In light absorbing liquids optical trapping of solid micro-objects but also gas bubbles can be achieved and explained by the mechanisms involving the hydrodynamic whirls formation. The various forms of these whirls, that arise due to optothermal Marangoni effect induced by laser light beam, are able to accelerate the objects movement, transport them and subsequently trap at the laser beam center but also close to it. The usual light gradient field force and scattering force solely are insufficient and even not adequate to properly describe the mentioned by us particle trapping effects as the trap potential extends to much larger distances that the beam waist. We will demonstrate the mechanism of optical trapping and transporting of gas bubbles and will discuss the physics of whirls formation in this case. The numerical modelling of Marangoni flows at the liquid-gas interface confirms the experimental findings. We also demonstrate a novel type of trapping of micro-objects that occurs inside a toroidal whirl induced by laser in dye-doped oil. This type of trapping is quite unusual but allows to transport objects immobilized far from the beam waist just avoiding their excessive heating.
We present the results of studies carried out for oversaturated solutions with common laser dye 4-
(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) and 3-(1,1-Dicyanoethenyl)-1-phenyl-4,5-
dihydro-1H-pyrazole (DCNP) nonlinear chromophore. We show that oversaturating the solution leads to formation of
crystals suspension resulting in strong Mie scattering and thus random laser operation can be observed. The formation of
aggregates can be induced be oversaturating the solution or by injection of non-solvent to the dye solution, leading to
reduction of solubility limit. Similar situation can be obtained for polymeric matrices for which small crystals are
precipitated during layer formation (solvent evaporation) when film is casted from the solution.
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.
In this article we present the results of studies carried out on the selected polymeric systems doped with luminescent dyes. Our studies focused on polymers like DNA-CTMA and PVK which were doped with common laser dyes Rh6G and DCM. We show that simple incorporation of highly luminescent dye into polymeric matrix can form ;efficient solid laser materials. Moreover, naturally occurring inhomogeneities of polymeric layers prepared by a drop casting process can scatter out pumping light in such a way that a feedback is introduced to the system and coherent and incoherent random lasing can be observed.
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.
The potential for photonic application of modified deoxyribonucleic acid with cationic surfactant cetyltrimethylammonium chloride, has been shown in many fields. Here we present results of detailed studies on random lasing achieved in a biopolymer based matrix loaded with luminescent dye. The random lasing originates due to the light scattering induced by formation of nanocrystals in the bulk biosystem. We show that lasing parameters for bio-polymeric system can be comparable with similar systems based on standard or π-conjugated polymers and may contribute to commercialization of polymeric lasers.
We present experimental results of studies of new organic compounds and hybrid systems which can be used for amplified spontaneous emission (ASE) and lasing applications. As dye molecules we have choose group of pyrazoline derivatives (PRD). Luminescence has been studied for solution of PRD in tetrahydrofuran. ASE has been studied for thin films of PRD in polymethyl methacryalte matrix. For the samples excitation we have used Nd:YAG nanosecond pulsed laser doubled in frequency (λ= 532 nm). We report on light amplification and spectral dye luminescence narrowing in function of different excitation pulse energy densities.
Pure deoxyribonucleic acid (DNA) is known to be soluble in water only and exhibits poor temperature stability. In
contrary, it is well known that the complex of DNA – with cetyltrimethyl ammonium (CTMA) is soluble in alcohols and
can be processed into very good optical quality thin films by solution casting and spin deposition.
Despite the success of DNA-CTMA, there is still need for new cationic surfactants which would extend the range of
available solvents for DNA complex. We test and present experimental results of influence of new surfactants based on
benzalkonium chloride (BA), and didecyldimethylammonium chloride (DDCA) for applications in all optical switching.
