: we characterize the motion of single functionalized chain dependent on the intensity of linearly polarized light illumination. We report various regimes of diffusion of the center of mass (CM) of the chain: subdiffusion, normal diffusion and superdiffusion. Presence/absence of those regimes depends on the light intensity and on the length of the chain. We report a surprising effect in the presence of light illumination: the longer the polymer chain the stronger superdiffusion sets in.
Recent experimental Dynamic Light Scattering (DLS) studies of the coil sizes of DNA-CTMA:Rh solutions have lead to numerical discrepancies with theoretical predictions amounting to one-two orders of magnitude.1 In this paper, which has partially character of a tutorial, we present the basic theoretical concepts underlying an analysis of the polymer coil sizes from DLS experiments. In particular, we discuss the limitations of those methods. We present a wormlike model of a polymer chain which is a promising candidate for inferring information about the spatial structure of the DNA chain from experimental data.
KEYWORDS: Light scattering, Random lasers, Systems modeling, Photons, Scattering, Chemical species, Biological research, Ray tracing, Polymers, Modeling
One of many photonic applications of biopolymers as functional materials is random lasing resulting from an
incorporation of highly luminescent dyes into biopolymeric matrix, which leads to a random but coherent light
scattering in amplifying medium. In spite of numerous theoretical and experimental studies the origin of the
coherence is still not clear and various scenarios are discussed. In particular, inhomogeneity of biopolymeric
layers can hypothetically promote the feedback in the scattering of the emitted light resulting in coherent and
incoherent random lasing. In this paper we analyze the light scattering in a model system of scattering centers
of circular shapes and various dimensions using ray-tracing techniques. In the second part, which has mostly a
tutorial character, we present the approach to the study of random lasing using a cellular automaton model of
Wiersma et al.
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.
We present trial calculations of surface light-induced patterns in photochromic azo-substituted polymers. Using microscope with nanopositioning stage various birefringence and surface structures have been recorded in photochromic azo-functionalized polymers. By systematic approach to the inscription experiment and controlling cw or pulsed laser light intensity, its polarization and beam scan speed we observed the dynamics of molecular photoorientation and its relation to mass transport. We discuss properties of holographically inscribed polarization gratings and analyze them spatially by monitoring of microscopic local diffraction efficiency. We report how azo-benzene molecules can work in other systems, i.e. azobenzene functionalized POSS molecules embedded in nematic liquid crystal.
We study the behavior of refracted angle for k-vector at the interface of uniaxial anisotropic media in the case of nanosphere dispersed liquid crystal (NDLC) matematerial. Finite Element (FE) calculations (COMSOL Multiphysics) are used to trace the propagation of the electromagnetic wave. Preliminary results on the influence of incident angle on refracted angle wave-vector are presented.
KEYWORDS: Polymers, Diffraction gratings, Monte Carlo methods, Molecules, Thin films, Diffraction, Holography, Data modeling, Modeling, Two wave mixing
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.
We discuss the effect of a negative refraction at the interface of uniaxial anisotropic media in the case of nanosphere dispersed liquid crystal (NDLC) matematerial. Finite Element (FE) calculations (COMSOL Multiphysics) are used to trace the propagation of the electromagnetic wave. We show that for chosen values of the parameters of nanospheres and of nematic liquid crystal (NLC) host negative refraction can be obtained for a wide range of incident angles.
In this paper the concept and design of infrared cloaking using nanosphere dispersed liquid crystal (NDLC)
matematerial in cylindrical geometry is presented for TM polarization of incident light. The influence of material
losses on the cloaking efficiency is discussed. The loss can be tuned by changing design parameters.
KEYWORDS: Polymers, Monte Carlo methods, Correlation function, Optical fibers, Probability theory, Modeling, Physics, 3D modeling, Chromophores, Liquids
Recently we have formulated a simple Monte Carlo model1 for the study of the photomechanical effect in polymeric fibers pumped with a linearly polarized laser beam.2–4 The model fiber is a host - guest system consisting of the polymeric matrix and azodye chromophores, which undergo multiple trans - cis - trans cycles when illuminated by linearly polarized light. Current paper is focused on the topic so-far neglected in,1 closely related to the hypothetical cooperative mechanism of stress relaxation2–4 – on the characterization of local voids in computer-generated polymeric matrix. We study void-size distributions and void-void correlation functions.
