Photorefractive Gratings Research Articles

Enhanced photorefractive performance of polymeric composites through surface plasmon effects of gold nanoparticles.

We investigated the photorefractivity enhancement of polymeric composites by introducing gold nanoparticles (NPs). The gold NPs enhance the photocharge generation rate of sensitizers through plasmon resonance coupling achieved between NPs and sensitizers. Systematic studies show that the presence of gold NPs has increased photocharge generation efficiency, photoconductivity, diffraction efficiency, refractive index modulation, and photorefractive (PR) grating formation rate. The gold-NP-doped PR sample exhibits 2 times larger photocharge generation efficiency and photoconductivity, and 2.5 times faster PR grating formation rate compared to the control sample without the NPs.

The Direct Measurement of the Photorefractive Grating on Anisotropic Self Diffraction Using Digital Holography

In this paper, the measurement of grating period in photorefractive anisotropic self diffraction by using digital holography technique is proposed. In our experimental setup, He –Ne laser beam with wavelength of 632.8 nm has been separated and then incident on photorefractive cerium doped barium titanate crystal to produce photorefractive index grating. The transmitted probe beam, which contains phase and amplitude has been expanded and recorded on digital camera. To explore the grating periods, both phase and amplitude of the images are reconstructed by numerical process using computer. Then the grating periods in photorefractive anisotropic self diffraction have been measured. The results show grating periods, holograms and their reconstruction.

Manipulation of fast light using photorefractive beam fanning

Light pulse group velocity manipulations due to the specific dispersion of a medium (so-called “slow” and “fast” light phenomena) can be obtained on the basis of several mechanisms. One of these techniques is two-wave mixing in a photorefractive crystal. This work presents a modification of this method, exploiting the strong beam fanning in Sb-doped Sn2P2S6 crystals. Our experimental results demonstrate a “fast light” behavior of Gaussian pulses transmitted through a Sn2P2S6:Sb sample. The phenomenon is due to the beam fanning (i.e., the self-diffraction of the incident beam on self-induced noisy photorefractive gratings) that ensures a significant depletion of the input beam. Due to the relatively fast photorefractive response of the Sn2P2S6:Sb crystals this depletion occurs with times in the range of 10–100 ms, depending on the beam intensity, and the “fast light” feature is observed. The temporal and amplitude characteristics of the output beam are measured in function of the intensity and polarization azimuth of the incident beam. Besides, a negative phase shift of the periodical output beam relative to a sinusoidal intensity-modulated input beam is also obtained experimentally. It is shown that the phase and amplitude relation between the input and output periodic signals are described by a simple analytical expression that takes into account the beam fanning strength (depletion factor) and its dynamics (depletion time constant). It is also demonstrated that the pulse advance (or phase shift of the modulated signal) can be regulated by the light polarization azimuth. The advantages of the proposed method are discussed.

Photorefractive long-period waveguide grating filter in lithium niobate strip waveguide

To fabricate a long-period waveguide grating (LPWG) filter in a simple, reliable and low-cost way, a novel LPWG scheme based on photorefractive grating is designed and theoretically analyzed in a Ti-diffused strip waveguide embedded in a Ti-diffused planar \(\hbox {LiNbO}_{3}\) waveguide. By means of effective index method and coupled-mode theory, the processing conditions of the planar and the strip waveguide have been calculated. The minimum 100 % filtering length \(L=1.40\) cm and the coupling coefficient \(\kappa =112.23\,\hbox {m}^{-1}\) are obtained. The photorefractive LPWG filter can be used in coarse wavelength division multiplexing system.

Dark current and light illumination effects on grating formation during periodic long-term operation in photorefractive polymers

Photorefractive grating formation dynamics in long-timescale writing and the effects of periodic writing through the control of writing beam irradiation or electric field application were investigated using typical photorefractive polymers. Both dark current and writing beam irradiation affected grating formation dynamics. Dark current in polymers changed the effective trap density over time through deep trap filling and/or detrapping and thus affected grating formation considerably. The writing beam irradiation also affected grating development in the presence of an electric field owing to the accumulation of filled deep traps. However, grating development recovered after the elimination of the electric field freed up the filled deep traps.