Organic nanocrystals (ONCs) similarly to inorganic ones show interesting size effects. It has been already observed that
their luminescence properties can be changed in terms of enhanced luminescence and spectral shift of fluorescence bands
as compared to bulk materials. Usually these effects are observed for much larger size of nanoparticles from 50 nm up to
1000 nm. Noncentrosymmetric and fluorescent organic crystals are rare but particularly interesting as they exhibit both
nonlinear optical properties like Second Harmonic Generation (SHG) and fluorescence. Such ONC's could serve as
fluorescent and SHG probes for bio-imaging purposes simultaneously. This could be beneficial for situation in which the
dynamics of ordering of biological systems has to be studied. Here, we report on 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-
dihydro-1H-pyrazole (DCNP) compound which forms nanocrystals, exhibits large shifts of fluorescence maximum with
size and strong SHG signals. When embedded in polymeric or biopolymeric matrix DNA-CTMA shows efficient
amplified spontaneous emission. The unique properties of this compound in its various forms from molecules, ONC's to
macroscopic single crystals are studied and discussed.
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 present results of studies of the systems containing photochromic molecules, for all-optical switching and amplified
spontaneous emission applications. The systems consisted of: a) deoxyribonucleic acid doped with different
photochromic molecules like Disperse Orange 3 or spiropyranes, and b) photochromic molecules of 4-heptyl-4'-
methoxyazobenzene showing nematic liquid crystalline properties close to the room temperature (above T = 34° C).
Experiments of dynamic birefringence switching were done in Optical Kerr Effect set-up, where for the sample
excitation chopped cw or picosecond pulsed lasers were used. An excellent switching times and reversibility of the
studied processes have been observed. The amplified spontaneous emission in luminescent dye doped biopolymeric
system was achieved under the sample excitation by UV light pulses (355 nm) coming from pulsed Nd:YAG laser.
All optical switching has been studied using the Optical Kerr Effect (OKE) configuration in a biopolymer matrix
containing a photochromic molecule. The biopolymer system consisted of a deoxyribonucleic acid blended with cationic
surfactant molecule cetyltrimethyl-ammonium chloride suitable for optical quality thin film fabrication. The excitation
beams inducing birefringence were delivered from a continuous wave laser at 473 and chopped using a variable
frequency chopper. Additionally auxiliary nanosecond pulses coming from Nd:YAG laser were used. The birefringence
was instantaneously monitored by a weak non-absorbed light from a cw He-Ne laser working at 632.8 nm under crossed
polarizer system. Excellent switching times in the range of microseconds and full reversibility of the studied processes
have been observed.ïýïýïý
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 present results of the amplified spontaneous emission (ASE) in the system based on dye-doped modified DNACTMA
(deoxyribonucleic acid with cetyltrimethyl-ammonium chloride) and lasing in distributed feedback Bragg (DFB)
laser composed of a two-layer system: a layer of modified (DNA) polymeric matrix containing dye superimposed on a
periodic relief structure formed in photochromic polymer layer. As photoactive and luminescent dye we used the wellknown
Rhodamine (Rh 6G). This layer covered a specially designed photochromic polymer layer in which a surface
relief grating (SRG) was inscribed by holographic method in order to form a Bragg reflector for photons. Thin film of
the DNA-CTMA:Rh6G/photochromic polymer was excited with 6 nanosecond laser pulses at λ = 532 nm wavelength.
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.
We present results of possible applications of a modified DNA-dye system for lasing and dynamic optical information
recording. The system consisted of bio-polymeric matrix made of deoxyribonucleic acid (DNA) substituted with cationic
surfactant molecule cetyltrimethyl-ammonium chloride (CTMA) and doped with a photochromic (Disperse Red 1) or
fluorescent (Rhodamine 6G) molecules.
Results of optical information recording were obtained in a typical degenerate two wave mixing experiment. For sample
excitation we used a linearly polarized light of λ = 514.5 nm delivered by an argon ion (Ar+) laser. For amplified
spontaneous emission measurements we used 6 ns pulses of 532 nm wavelength delivered by Nd:YAG laser doubled in
frequency.
Depth and profile information of one or two-dimensional photonic crystals can be obtained through
measurements of reflective diffractive patters obtained from the structures and subsequent numerical analysis. The
technique is known as a scatterometry. The method is non-invasive and fast, and competitive to the alternatives of AFM,
SEM etc. In our paper we presented results of investigation 1D photonic crystal fabricated in GaN with period
Λ = 400 nm, fill factor ff = 50% and depth d = 400 nm. Using computer algorithm of Rigorous Coupled Wave Analysis
(RCWA) and measuring diffracted light we extracted the profile parameters of Λ = 420 nm, ff = 51%, d = 400 nm.