KEYWORDS: Polymers, Diffraction gratings, Monte Carlo methods, Molecules, Diffraction, Thin films, Molecular interactions, Modeling, Data modeling, Correlation function
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.
We discuss the concept of infrared cloaking using nanosphere dispersed liquid crystal (NDLC) matematerial in
cylindrical geometry for TM polarization. The system consists of layers of NDLC with different values of ordinary
refractive index and the same value of extraordinary refractive index of liquid crystal host. Finite element
calculations (COMSOL Multiphysics), which include the Poynting vector calculations, show that scattering from
the hidden object is limited in the presence of the layered cloak.
We discuss the concept of infrared cloaking using nanosphere dispersed liquid crystal (NDLC) matematerial in
cylindrical geometry for TM polarization. The system consists of six layers of NDLC with different values of
ordinary refractive index. Finite element calculations (COMSOL Multiphysics) show that scattering from the
hidden object is strongly limited in the presence of the cloak.
KEYWORDS: Polymers, Monte Carlo methods, 3D modeling, Physics, Polarization, Optical fibers, Fiber lasers, Laser beam propagation, Systems modeling, Time metrology
We present a simple Monte Carlo modeling of the light-driven expansion in a host-guest system polymer -
azodye, which is the main mechanism responsible for the photomechanical effect in a dye-doped polymer fiber
illuminated with linearly polarized laser light. Bond-fluctuating model of polymer matrix is used, the concept of
void - free volume in the matrix - is quantified. The role of the dyes which are in trans state oriented along the
beam propagation direction in the elongation of the system is studied via a non-thermal kinetic Monte Carlo
process. Preliminary results show that upon illumination the system expands while in the dark period it shrinks.
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.
We discuss the concept of infrared cloaking using nanosphere dispersed liquid crystal (NDLC) matematerial in
cylindrical geometry. Preliminary results show that NDLC is a promising candidate for cloak design. Monte
Carlo simulations are used for the design.
We use the recently formulated hypothesis of semi-intercalation of an azo-dye Disperse Red 1 (DR1) into a biopolymeric
material made of deoxyribonucleic acid (DNA) complexed with the cationic surfactant hexadecyltrimethylammonium
chloride (CTMA)1-3 to model the unique photochromic properties of the DR1:DNA-CTMA system.
First results of kinetic Monte Carlo simulations accurately reproduce4 the main experimental results5 of laser
dynamic inscription of diffraction gratings in this photochromic material: short response time, low diffraction
efficiency, single-exponential kinetics and flat wavelength dependence. Results of systematic MC studies are
presented. The question of extending the model of paper4 by including into it probabilistic features of local free
volume in DNA matrix is discussed.
Monte Carlo studies of the field induced complex refractive index changes in nano-dispersed nematic liquid crystals
exhibiting negative - positive refractive indices1, 2 have been performed for various cases of applied field
strengths,3 anchoring profiles and temperatures over a broad spectral regime. The resultant induced spatially
inhomogeneous molecular order and the corresponding metamaterial properties are obtained for various wavelengths,
applied field strengths, anchoring forces and temperatures below and above the Freedericksz transition.
In general, the director axis reorientation and the resultant refractive index distribution are spatially inhomogeneous,
even under an uniform applied field. The detailed computation have identified parameter sets for obtaining
negative index of refraction and maximal index modulations that can be more than an order of magnitude larger
than those obtainable from pure NLC systems.
Recently, we have discussed anchoring forces and the electric field as new control parameters for negative-
positive refraction index tuning in nanosphere dispersed nematic liquid crystal (NDLC). The present study is
focused on calculation of the amplitude modulation of the refractive index caused by amplitude variation of
anchoring forces and spatial modulation of anchoring forces. Preliminary results indicate that, similarly to
case studied earlier,1 refractive index amplitude modulation can be significantly larger as compared with a
conventional liquid crystal (LC) system. The inhomogeneous molecular order in nematic liquid crystal (NLC)
cells is modelled using Monte Carlo simulations with the Lebwohl-Lasher effective Hamiltonian with the Rapini-
Papoular term for anchoring forces.
