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.
Phototropic liquid crystals (PtLC), in which the phase transition can be controlled by the light, are a new class of liquid crystal materials possessing number of potential applications, especially in photonic devices. So far a significant majority of PtLC materials has been realized by the doping a classical liquid crystal with a photochromic dye. Here we report PtLCs comprising a single compound. Liquid-crystalline and photochromic properties have been accomplished in alkylo-alkoxy derivatives of azobenzene. Such compounds show a rich polymorphism which can be controlled by the light. The phenomenon of the photochemical phase transition has been investigated by means of holographic grating recording.
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.
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 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.
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.
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 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).
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.
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 .
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