Possibility of application of our method for analysis 2D photonic crystals is discussed also.
In this paper we present results of possible applications of a modified DNA-dye system for dynamic processing of
optical information like phase conjugation or optical correlation. The system consisted of bio-polymeric matrix made of
deoxyribonucleic acid (DNA) substituted with cationic surfactant molecule cetyltrimethyl-ammonium chloride (CTMA)
and doped with a photochromic Disperse Red 1 dye. Fast dynamics (a single millisecond rise and fall times) of
diffraction grating formation were obtained in a typical degenerate two or four wave mixing experiments. For sample
excitation we used a linearly polarized light of λ = 514.5 nm delivered by an argon ion (Ar+) laser. Complete reversibility
of the signal generation with no residual light diffraction even after longer time exposures (up to few hours) was
observed.
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.
Third-order nonlinear optical properties were investigated for the system incorporating the Disperse Red 1 dye and the deoxyribonucleic acid - cetyltrimethylammonium complex (DNA-CTMA). The interaction of the dye with the DNA chains does not lead to major changes of the nonlinear optical effects due to the DR1 dye. Polarization dependences of the nonlinear absorption in this system reveal, however, some indications that the interactions may lead to ordering of the dye molecules against the DNA chains.
DNA is emerging as a novel exciting photonic polymer material due to its unique double-helix structure and the ability to
act as a host capable to be aligned itself and capable of inducing orientation of nonlinear optical (NLO) chromophores.
Physical and optical properties of DNA are remarkably modified with the alteration of the nucleic acid counter-ions. We
determined optical properties of salmon-derived DNA and DNA complexed with cetyltrimethyl-ammonium (CTMA)
surfactant in solutions and films. Absorption coefficients derived for an average nucleotide formula weight indicated
DNA of high purity. Prism coupling measurements showed a large birefringence in refractive indices in the direction
parallel and perpendicular to the surface plane of the films indicating anisotropic alignment of DNA molecules. Almost
isotropic refractive indices were measured in DNA-CTMA films indicating disorder in orientation of DNA-CTMA
molecules in the films. Doping with about 5 wt% Disperse Red 1 (DR1) essentially did not change this very weak
birefringence in the DNA-CTMA films. Optical properties of DNA films were sensitive to environmental humidity while
the DNA-CTMA films were less susceptible to it. The Z-scan technique using femtosecond pulsed laser system was
employed to determine the NLO properties of DNA in solutions in the 530-1300 nm wavelength range.
Two-dimensional (planar) photonic crystal waveguides give a possibility to propagate a light beam at narrow angles with small or no energy losses. Line and point defects introduced into the lattice modify the photonic structure of the crystal, which further leads to the possibility of designing more advanced integrated optical structures, such as strip waveguides, splitters or emitters. In our research we adopted Electron Beam Induced Deposition technique to produce the point and the line defects in a photolithographic pattern of a photonic crystal. First, we produced a pattern of holes in a positive photoresist film by two-beam interference lithography1. Then we utilised EBID technique to fill the selected holes, by adopting SEM Hitachi S 570 device. As a process precursor we used diluted vapour of trimethylpentaphenyltrisiloxane, which is the dominant constituent of diffusion pump oil2.
Focused electron beam locally decomposes precursor molecules, which leads to solid material deposition. Composition of deposited structure is a mixture of amorphous carbon and some polymers. By the beam scanning in a line mode, the line of carbon can be deposited. Such a line defect in photoresist can act as a protecting mask during the further etching process. This controllable and high-resolution method can be used to fabricate W1, W2 and W3 types of channel waveguides. The best EBID resolution obtained in the selected setup gives lines with width of 15-25 nm.
