Degenerate Two-wave Mixing Research Articles

Ultraslow light propagation in photorefractive SBN:75

Slowing down of light pulses to ∼1.9 mm/s has been obtained theoretically in SBN:75 via degenerate two-wave mixing (TWM) configuration. The optical delay has been tuned by varying the input pulse width & beam crossing angle. At the optimum beam crossing angle (2θ ∼ 10◦), very high TWM gain ∼107 has been achieved. Maximum fractional delay (FD) of 1.04 and 0.20 is obtained for 150 ms pulse in case of SBN:75 and SBN:60 respectively. Further, the optical delay, group velocity, FWHM, pulse broadening and amplification in case of SBN:75 is compared with SBN:60. SBN:75 showed minimal pulse broadening, large amplification (∼105), five times more FD and two degrees of higher optical delay than that of SBN:60.

Modeling of Nonlinear Optical Phenomena in Host-Guest Systems Using Bond Fluctuation Monte Carlo Model: A Review.

We review the results of Monte Carlo studies of chosen nonlinear optical effects in host-guest systems, using methods based on the bond-fluctuation model (BFM) for a polymer matrix. In particular, we simulate the inscription of various types of diffraction gratings in degenerate two wave mixing (DTWM) experiments (surface relief gratings (SRG), gratings in polymers doped with azo-dye molecules and gratings in biopolymers), poling effects (electric field poling of dipolar molecules and all-optical poling) and photomechanical effect. All these processes are characterized in terms of parameters measured in experiments, such as diffraction efficiency, nonlinear susceptibilities, density profiles or loading parameters. Local free volume in the BFM matrix, characterized by probabilistic distributions and correlation functions, displays a complex mosaic-like structure of scale-free clusters, which are thought to be responsible for heterogeneous dynamics of nonlinear optical processes. The photoinduced dynamics of single azopolymer chains, studied in two and three dimensions, displays complex sub-diffusive, diffusive and super-diffusive dynamical regimes. A directly related mathematical model of SRG inscription, based on the continuous time random walk (CTRW) formalism, is formulated and studied. Theoretical part of the review is devoted to the justification of the a priori assumptions made in the BFM modeling of photoinduced motion of the azo-polymer chains.

Nonlinear all-optical light valves fabricated on mesoscopic Ti-, Si-substrates

Nonlinear optical properties are investigated in new composite materials. The investigated samples have been (i) a glass substrate covered by mesostructured (SiO2 or TiO2) film filled with the dye Rhodamine 6G, and (ii) a hybrid liquid crystal (LC) cell made on such mesostructured film. The nonlinear optical coefficients have been defined by two different techniques: the degenerate two-wave mixing in the regime of Raman-Nath self-diffraction, and the Z-scan method. The nonlinear optical absorption in the hybrid LC cell is strengthening under continuous laser radiation. The hybrid LC cell is perspective for applications in optical limiting and light-control-light LC valves.

On the optical bistability of ferroelectrics via two-wave mixing: Beyond the slowly varying envelope approximation

In this paper, we investigate the Optical Bistability (OB) resulting from a degenerate two-wave mixing process in a single layer of Kerr nonlinear ferroelectric material. The nonlinear process is mathematically described by a nonlinear coupled-wave (CW) equations propagating across the nonlinear medium. The system is solved numerically with and without employing the Slowly Varying Envelope Approximation (SVEA). The effects of layer thickness, operating frequency, and the intensity of the probe beam on the bistable response are investigated in both cases with and without SVEA. We have found that there is a significant difference between both solutions (with and without SVEA) in the analysis of the CW theory of two-wave mixing for OB. The SVEA solution fails to predict the bistable response for various combinations of the input parameters. Furthermore, the solution with SVEA overlooks the detection of several bistable response periods, reduces the switching contrast of the detected OB, and shows only higher threshold values of bistable response. A comparison between our theoretical results and some available experimental data is also shown.

Pulsed laser deposited barium titanate thin film for tunable optical delay application

The thin film of barium titanate was fabricated by pulsed laser deposition (PLD) technique. Tetragonal phase in the film has been confirmed by XRD and laser Raman spectra. The film thickness estimated from the analysis of the fringes observed in UV–visible spectra was ∼3.3μm. Barium titanate thin film was subjected to Optical delay studies via degenerate two-wave mixing at room temperature, using 200ns pulse of 2nd harmonic of Nd:YAG laser. Sub as well as superluminal pulse propagation has been observed by tuning the film orientation. The optical delay has been compared for BTO crystal and thin film.