Khoo et al.1 have introduced the concept of tuning of negative refractive index using nanosphere dispersed
nematic liquid crystal (NDLC). Recently,2, 3 we have discussed anchoring forces as a new control parameter for
negative-positive refraction index tuning in NDLC. In particular, we have calculated3 the phase diagrams in
variables electric field - anchoring force for real and imaginary parts of permittivity and permeability for NDLC
using averaged (global) value of refractive index for the inhhomogeneous system. In current paper we analyze an
influence of anchoring forces and of spatial modulation of electric field on local distribution of negative refraction
index in NDLC. The study constitutes a generalization of homogenous isotropic dielectric layer approximation
used in papers.1 Inhomogeneous molecular order in planar NLC cells is modeled using Monte Carlo simulations
with Lebwohl - Lasher effective hamiltonian and Rapini - Papoular term for anchoring forces.
Khoo et al.1, 2 have shown that nanosphere dispersed nematic liquid crystal (NDLC) constitutes a new type of
metamaterial with index of refraction tunable from negative to positive values. Recently3 we have combined this
approach with Monte Carlo simulations of inhomogeneous molecular order in planar NLC cells. Lebwohl - Lasher
effective hamiltonian with Rapini - Papoular term for anchoring forces was used. Electric field and amplitude
of anchoring forces are control parameters which determine the profiles of order parameter. In this paper we
study, using the same approach, local spatial distribution of refractive index in NDLC planar cell. We show
that NDLC material consists of layers with negative-zero-positive index of refraction. The spatial organization
of those layers strongly depends on incident light wavelength. The role of spatially modulated external electric
field for tuning of refractive index of NDLC is briefly discussed.
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.
The origin of a strongly nonlinear dependence of second order nonlinear optical properties of poled thin films
of a guest-host system consisting of charge transfer chromophores in a polymer matrix, on concentration of
chromophores, remains unknown. Recently Rau et al.1, 2 have provided evidence based on optical microscopy
in favor of a new mechanism of relaxation due to an accelerated chromophore aggregation during the heating
which facilitates chromophore diffusion. Aggregation is an important factor which decreases the susceptibility.
Kinetic Monte Carlo technique is used for a study of a poled guest-host system (polymer matrix with dipoles)
to analyze the local free volume distribution which determines the ability of a dipole to reorient. The simulations
show a large degree of inhomogeneity in this parameter, which results in a jamming effect for both orientational
and translational diffusion of the dipoles. The applicability of point dipoles approach at high loads is critically
discussed. The emerging physical picture of the poling allows an interpretation in terms of an aggregation
observed by Rau et al.
The semi-macroscopic mechanisms responsible for the surface relief gratings (SRG) formation on azobenzene-containing
films are far from deep understanding.1 We present the results of experimental studies of SRG in
PMMA and PVK polymeric matrices. The Monte Carlo (MC) kinetics of polymeric movements is reported for
recently proposed model,2 which mimics the effect of mass transport along the direction of light modulation,
resulting from multiple trans ↔ cis photoisomerisation cycles of functionalized dyes. We show that the model,
symmetric on "microscopic" level, leads to a directed mass transfer from bright to dark places. Preliminary
studies show that the centers of mass of polymer chains undergo a normal diffusion under the light illumination.
No global light-induced ordering of polymeric chains was detected. MC studies were performed for a system
consisting of half a million of model monomers.
The mechanisms responsible for a strongly nonlinear dependence on concentration of electro-optic coefficient in
poled thin films of a guest - host system, consisting of charge transfer chromophores in a polymer matrix, are in
the center of interest since last decade. This effect is probably due to an aggregation of chromophores (dipoles),
leading to a cancellation of second-order nonlinear optical susceptibilities. Recently, a direct observation of a
process of aggregation of dipoles was reported by Rau et al. An important role for the relaxation is played
by an accelerated chromophore aggregation due to heating. We analyze the Monte Carlo kinetics of a poled
guest-host system using simulation methods developed for studies of diffraction gratings formation and erasure.
Preliminary results indicate a presence in the system of quasi one-dimensional (1D) object - spatial curves - along which the dipoles are aligned. Some of the lines form closed loops, leading to a decrease of nonlinear
optical properties. Monte Carlo kinetics of simple processes related to poling of host-guest and purely dipolar
systems is presented and discussed.
