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We provide evidence that the friction of graphene films has a directional dependence in the interior and at the edges of the graphene sheets. Graphene fluoride was shown to have lower friction coefficients than graphene at almost all angles either inside or at the edges of the samples. At the edges, graphene and graphene fluoride were shown to have a dependence on the scan direction. It is unexpected with respect to all reported results as of today. We have experimentally proven that graphene and graphene fluoride manifest different coefficients of sliding friction at the edges (graphene fluoride from 5.8×10−3 to 4.9×10−1; graphene from 8.2×10−3 to 3.3×10−1) of a sheet sample versus the interior (graphene fluoride from 5.1×10−3 to 1.5×10−1; graphene from 2.5×10−2 to 2.3×10−1) under ambient humidity conditions (~60%). Atomic force microscopy (AFM) was used to prove the friction coefficients have distinct directional dependence between graphene and graphene fluoride. It is assumed that differences in friction coefficients between those in literature and our results for graphene fluoride were due to ambient experimental conditions. Further investigation of the effects of water on friction between the AFM tip and graphene fluoride is warranted.

A technique, based on amplitude modulation of Atomic Force Microscope (AFM) tips, in the absence of an applied bias voltage, is reported in this study. Under ambient humidity conditions, ultra-sharp n- doped silicon tips (spike radius ~1 nm) oscillating at 160–250 kHz generate raised nanostructures ~50–200 nm wide and ~2 nm high on undoped or p -doped Si (100) surfaces pretreated with certain neat organic solvents. The solvents in the present work are propan-2-ol, butan-2-ol, or toluene. AFM is used to characterize the nanostructures which are found to be stable for at least 96 h. It is suggested that mechanical stress associated with the oscillatory Hookean energy (~5–15 eV) of the tip promotes cleavage of residual solvent bonds on the surface. This bond cleavage, and the presence of surface defects, which may be critical in the formation of surface-solvent bonds (specifically Si–O–C or Si–C) to create the observed nanostructures, is discussed. The process described here to create raised nanostructures is distinctly different from all other reported techniques to date.

We report a novel and easily accessible method to chemically reduce graphene fluoride (GF) sheets with nanoscopic precision using high electrostatic fields generated between an atomic force microscope (AFM) tip and the GF substrate. Reduction of fluorine by the electric field produces graphene nanoribbons (GNR) with a width of 105-1,800 nm with sheet resistivity drastically decreased from >1 TΩ·sq. -1 (GF) down to 46 kΩ·sq. -1 (GNR). Fluorine reduction also changes the topography, friction, and work function of the GF. Kelvin probe force microscopy measurements indicate that the work function of GF is 180-280 meV greater than that of graphene. The reduction process was optimized by varying the AFM probe velocity between 1.2 μm·s -1 and 12 μm·s -1 and the bias voltage applied to the sample between -8 and -12 V. The electrostatic field required to remove fluorine from carbon is ~1.6 V·nm -1 . Reduction of the fluorine may be due to the softening of the C-F bond in this intense field or to the accumulation and hydrolysis of adventitious water into a meniscus.

Direct experimental measurements of the thermo-optic for fixed temperature intervals (20–200 °C, 200–500 °C, 500–660 °C, 660–780 °C) in fused silica fiber containing fiber Bragg gratings (FBGs) were conducted. The diffraction efficiency of a FBG fluctuated with temperature between 2.01 × 10 − 4 and 0.17 × 10 − 4 while the temperature shift of the Bragg's peak was monitored between 1300 and 1311 nm with sub-Angstrom precision. Numerical simulations were focused on FBG's diffraction efficiency calculations accounting for the temperature drift of the gratings, and found to be in excellent agreement with obtained experimental data. It was found that the first-order thermo-optic coefficient changes between 1.29 and 1.85 × 10 − 5 K − 1 for the linear fit and at T = 0 °C its value was found to be close to 2.37 × 10 − 5 K − 1 for the polynomial fit of experimental data. The average thermo-optic coefficient undergoes a minimum in the vicinity of 440 °C. Additional observation indicates a negative sign of the second-order thermo-optic coefficient. The value of thermal expansion coefficient was much less (0.5 × 10 − 6 K − 1 ) than that for the average thermo-optic coefficient. Based on the energy dispersive spectroscopy it was determined that thermal erasing of the FBGs at a temperature around 780 °C corresponds to germanium monoxide diffusion out of core in silica-based fibers.