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

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

Effect of Pump-Probe Polarization and Crystal Orientation on Propagation of Flat Top Pulse in ${\rm Ce{:}BaTiO}_{3}$ via Degenerate Two-Wave Mixing

The propagation of flat top pulse is studied inside the crystal via degenerate two-wave mixing as a function of various polarization states of a pump-probe beam. Subluminal as well as superluminal pulse propagation is observed by tuning the crystal orientation for S-S and P-S polarization states of a pump-probe laser beam.

Optical vortex interaction and generation via nonlinear wave mixing

Optical vortex beams are made to interact via degenerate two-wave mixing in a Kerr-like nonlinear medium. Vortex mixing is shown to occur inside the medium, leading to exchange of topological charge and cascaded generation of vortex beams. A mean-field model is developed and is shown to account for the selection rules of the topological charges observed after the wave-mixing process. Fractional charges are demonstrated to follow the same rules as for integer charges.

Effect of Pump Intensity on Slowing Down of Light in ${\rm Ce{:}BaTiO}_{3}$ Crystal via Degenerate Two-Wave Mixing Using Chopped Pulses

The slowing down of light in Ce:BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> at room temperature via degenerate two-wave mixing for chopped pulses is reported. Delay of 0.9 ms is observed for chopped pulses of duration 8 ms, corresponding to a group velocity of 5.5 m/s. Effect of pulse width of signal beam and pump intensity onto the slowing down action of light in the crystal is studied. The building up and decay time of photo grating is also recorded as a function of pump beam intensity.

Polarization Dependence of Holographic Grating Recording in Azobenzene-Functionalized Polymers Monitored by Visible and Infrared Light

The holographic grating recording process in thin films of amorphous azobenzene-functionalized polymers has been widely reported in the literature. In spite of the many reports, little is known about the mechanisms responsible for different temporal behaviors of the diffraction efficiency during long recording times. Here, we report on experimental studies of the diffraction efficiency changes during the holographic diffraction grating recording process in photochromic polymer. The gratings were inscribed for four different polarization combinations of the recording beams: s-s, p-p, s-p and right to left circular polarization (RCP-LCP) employing the degenerate two-wave mixing technique. The grating recording process was simultaneously monitored by three different wavelengths: 514.5 nm (writing) and 632.8 and 904 nm (reading). The temporal evolution of the diffraction efficiency (for all polarization configurations and for each wavelengths) was precisely fitted within the model, which assumes simultaneous formation of the absorption grating and three coupling phase gratings shifted by 0 or pi with respect to each other. Two of the phase gratings originate from the refractive index grating changes in the bulk (volume) of a material and the third one from the surface relief modulation. The model enabled us to extract relevant parameters for each grating such as the build-up time constant, its final amplitude, and the phase shifts between phase gratings. Performed studies and the discussion of results revealed the main differences in the diffraction grating formation process for s-s, p-p, s-p, and RCP-LCP polarization configurations.

Degenerate two-wave mixing via a dynamic grating in Fe3O4 nanoparticle suspensions

Optical-gradient-force-induced spatially inhomogeneous disturbances of the dielectric permittivity of a suspension of spherical nanoparticles are analysed in the Maxwell Garnett approximation. Degenerate two-wave mixing in such media is shown to cause the formation of a spatial nanoparticle grating and the associated permittivity grating in the colloid. Relations are derived for the complex coupling constant of the waves. Codirectional and contradirectional two-wave mixing in suspensions of light-absorbing nanoparticles is considered. The two-beam coupling gain at 640 nm in various suspensions of Fe3O4 nanoparticles may reach ∼10 cm-1.

Photoinduced Holographic Gratings in Azobenzene-Functionalized Poly(amideimide)s

We present the diffraction grating recording in a series of poly(amideimide)s functionalized with methyl-substituted azobenzene chromophore as side group and correlate the results with the molecular structure. Poly(amideimide)s, differing in their backbone structure and in position in which azobenzene group is attached to polymer chain, were synthesized and characterized by the spectroscopic methods. Diffraction gratings were fabricated using degenerate two-wave mixing technique with cw Ar+ laser working at λ=514.5 nm and monitored by light diffraction of low power He-Ne probe beam λ=632.8 nm in a transmission mode. Two qualitatively different configurations of the recording beam polarizations were used s-s and s-p. The dynamics of diffraction grating inscription was described by mono- or biexponential growth functions. The surface relief grating formation was also examined in poly(amideimide)s. They were studied by atomic force microscope and revealed the strong dependence on the writing beams polarization. Additionally the thermal stability of the photoinduced surface gratings was checked.