The use of the holographic lithography method for sub-nano pattering of photoresist layer deposited on bare sapphire substrate as well as on GaN grown by metaloorganic vapour phase epitaxy on Al2O3 is reported. Positive photoresist Shipley SPR700 was first diluted with photoresist thinner and then spin-coated on prepared substrates to obtain layers of final thickness of 227nm. Thin photoresist layer was exposed in the holographic setup with wavelength of 355nm to produce the surface relief grating. After development SEM observations reveled well-defined valleys and ridges of diffraction grating in SPR700 deposited on gallium nitride layer whereas the whole structure on sapphire was strongly affected by the speckles created by reflection from the unpolished back surface of the sapphire substrate. Latter, we confirmed with transmission spectroscopy, that even small amount of light transmitted through the substrate, which is back reflected by the unpolished back-surface of sapphire, canstrongly disturb nano-sized features in photoresist.
Light-induced optical anisotropy have been studied by laser grating inscription method within the group of novel azobenzene functionalized polymers. Undergoing light-induced trans-cis-trans photoisomerisations azo-benzene groups were covalently bonded to the main chains of polymers. Two-wave mixing technique was employed to inscribe the amplitude and phase gratings in these polymers. Using two light polarization geometries s-s an s-p we were able to study differences in grating properties as well as ability of surface relief grating formation in studied class of polymers. The supplementary technique of moving grating was used as a tool for obtaining information on contribution of various reorientations of chromophores and main chains to light diffraction on the obtained periodic structures.
Novel azobenzene functionalized poly(amide-imide)s (PAIs) having azo-benzene group in side chain of polymers were prepared. FTIR spectroscopy and elemental analyses confirm structure and the existence of azo-benzene chromophores in obtained polymers. The relationship between structure of polymers and their physical and optical properties was studied. Thermal behaviour such as glass transition temperature and thermal stability were measured. The influence of the main chain structure of poly(amide-imide)s on polarization gratings recording using standard degenerate two-wave mixing technique with 514.5 nm Ar+ laser line has been investigated.
Application of photonic crystals in the future photonic integrated circuits (PICs) is one of the most interesting issues in modern photonics. With the photonic-crystal-based PICs it will be possible, at last, to realize compact in size, multicomponent optical integrated circuits. Nanostructured materials with ordered arrays of holes or rods are practical realization of photonic band-gap concept. In this paper we present a fabrication method for periodic arrays with openings of arbitrary shape and size. The method is based on exposition of thin photoresist film with two interfering laser beams 3rd harmonics of Nd-YAG laser source and positive photoresist has been used. Two examples of periodic structures are described here: obtained with 75° and 90° substrate rotation. In both cases the starting structure was one-dimensional diffraction grating with period of 1.26 μm. Two different patterns has been obtained in photoresist as a result of the test exposures. In one case nearly circular openings with diameter of 780 nm has been obtained. Periodicity of the resulting array, grid pattern, shape and size of the openings can be varied by adjustment of exposure parameters which can lead to fabrication of two-dimensional photonic crystal.
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.
Dynamic optical reconstruction of digital binary holograms projected on optically addressed liquid crystal spatial light modulators with the use of computer driven multimedia projector is described. The high spatial resolution, sensitivity and full reversibility of manufactured spatial light modulators is achieved by the use of photoconducting PVK:TNF polymer layer serving as a transducer of incoming light intensity pattern into modulation of refractive index inside adjacent LC layer. Linearly polarized laser light reconstructs the phase holograms at the video-rate. The advantages and drawbacks of the presented system are discussed.
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.
Simple liquid crystal panel equipped with a polymeric photoconducting layer can be used for displaying dynamic holographic images. It is sufficient to compute the hologram of the object and reconstruct the wavefield optically. This can be done by projection of the binary hologram onto liquid crystal panel with the help of standard video-projector. Illumination of the photoconducting polymeric layer by a white light interferogram leads to tiny molecular rearrangements within the bulk of the liquid crystal layer which form a refractive index grating. They occur as a result of spatially modulated electric space charge field produced in a polymer. Short holographic films displayed at video-rates are achievable with the system based on PVK:TNF polymer and planar nematic liquid crystal mixture. The underlying electrical and optical processes as well as characteristics, performances and limitations of the system are discussed.