A nanolithography technique based on the lateral displacement of electrically biased AFM tip was developed for nanostructures formation of 30–100 nm in width and 1–10 nm in height in the polystyrene (PS) films. It was demonstrated that the nanostructures patterned in annealed PS films (90K Mw) show slow exponential relaxation between 55 and 265 h depending on their size. Relaxation of the nanostructures in non-annealed films usually occurred in minutes. It was observed that in the annealed samples a negative electric charge accumulated in the areas where the nanostructures formed while in the non-annealed samples only the positive charge in exposed areas was detected using the electric force microscopy. After 320 h of monitoring under the humidity maintained between 25 and 27% it was suggested that slow dynamical changes of the nanostructures can be attributed to the negative electric charge dissipation in the annealed samples.

A time-resolved method for tip’ retraction at μs-scale away from dielectric surfaces has been developed. Analysis of the forces in the system comprising AFM tip, water meniscus, and polymer film suggests that an electrostatic repulsion of the tip from the surface in the double-layered (water and polymer) system, and water condensation in the tip–surface junction are the dominant factors enabling the mechanical work for tip retraction. Nanostructures of 5–80 nm height are formed in polymeric surfaces as a result. This interesting physical phenomenon could be used for nanostructures patterning in polymeric materials at enhanced aspect ratio.

A minor polymer was deformed into lamellar and fibrillar morphological forms in a chaotic mixer, which rendered the resultant immiscible blend electrically conductive along the flow direction. This was demonstrated using a blend consisting of 10 wt% polypropylene (PP), polyamide 6 (PA6), and 1 wt% conductive carbon black (CB) particles. It was found that PP-phase containing CB particles deformed into lamellar and fibrillar morphological forms produced continuous networks in the flow direction, and provided conductivity by double percolation. Breakup of PP fibrils into droplets destroyed the continuous conductive networks, although conductivity was sustained purportedly due to migration of CB particles from the bulk to the surface of closely spaced PP droplets. This was augmented by the formation of much smaller PP droplets in the presence of CB particles. On continued mixing, the blend eventually turned into insulator as CB particles migrated from the polymer–polymer interfaces to PA6 phase. POLYM. ENG. SCI., 46:19–28, 2006. © 2005 Society of Plastics Engineers

We report the observation of anomalously high currents of up to 500µA during direct oxide nanolithography on the surface of n-type silicon {100}. Conventional nanolithography on silicon with an atomic force microscope (AFM) normally involves currents of the order of 10−10 –10−7 A and is associated with ionic conduction within a water meniscus surrounding the tip. The anomalous current we observe is related to an electrical breakdown resulting in conduction dominated by electrons rather than ions. We discuss the electron source during the AFM-assisted nanolithography process, and the possibility of using this breakdown current for nanoscale parallel writing.

Full-length single-walled carbon nanotubes (SWNTs) were rendered soluble in common organic solvents by noncovalent (ionic) functionalization of the carboxylic acid groups present in the purified SWNTs. Atomic force microscopy (AFM) showed that the majority of the SWNTs ropes were exfoliated into small ropes (2−5 nm in diameter) and individual nanotubes with lengths of several micrometers during the dissolution process. The combination of multiwavelength laser excitation Raman scattering spectroscopy and solution-phase visible and near-infrared spectroscopies was used to characterize the library of SWNTs that is produced in current preparations. The average diameter of metallic nanotubes was found by Raman spectroscopy to be smaller than that of semiconducting nanotubes in the various types of full-length SWNT preparations. This observation sheds new light on the mechanism of SWNT formation.

Mechanistic aspects of the reductive dehalogenation of trichloroethylene using zerovalent iron are studied with three different surface characterization techniques. These include scanning electron microscopy, surface profilometry, and atomic force microscopy. It was found that the pretreatment of an iron surface by chloride ions causes enhancement in the initial degradation rates. This enhancement was attributed to the increased roughness of the iron surface due to crevice corrosion obtained by pretreatment. The results indicate that the “fractional active site concentration” for the reactive sorption of trichloroethylene is related to the number of defects/abnormalities present on the surface of the iron. This was elucidated with the help of atomic force microscopy. Two possible mechanisms include (1) a direct hydrogenation in the presence of defects acting as catalyst and (2) an enhancement due to the two electrochemical cells operating in proximity to each other. The result of this study has potential for further research to achieve an increase in the reaction rates by surface modifications in a practical scenario.