Holographic grating recording in azobenzene functionalized polymers

Abstract A group of 22 polymers have been synthesized to test their suitability for recording holographic gratings. Polyamides, polyimides, polyesters and their combinations were functionalized with pendant azobenzene groups containing single or double N=N. The polymers were studied using a standard degenerate two-wave mixing technique, which enables measurement of light-induced periodic modification of polymer refractive index and absorption coefficient by analysis of the diffracted light. Two qualitatively different configurations of the holographic polarization recording were used, s-s and s-p. The relationship between structural properties of polymer matrix and azobenzene groups and the holographic grating recording kinetics and light diffraction efficiency was investigated.

Monte Carlo Based Design of Photonic Processes in Azopolymers

ABSTRACT Monte Carlo kinetics of diffraction efficiency evolution in a process of a pulsed diffraction grating inscription in a model system consisting of a polymer doped with azo-dye is presented. A comparison between simulations and degenerate two-wave mixing (DTWM) experiment is given. A good qualitative agreement of those results supports the concept of Monte Carlo based analysis and design of temperature-dependent photonic processes in azopolymers.

Temperature Dependence of the Kinetics of Diffraction Gratings Formation in a Polymer Matrix Containing Azobenzene Chromophores: Monte Carlo Simulations and Experiment

We use the framework of recently formulated kinetic model of temperature-dependent 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 to study general features of temperature dependence of time evolution of diffraction efficiency close to the glass transition temperature T g of the polymer matrix. Bond fluctuation Monte Carlo simulations are used. Basic kinetic results found in simulations are in agreement with preliminary experimental results obtained in degenerate two-wave mixing (DTWM) experiment. We discuss, using Monte Carlo results, the concept of temperature controlled writing of diffraction gratings in a temperature interval close to Tg.

On the efficient mixed amplitude and phase grating recording in vacuum deposited Disperse Red 1

We report on two-dimensional optical storage photochromic material made of Disperse Red 1 (DR1) thin film vacuum deposited on glass substrate. Holographic mixed amplitude and phase gratings were recorded in highly organized DR1 thin molecular films with 488 and 514.5 nm cw Ar + laser light using degenerate two-wave mixing technique. Self-diffraction efficiency measured within Raman–Nath scattering limit reached 3.5%. The light scattering was mostly due to presence of periodic absorption changes (amplitude grating) as it was established from grating translation technique. Kinetics of grating recording was studied as a function of incident light intensity allowing to formulate the possible mechanism of grating build-up process in the studied film. Polarizing microscopic studies revealed the presence of light induced optical anisotropy possibly connected with heat-induced crystallization. Absorption grating profiles were analyzed in relation to exposure conditions.

Self-action of nanosecond light pulses in CdTe in degenerate two-wave mixing

Measurements are presented for the photorefractive effect in CdTe using 10-ns pulses at a wavelength of 1.06 μm. Different self-action effects that accompany two-beam coupling are discussed.

Multiple bistability in an optical fibre double-ring resonator by using degenerate two-wave mixing

A theoretical study is presented of multiple bistability in a single-mode optical fibre double-ring resonator using an all-fibre scheme involving degenerate two-wave mixing. Two successive bistable hysteresis loops (one is a normal type and another an inverting type) are formed in the input-output transmission curve. The dependence of the multiple bistability on major parameters of the fibres and the couplers is investigated.

Optical bistability in a one planar 3 × 3 coupler fiber‐ring resonator with degenerate two‐wave mixing

AbstractA theoretical study of optical bistability in a Type 1 one (planar 3 × 3) coupler fiber‐ring resonator with an all‐fiber scheme, Scheme B, of degenerate two‐wave mixing (DTWM) is presented. The bistability is caused by the DTWM, the inherent positive feedback in the fiber‐ring resonator and the nonlinear dependence of the circulating intensity of the fiber‐ring resonator on the DTWM gain. The relations between the bistability and various parameters of the fibers and the planar 3 × 3 coupler are inuestigated. A comparison between the bistability in such a jiber‐ring resonator and that using an equilateral triangular 3 × 3 fiber coupler is also made. © 1994 John Wiley &amp; Sons, Inc.

On the existence condition for bistability in a one-coupler optical fiber ring resonator by using degenerate two-wave mixing

The existence condition for bistability, B2G > 1, in a one-coupler single-mode optical fiber ring resonator using two different all-fiber schemes of degenerate two-wave mixing (DTWM) is studied in detail. It is shown that the existence condition for bistability is in general a necessary condition only, not a sufficient condition for the occurrence of the DTWM bistability. © 1994 John Wiley & Sons, Inc.