The dynamic holography method is applied to determine the response time for trans-cis-trans izomerization process in DR1 chromophore embedded in a solid polymer matrix. The experiments were performed on free standing films using a pulsed ps laser for grating writing and a cw laser for it reading. It is found that the writing process may be well described by a mono-exponential curve with a time constant below ms while the decay process is more complicated and can be approximated by a bi-exponential equation with two response times, significantly larger than for the writing process.
In this paper we present experimental results of study of liquid crystal panels (LCP) designed for dynamic holography, with new photoconducting and photorefractive elements as their parts. We used either microcrystals or photochromic molecules which were added to the liquid crystal mixture, alternatively photoaligning polymers or polyvinyl carbazole doped with trinitroflourenone (PVK:TNF) and polyoctylthiophene as photoconducting layers were employed. Studies of light diffraction efficiency (η) were made in a typical degenerate two-wave mixing experiments (DTWM). We report here results of maximum diffraction efficiencies obtained for different type of LCPs. The highest η was measured in LCP with PVK:TNF layer (η=32%) and the lowest were reported for LCP containing microcrystals (η=0.01%). Best of developed LCPs were used as media for dynamic holographic applications. Elimination of phase distortion in degenerate four-wave mixing (DFWM) experiment, reconstruction of binary holograms and optical correlation are only few representative examples of applications demonstrated recently in our laboratory.
The performance of photorefractive hybrid panels made from photoconducting polymer ane nematic liquid crystal layers are reviewed and discussed. Their properties were studied under pulsed and cw illumination. The dynamic holography experiments with pulsed laser beams give information on the charge mobility in the thin photoconducting layers. The use of panels to visualize the phase objects introducing small distortions to plane wave front (such as e.g. turbulence of air) through the well-known Zernike filtering is also shown. The advantage of such nonlinear Zernike filter over conventional one is that no precise optical adjustment is necessary and the filtering is relatively easy to control by tuning the externally applied field to the modulator and/or varying the incoming light intensity.
In this paper we present experimental results on the influence of an external light on phase grating recording process in nematic liquid crystal cell with a photoconducting polymeric layer (PVK doped with TNF). Depending on voltage applied to the cell the external light can amplify on attenuate the diffracted into first order light power measured in degenerate two-wave mixing experiment. From the time delay between the opening of the external light and the moment of diffraction signal change we deduce an information about the effective charge carriers mobilities in PVK:TNF polymeric layer. We also discuss and present simple explanation of the observed effect.
In this paper relation between the diffraction efficiency in LC dye doped cell in two wave mixing system and the applied voltage parameters had been described. The goal of this work was increase of diffraction efficiency using low frequency AC voltage. The LC cells used in the experiments were filled with pure and dye-doped liquid crystal mixtures. In this system we obtained diffraction efficiency increasing about five to eight times.
In the present work we focused our attention on studies of PVK:TNF hybrid polymer liquid crystal panels under short pulse laser illumination conditions. The diffraction gratings in a LC panel were induced by crossed beams generated by doubled in frequency Nd:YAG laser ((lambda) equals 532 nm) delivering pulses of 20 ps duration. So induced gratings were read by a cw laser radiation coming from a weak power He-Ne laser working at (lambda) equals 632.8 nm. The temporal evolution of intensity of first order diffraction measured in PVK:TNF hybrid liquid crystal panels shows many interesting features and complexity dependent on various experimental conditions. The substantial diffraction is observed already in time less than 1 ms after the pulse and the grating decay is completed within hundreds of milliseconds. At least three different steps of grating build-up can be distinguished which depend in various ways on the experimental conditions. A tentative mechanism of the observed responses is discussed in connection with the photoconductive properties of polymeric layers and the optical and electrical properties of the used liquid crystal E-7 (Merck).
The possibility of diffraction pattern generation in LC (Liquid Crystal) cells working as a dynamic write-read media was proven. Recording of holographic gratings formed by interfered He-Ne laser beams onto LC-cells with pure and dye-doped mixture have been investigated. The possibility of optoelectronic reconstruction of digitally stored microinterferograms allow us to adopt them in multimedia application. The hologram of virtual or real object was reconstructed by coherent illumination of optically addressed spatial light modulator (OASLM) containing holographic grating. The results obtained from LC cells of standard twisted nematic type were examined. As a result of examination of physical properties of nematic-type liquid crystal mixtures, new method of realization of LC cells were proposed. The experimental results of replacing TN cells with polivinylcarbazole (PVK) photosensitive layer cells have been discussed.