The placement accuracy and resolution of direct-write patterning tools, in particular the atomic force microscope (AFM), is considered for application to fabricating multi-passband integrated optical niters. Because of its simpler fabrication a grating structure is proposed that consists of identical stripes that are non-periodically spaced. The recently developed pseudorandom encoding method from the field of computer generated holography is modified to effectively assign analog reflectances at each point along the grating by selective withdrawal and offsetting of the stripes from a periodic spacing. An example filter designed by this method has two 1.5 nm bandwidth passbands and –23 dB of rejection for lightly coupled stripes. As with single band filters, the passbands broaden as the coupling increases. A calculation of the coupling coefficient of stripes on a fundamental mode, slab waveguide indicate that stripes on the order of 100 nm in depth and width support low insertion loss, multipassband filtering applications at visible wavelengths. Lines of these dimensions patterned with an AFM on (110) silicon indicates the feasibility of fabricating these filters. These conclusions are specific to current AFM’s that are limited to writing fields of 100 μm. Increased rejection and decreased passband widths will result from incorporating precise field stitching into future AFM’s.

linesymm can be generated which are beyondthe resolution limit of light-optical investigation methods. We report on the first direct experimentalobservation of such higher harmonics in deuterated (PMMA) by the nondestructive method of diffractionof neutrons at a wavelength of 1.2 nm. Diffraction efficiency of the second harmonic was found to be200–500 times weaker than that of the first order. The origin of higher harmonics is attributed to thenonlinearities of the processes of chain growth and of termination. [S0031-9007(98)05599-9]

SiO20 crystals in the diffusion regime. We find unusually high cross-talk for two gratings with close spatial frequencies at wavelengths 488 and 476 nm. The results indicate that the density of free charge carriers does not directly follow light modulation at low spatial frequencies.

A new experimental method for the study of resonance effects and space-charge wave excitation in photorefractive Bi 12 SiO 20 crystals by using a combination of frequency detuning and phase modulation technique has been developed. The accuracy of the method allows a detection of resonance peaks of diffraction efficiency within 0.5 Hz. Numerical simulations of the nonlinear differential equations describing the behaviour of the space-charge waves in photorefractive crystals have been performed and found to be in a good agreement with experiment. We have measured the photocurrent through the crystal and revealed its resonance dependence. A minimum of electric current through the sample corresponds to the main resonance detected by phase modulation technique.

It is well known that when two beams (object and reference) are incident on a recording material, high order diffraction may be observed during both the writing and reading of the hologram. These higher or non-Bragg orders contain the phase conjugate and the phase amplified versions of the object. In this talk, the utility of Bragg and non-Bragg orders during holographic construction and reconstruction using photorefractive materials and photothermoplastics is investigated with applications to object shape determination and deformation. Specifically, we discuss applications of non-Bragg orders in the 3d reconstruction of objects, and holographic interferometry using Bragg order to determine object attitude and deformation. The advantages of using non-Bragg orders (viz., phase conjugate and phase doubling) are also discussed.

In this paper we derive the underlying equations for real time holographic interferometry through selfdiffraction, and demonstrate experimentally on how to employ the different Bragg and non-Bragg diffraction orders to perform one shot determination of the phase deformation, or equivalently, the depth information of a deformed object. We also show that the various diffracted orders may be phase shifted with respect to each other with a constant phase shift which depends on the phase mismatch and the length of the recording material. This technique can therefore have an obvious advantage over traditional phase-shifting holography due to the elimination of the piezo-shifted mirror (inducing the different phase shifts) which limits the traditional technique to slow deformations.

Amplitude modulated electrostatic lithography using atomic force microscopy (AFM) on 20–50 nm thin polymer films is discussed. Electric bias of AFM tip increases the distance over which the surface influences the oscillation amplitude of an AFM cantilever, providing a process window to control tip-film separation. Arrays of nanodots, as small as 10–50 nm wide by 1–10 nm high are created via a localized Joule heating of a small fraction of polymer above the glass transition temperature, followed by electrostatic attraction of the polarized viscoelastic polymer melt toward the AFM tip in the strong (108–109 V/m) nonuniform electric field.