Analytical (elastic medium approximation) and numerical (Monte Carlo and molecular dynamics) modelling of
an inhomogeneous spatial order in nematic liquid crystal (NLC) molecules in confined systems (e.g. in planar or
twisted NLC cells) is widely used for molecular engineering of optical devices based on inhomogeneous spatial
distribution of refractive index. We present an overview of recent results obtained viaMonte Carlo approach based
on model Lebwohl-Lasher Hamiltonian, concerning optical aspects (index of refraction, diffraction efficiency,
transmission) of inhomogeneous molecular order in nematic liquid crystals in confined geometry. The role of
anchoring forces and of their fluctuations is discussed, in particular for transmission in Twisted NLC (TNLC)
cells. Preliminary results on an influence of molecular order on tunable negative- and zero-index nanosphere
dispersed liquid crystals (NDLC) are presented. The phase diagram for real part of effective permittivity in
variables external electric (magnetic) field - anchoring strength is calculated.
DNA-based materials offer new possibilities for holographic information inscription for photonic applications, in
particular because of a very short operational time. Recent results for dynamic holographic recording/erasure of
gratings in DR1:DNA-CTMA thin films are presented. We discuss the possibilities and perspectives of numerical
modelling of underlying fast "microscopic" physical processes, in a framework of a kinetic Monte Carlo simulation
method developed recently for studies of various aspects of inscription/erasure of gratings in azopolymers with
doped and functionalized dyes.
Statistical mechanics of an inhomogeneous NLC molecular alignment in NLC cells with complicated intermolecular and molecule-wall (anchoring) interactions can be easily studied using Metropolis Monte Carlo simulation method for Lebwohl-Lasher lattice system.1 Those studies have elucidated some microscopic aspects of the role of anchoring forces for local and global molecular ordering, leading to novel features in the external electric field (E) - anchoring strength (α) phase diagram for refractive index or an unexpected maximum of diffraction
efficiency for low values of anchoring strength α in planar NLC cells.2 Quite recently, this approach was used for a study of transmission of light through a twisted NLC cell as a function of applied voltage and light wavelength, for some values of α.3 First results show interesting generalizations of known analytical results of Yeh and Gu.4 In this paper we address a question of the importance of fluctuations of local rubbing directions and of fluctuations of anchoring strength for optical properties of NLC based optical systems. Preliminary results for chosen systems are presented.
We present the first Monte Carlo simulations of the mechanism of two-photon induced molecular orientation by photo-isomerization of chromophores in solid polymer films. Two-photon absorption (TPA) occurring at intense laser pulses creates molecular orientation of photo-isomers proportionally to the square of the excitation intensity and the fourth power of the cosine of the angle θ between the TPA-transition dipole moment and the direction of the polarization of the excitation light. Photo-selection in cos4θ will result in the orientational redistribution of the chromophores differently from the well-known one-photon absorption process. Kinetic Monte Carlo model is proposed and illustrative "microscopic" results related to diffraction gratings inscription are presented.
Monte Carlo Metropolis simulations of electro-optical phenomena in nematic liquid crystals (NLC) proposed in papers[1, 2] are used to calculate the profiles of an inhomogeneous ordering of NLC molecules in twisted nematic systems. The important role of low anchoring forces is clearly demonstrated. The dependence of transmission of light propagating through a twisted NLC cell on applied electric field and light wavelength generalizes known result[3] for an infinitely strong anchoring.
KEYWORDS: Monte Carlo methods, Molecules, Polymers, Diffraction gratings, Diffraction, Chromophores, Modulation, Modeling, Two wave mixing, Systems modeling
We propose a simple exactly solvable kinetic model of trans
↔ cis processes accompanying build-up of diffraction gratings in a polymer matrix doped with azobenzene chromophores, illuminated with spatially modulated and linearly polarized light in degenerate two-wave mixing experiments. This model mimics the essential features of a more realistic kinetic model studied recently using Monte Carlo simulations. A qualitative agreement between an experiment, Monte Carlo simulations and simple kinetic modelling implies that basic photoisomerisation processes during diffraction grating recording are of relatively simple nature. In particular, two exponential dependence of diffraction efficiency versus time measured in experiments and found in Monte Carlo simulations is easily
described by the model.
KEYWORDS: Polymers, Diffraction gratings, Monte Carlo methods, Molecules, Diffraction, Systems modeling, Modulation, Chromophores, Two wave mixing, Process modeling
We review, in a light of recent results, the relevance of Monte Carlo simulations of simple kinetic models of processes accompanying the recording and erasure of diffraction gratings in a polymer matrix doped with azobenzene chrompohores under the illumination with spatially modulated and linearly polarized light, in modelling the temperature-dependent processes observed in real degenerate two-wave
mixing (DTWM) experiments. We comment on the perspectives of Monte Carlo based design of effective photonic devices operating on photochromic systems.
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