A novel liquid crystal panel suitable for real-time holography and exhibiting a very high exponential gain coefficient is presented and discussed. It consists of a nematic liquid crystal layer sandwiched between photoconducting polymeric layers. Under exposition to sinusoidal light intensity pattern the panel shows an efficient formation of the refractive index grating. Three different photoconducting polymers were used: poly(3-octyl thiophene), poly(3-octyl thiophene) functionalized with disperse red #1 and polyvinyl carbazole (PVK) doped with TNF. The first order diffraction efficiency, measured in degenerate two wave mixing experiments, depends on the photoconductor used and reaches 44% with PVK:TNF. The panel shows also the ability to switch energy from beam to beam. In the two-wave coupling experiment multiple orders of diffraction are present and a very high two-beam coupling gain is obtained. The largest value of net gain g equals 12 and the corresponding exponential gain coefficient (Gamma) equals 3700 cm-1 were again observed with PVK:TNF. This was achieved in samples biased by a dc external electric field and tilted with respect to the beam incidence bisector at 15 degrees. The time constants of grating formation and its erasing in the studied system depend on the applied voltage and can be made as short as few milliseconds in favorable conditions.
We have described the relation between LC parameters and induced diffraction gratings recorded by light intensity pattern in dye-doped nematic liquid crystals sandwiched between two coating glass plates covered with ITO conductive and polyimide orientation layers. In our investigations we used the LC cells containing planary oriented nematic mixtures (low-angle rubbing, tilt 1.5 - 2%) doped with the antraquinone dyes (0.1 - 1% w/w). Nematic liquid crystal mixtures with different electrooptical threshold, optical anisotropy, viscosity and conductivity have been investigated. The influence of resistivity, optical anisotropy of LC mixtures and polyimide layers on diffraction efficiency and other parameters have been discussed.
Mechanism of holographic dynamic grating recording in a thin cell with dye doped nematic liquid crystal under laser light has been proposed. Nematic liquid crystal with dye is sandwiched between two coating glass plates covered by ITO conductive and polyimide orientation layers. In this experiment, two interfering laser beams induced spatial modulation of refraction index and form interference pattern in the liquid crystal cell. The recording mechanism is linked with DC electric field driven reorientation of nematic director and induces the bulk photoconductivity of the system.
The high-performance photorefractive polymer hybrid liquid crystal structures are investigated in detail by use of optical tow-wave mixing technique. In such structures an amplification of laser light can be realized by two-beam coupling mechanism. Photorefractive properties of the structures rely on the spatial light induced charge carrier density modulation in thin film of photoconducting polymer. The spatially modulated space-charge field induces efficient reorientations of molecules of adjacent to polymeric layer nematic liquid crystal forming, in this way, the refractive index grating. Efficient energy transfer between the incident and the higher-order diffracted beams has been observed in dynamic self-diffraction process on thin phase gratings at oblique light incidence. The phase shift between the light intensity pattern the light intensity pattern and the refractive index modulation grating, responsible for the energy exchange, can be controlled by the applied electric field enabling reversal of direction of energy flow. Energy transfer and diffraction efficiency are found to exhibit different response characteristics with fringe spacing, voltage, cell parameters and light modulation-depth. The net exponential gain coefficient reported for the structures amounted to (Gamma) approximately equals 2600 cm$_-1) and was obtained at driving voltages of the order of 1 V/micrometers .
Self-diffraction of light on transient holographic index gratings has been measured in dye-doped twisted nematic liquid-crystal cells. The mechanism of gratings formation is connected with bulk photoconductivity which results in electric field induced torques causing local director axis reorientations. Time evolution of grating recording, voltage and polarization-configuration dependencies were examined. In the self-diffraction experiments the beam coupling effects were observed.
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