Fiber Bragg gratings (FBGs) embedded in conventional fibers may serve as temperature sensors over a wide temperature range and withstand temperatures around 1200 K. A variety of linearly polarized (LP) modes for the wavelengths between 400 and 700 nm may be sustained in fibers with and without FBGs. The composition of the LP modes and their competition is instrumental for understanding physics of thermo-optics and thermal expansion effects in silica-based fibers. The first objective of this work was to model mathematically the competition between LP modes and modal distribution using the solutions of Bessel equations for the fibers with and without the gratings. Computer generated modes were constructed and the cut-off V-numbers (and Eigen values W and U ) were determined. Theoretical results then were compared with experimental observations of LP modes for two separate ranges of temperatures: 77– 300 K and 300-1200 K. To study the formation of LP modes over the first temperature range, liquid nitrogen was used to cool down the fiber and a thermocouple was used to monitor the temperature of the fiber. Real time recording of the modal structure was performed using digital imaging and data acquisition instrumentation. To study LP modes between 300– 1200 K, the fibers were inserted into a tube furnace with temperature control. The wavelength of the infrared radiation was reflected by a FBG and detected by an optical spectrum analyzer. Radiation at the visible wavelength propagated through the fibers, and transmitted visible light was collected, analyzed and recorded with a CCD camera to monitor distribution of the LP modes in the samples with and without the FBGs.

Dynamical sensing based on combination of classical optical effects and atomic force microscopy (AFM) presents challenge for analysis of the forces at the nanoscale and beyond. An interference effect between light reflected from an AFM cantilever and highly reflective silicon surface of the calibration grating was studied for relative humidity (RH) varied between 9 and 60%. Force-distance analysis indicates on separation of capillary, van der Waals, adhesion, and electrostatic forces. The measurements performed in contact AFM mode suggest that the period of interference pattern observed in displacement curves is a function of humidity and varies between 293 nm at RH = 9% and 335 nm at RH > 50% with standard deviation less than 8 nm. Clear change of the interference period suggests that other than hardwarerelated factors may be involved in the formation of the interference in force-distance curves.

We report an approach for monitoring tip-surface interactions. The approach is based on power spectrum analysis of atomic force microscope (AFM) tip oscillations during scanning probe nanolithography on Si surfaces. A single mode harmonic oscillator model allows us to determine the main characteristics of the AFM tip-surface interaction: the amplitude of oscillation, the resonant frequency, and the damping factor, during the writing process. We measure these quantities in scanning probe anodization versus the bias voltage. By fixing the length of lithographically patterned lines, and the energy deposited into each line, we search for trends which may reveal the major factors controlling the quality of AFM-written nanostructures. Our data are consistent with the concepts that a water meniscus and electrostatic tip-surface interactions dominate contact AFM lithography.

A pulsed dye laser with an intracavity photorefractive element is reported. It is shown that the element can be used for storage of spectral information. New effects for this system such as spectral equalization and oscillation-frequency autosweeping are demonstrated.

Phase-conjugate images in the forward direction are obtained by a new type of four-wave interaction and by the Heer-Griffin type of three-wave interaction. The nonlinear medium is bismuth silicon oxide in which the interaction is optimized by detuning the input beam(s). Measurements are made for several test objects and interbeam angles.

Nanoscale surface modification is reported for styrene butadiene rubber using an electrically biased conducting atomic force microscope tip. Under appropriate bias conditions, the local electric field magnitude is of the order of 10 8 ‐10 9 Vm 1 , which is sufficiently large to initiate cross-linking in the rubber. Peaklike surface features, surrounded by a circular trough and a raised ring, are created by careful and controlled retraction of the biased tip. The features’ aspect ratios can be controlled by modifying the tip retraction protocol, tip geometry, and bias voltage. Typical feature dimensions reported here vary from approximately 0.5‐10-nm high and up to several hundreds of nanometer in diameter. Although the temperature of the rubber is above the glass transition and the rubber is in a viscous state, the features are stable over a period of several days once created—which is believed to be due to cross-linking of the rubber during feature formation. Modeling of the electric field distribution in the vicinity of the tip is presented, which strongly supports the assertion that the resulting nonuniform electric field induces nanostructure formation and initiates cross-linking. A mechanism is proposed whereby source material is redistributed in the proximity of the tip/surface region to form the observed features.

A Collaborative Center in Polymer Photonics (CCPP) that utilizes the collected research expertise of the AFRL Materials and Manufacturing Directorate and The University of Akron College of Polymer Science and Polymer Engineering in the development of polymers for photonic systems has been established. The Collaborative Center was formed so as to augment the Air Force Materials Laboratory in-house photonics research through team work with The University of Akron's polymer program (ranked second in the Nation by U.S. News and World Report). To this end, research teams made up of scientists from both institutions have been set up to pursue topics of mutual interest. This was made possible by funding by AFOSR ($900 k), AFRL ($900 k), and The University of Akron ($900 k). The Center has the capability to respond rapidly to new research directions as the needs of the Air Force change. It is also in a position to leverage resources from other sources such as the State of Ohio.

Elementary volume phase holograms can be recorded in poly(methyl methacrylate) doped with a photoinitiator using a holographic two-beam set-up. The photopolymerization process of the residual monomers in the polymer matrix turns out to be inherently non-linear. Consequently, illumination with a sinusoidal light intensity pattern results not only in a grating with the fundamental spatial frequency but also with higher harmonics. As the absorption of the photodoped material steeply rises for wavelengths below 320 nm, structures with periods below about 105 nm cannot be read out by light diffraction in Bragg geometry, the limit for Laue geometry is even 160 nm. This poses the following question: Can we induce gratings by light which are below the optical resolution limit by the non-linear recording mechanism and how can we prove it?

We apply non-local density functional formalism to describe an equilibrium distribution of the water-like fluid in the asymmetric nanoscale junction presenting an atomic force microscope (AFM) tip dwelling above an arbitrary surface. The hydrogen bonding dominating in intermolecular attraction is modeled as a square well potential with two adjustable parameters (energy and length) characterizing well's depth and width. A water meniscus formed inside nanoscale junction is explicitly described for different humidity. Furthermore, we suggest a simple approach using polymolecular adsorption isotherms for the evaluation of an energetic parameter characterizing fluid (water) attraction to substrate. This model can be easily generalized for more complex geometries and effective intermolecular potentials. Our study establishes a framework for the density functional description of fluid with orientational anisotropy induced by non-uniform external electric field.

An exact analytical solution, based on the method of images, is obtained for the description of the electric field between an atomic force microscope (AFM) tip and a thin dielectric polymer film (30 nm thick) spin coated on a conductive substrate. Three different tip shapes are found to produce electrostatic pressure above the plasticity threshold in the polymers up to 50 MPa. It is shown experimentally that a strong nonuniform electric field s53108–53109 V m−1d between the AFM tip and polymer substrate produces nanodeformations of two different kinds in planar polymer films. Nanostructures (lines and dots) 10–100 nm wide and 0.1–5 nm high are patterned in the polymer films by using two different experimental techniques. The first technique relies on electric breakdown in the film leading to polymer heating above the glass transition point followed by mass transport of softened polymer material towards the AFM tip. The second technique is believed to be associated with plastic deformation of the polymer surface at the nanoscale. In this case the nanostructures are experimentally patterned in the films with no external biasing of the AFM tip, and using only the motion of the tip. This suggests an additional nanomechanical approach for nanolithography in polymer films of arbitrary thickness.

We have developed the all-optical EO-modulator using a pulsed photogalvanic power supply driven by light illumination (coherent and incoherent). Generated by ferroelectric crystal with the photogalvanic effect (Fe-doped LiNbO 3 ), electrical pulses were used for driving a standard EO-modulator, based on transversal EO effect in Bi 12 SiO 20 (BSO) crystals. Both parallel and serial connections of photogalvanic crystal (LN), BSO and oscilloscope were tested. The depth of EO-modulation was very sensitive to the impedance of the connected cables, which implies the existence of transmission-line resonances. Secondly, a more compact version of EO self-modulation is realized with green (wavelength λ= 532 nm, P = 100mW) solid-state CW laser. In this case, reflection of the CW laser from the LN crystal was modulated in time. Pulsating optical reflection was correlated with the pulsating electrical signals. We report both modes of operation: (1) as pulsed high-voltage power supply, and (2) as compact-pulsed optical modulator. We have described these pulsations using model of photogalvanic effect and ferroelectric emission.

The past decade has witnessed an explosion of techniques used to pattern polymers on the nano (1–100 nm) and submicrometre (100–1,000 nm) scale, driven by the extensive versatility of polymers for diverse applications, such as molecular electronics1,2, data storage3, optoelectronics4, displays5, sacrificial templates6,7 and all forms of sensors. Conceptually, most of the patterning techniques, including microcontact printing (soft lithography)8, photolithography9,10, electron-beam lithography11, block-copolymer templating12,13 and dip-pen lithography14, are based on the spatially selective removal or formation/deposition of polymer. Here, we demonstrate an alternative and novel lithography technique—electrostatic nanolithography using atomic force microscopy—that generates features by mass transport of polymer within an initially uniform, planar film without chemical crosslinking, substantial polymer degradation or ablation. The combination of localized softening of attolitres (102–105 nm3) of polymer by Joule heating, extremely non-uniform electric field gradients to polarize and manipulate the soften polymer, and single-step process methodology using conventional atomic force microscopy (AFM) equipment, establishes a new paradigm for polymer nanolithography, allowing rapid (of the order of milliseconds) creation of raised (or depressed) features without external heating of a polymer film or AFM tip–film contact.

We report ferroelectric electron emission (FEE) from LiNbO3 crystal under laser radiation. The FEE is detected through electric pulses sequence of 1-10 msec depending on laser intensity. A preliminary analysis of the data indicates on very low, if any threshold of laser radiation required for light conversion into the FEE.

A study was made of the influence of an amphiphilic macrocyclic nickel(II) complex on the electrical and electro-optic properties of liquid-crystalline systems based on ZhK-440I. It was shown that addition of this complex leads to a decrease in the electrohydrodynamic instability threshold and formation of unusual tetragonal ordered structures in the systems studied.

We have observed nonlinear optical and photoelectric responses in suspensions of microorganisms (different species of algae). Also we have seen a self-organization of the laser scattering in the circular rings for different tested wavelengths (633, 474, and 515 nm with power /spl sim/0.05-0.1 mW). A light beam from a He-Ne laser at 633 nm was used to look at special structures generated by a suspension of Chlamydomonas renhardtii. The incident power, 6 mW of unexpanded beam with a diameter of 2 mm, resulted in algae moving along helical trajectories away from the light spot. First, the algae began to move clockwise and then, after 1-2 min they moved counterclockwise. If a small spot was brought on the cuvette, the algae manifested negative phototaxis such that all the algae concentrated in the upper and bottom parts of the cuvette after moving away from the illuminated area. The transmission increased to about 1.3 times for the beam at wavelength 633 nm. No positive phototaxis was detected at any laser wavelength. A current of about 100-120 nA was detected under light illumination of 10 mW/cm/sup 2/ and without running fringes. After the grating began to move, there was a distinct change of electric current up to 900-1000 nA. The change of direction of running fringes resulted in the current changing its sign. The value of the current does not depend on the speed on the running fringes, which was varied from 0 to 10/sup 3/ /spl mu/m/sec. Further, there was a drop of of current with time. After 2-3 h of illumination of the cuvette with running fringes, there was a degradation of the current down to 100 nA corresponding to illumination without running fringes.

It is shown that two pump beams incident upon a bismuth silicon oxide crystal give rise to a subharmonic beam bisecting the pump beam. Simultaneously a signal beam injected between the pump beams may be strongly amplified at the expense of the pump beams. It is shown that the amplified signal beam grows with a faster time constant than the subharmonic beam and that there is competition for gain in the steady state regime.

The interaction of three forward beams in a BSO crystal is investigated under conditions when the two pump beams are anti-symmetrically detuned and the signal beam is phase modulated. For sinusoidal phase modulation the signal gain is shown to be dependent on the instantaneous frequency detuning. Single and double maxima in gain are obtained depending on the voltage amplitude applied to the piezoelectric mirror. For triangular phase modulation a slight asymmetry is found in the gain versus detuning curve.

We report the study of cross talk between two competing modulated gratings in photorefractive Bi12SiO20. Zero cross-talk technique applies to exactly measure the reduction factor in diffusion regime. We suggest that the reduction factor is associated with a long-distance photoelectron path (100 µm) in the crystal.

Experimental observations on the structure of photorefractive subharmonic domains in Bi12SiO2o (BSO) generated by a running intensity grating are described. The near field structure of the domains explains experimental observations of far field beam broadening and shift and the difference between the subharmonic beams in the beamcoupling and non-beam coupling geometries of BSO. The phenomena called ”low frequency resoance” and ”subharmonics” are described as analogous to second harmonic generation and degenerate optical parametric oscillation in ordinary χ(2)-nonlinear optics.

We have suggested a mathematical model of bio-optical nonlinearities for the suspensions of motile microorganisms (MO). Modeling of photophobic and photokinetic types of the photoresponse of MO suspensions allow us to predict effects of aggregation and 'hole burning' in the spatial distribution of MO concentration. Theoretical description of nonlinear diffraction may help with the diagnosis of the vitality of MO culture and for characterization of photomovement.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recent measurements revealing new aspects of the nature and physics of space-charge waves in photorefractive Bi12SiO20 crystals are presented. A Bessel-function family of space-charge wave resonances are shown to exist for excitation by a frequency modulated running interference pattern. It is also demonstrated that space-charge waves may continue into regions of the crystal not subjected to the driving light field. Subharmonic space-charge gratings are shown to have an internal structure that for running waves moves with a group velocity opposite the driving optical interference pattern, which is moving with the phase velocity of the space-charge field and the associated refractive-index grating. We also describe how the subharmonics evolve dynamically from a speckle-like pattern towards a regular wave structure. Finally, self-excitation of spontaneous scattered beams ins described, and it is demonstrated that space-charge waves with a long decay time and a high Q-factor are associated with this phenomenon.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Using anomalous large diffusion we report a recording of reflection type gratings in a PVK-based photorefractive polymer without any external electric field. Diffraction efficiency of the gratings was measured to be 7%. An efficient modulation of beams during two-beam coupling up to 12% was demonstrated by using phase mutilation technique. This practical scheme can be used in dynamic vibration measurements.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We provide a direct experimental observation of space charge waves in photorefractive crystals with point group 23 based on their penetration into an area with uniform light illumination. Those are the results of excitation of eigen modes of space charge filed excited by moving light fringes. We have found experimental conditions when the quality factor of the space charge waves attempts dramatical increase with respect to what current theory predicts. This leads to self- excitation of the waves and results in self-oscillation of light. The competition between space charge waves and fundamental grating results in appearance of K/2, K/4 components in the fundamental grating and to spontaneously occurring beams called subharmonics. The subharmonic beams are generated in a pattern of domains that evolve dramatically as they move through the crystal. The domains are result of non- homogeneity of the space charge waves and separated by narrow boundary regions, where the phase of subharmonics changes by pi. We experimentally prove the domains move with the group velocity of running space charge waves.

We report a direct observation of space charge waves in photorefractive crystals with point group 23 (sillenites) based on their penetration into an area with uniform light illumination. It is shown experimentally that the quality factor of the waves increases substantially with respect to what current theory predicts [B. Sturman et al., Appl. Phys. A 55, 235 (1992)]. This results in the appearance of strong spontaneous beams caused by space charge wave self-excitation.

An analytical solution of the photoconductive material equations for dynamic holographic gratings of arbitrary contrast has been obtained. A method of measuring high-contrast correlation functions has been suggested and tested experimentally. Good agreement with the analytical expression for the current at high contrast and experiment is found. An interesting effect of the holographic transversal photocurrent is predicted and a number of high-contrast-related topics are discussed.

The physical mechanisms of the light interaction with different materials are discussed. The modulation of refractive index and conductivity gives a physical basis for the different applications: Information Storage, Image Processing, Material Characterization and Control. A concept of the dual-use devices is formulated, based on the simultaneous use of the light-induced modulation of the optical and electrical material parameters. The latest results in the high-contrast dynamic holography are presented with emphasis on nonlinear interaction of the optical and space-charge waves.

We report the effect of Asymmetry of Bragg reflective band of Cholesteric Liquid CRystal caused by the excited doping molecules of 4-phenilchalkon. Two possible reasons giving the explanation of the effect are nonuniform concentration of khalkon molecules in the bulk and absorption of the molecules on the inner surfaces of the cells. The influence of nonuniform variations of pitch step and the refractive index modulation on the shape of Bragg band are studied numerically. It has been shown that the variations of pitch step are probably the reason of asymmetry of the Bragg band. The used model is based on solving the equations of the Theory of Dynamical Reflection for the wave propagates along the cholesteric director axis.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.