Achieving enhanced gain in photorefractive polymers by eliminating electron contributions using large bias fields

Photorefractive polymers have been extensively studied for over two decades and have found applications in holographic displays and optical image processing. The complexity of these materials arises from multiple charge contributions, for example, leading to the formation of competing photorefractive gratings. It has been recently shown that in a photorefractive polymer at relatively moderate applied electric fields the primary charge carriers (holes) establish an initial grating, followed by a subsequent competing grating (electrons) resulting in a decreased two-beam coupling and diffraction efficiencies. In this paper, it is shown that with relatively large sustainable bias fields, the two-beam coupling efficiency is enhanced owing to a decreased electron contribution. These results also explain the cause of dielectric breakdown experienced under large bias fields. Our conclusions are supported by self-pumped transient two-beam coupling and photocurrent measurements as a function of applied bias fields at different wavelengths.

Photorefractive effects in polymer dissolved liquid crystal composites dopes with fullerene derivatives

We investigated the photorefractive performance of the polymer dissolved liquid crystalline composite (PDLCC), in which liquid crystalline polymer and low-molar-mass liquid crystal are miscible without phase separation, doped with three kinds of fullerene derivatives with different length of alkyl groups. The photorefractive performance was improved for the photorefractive PDLCC doped with fullerene derivatives with long length of alkyl groups. The photorefractive grating formation originates in the cooperative reorientation of the liquid crystalline director and the space charge field was estimated using the elastic continuum theory and the field for the PDLCC doped with the functionalized fullerene with longer alkyl side groups was larger than that for the PDLCC doped with conventional fullerene C60.

Interband photorefractive effect in semiconductors with hot-electron transport at arbitrary modulation depth

Hot-electron transport in photorefractive semiconductors strongly affects the recording of photorefractive grating. However, the theoretical description of this behavior has been always made under small signal approximation. In this paper rigorous numerical investigation of stationary and transient behavior of photorefractive grating induced in the semiconductor quantum wells structure is presented. The calculations were performed in the framework of the band transport model for the case of high modulation depth. The comparison between numerical and approximated analytical solutions is given as well as between linear and nonlinear transport model. Obtained results allow to verify some statement formulated in few earlier works concerning beneficial effects of electron transport nonlinearity.

Effects of transient dark currents on the buildup dynamics of refractive index changes in photorefractive polymers excited by pulsed voltage

We study the influence of a transient dark current on the buildup dynamics of photorefractive index gratings, which are excited right after the onset of a pulsed voltage, by using low glass-transition temperature photorefractive polymers under a heat-assisted condition. We conclude that the development of photorefractive index gratings is majorly controlled by changes in the transient dark current, through the suppression of a saturated photo-induced space-charge electric field. We also show that this influence can be estimated reasonably well by a simple analysis of the measured transient current traces.

High‐Performance Photorefractive Composite Based on Non‐conjugated Main‐Chain, Hole‐Transporting Polymer

AbstractThe photorefractivity of a non‐conjugated main‐chain polymeric composite, composed of electron‐rich N,N‐di‐toly‐N,N′‐diphenylbiphenyldiamine doped with 2‐{3‐[(E)‐2‐(piperidino)‐1‐ethenyl]‐5,5‐dimethyl‐2‐cyclohexenylidene}‐malononitrile (P‐IP‐DC) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM), is studied using two‐beam coupling and degenerate four‐wave mixing at 633 nm. The N,N′‐di‐toly‐N,N′‐diphenylbiphenyldiamine‐based composite shows a gain coefficient of 215 cm−1 at 80 V μm−1 with s‐polarized beams and a diffraction efficiency of 67% at 30 V μm−1, with a response time of the diffraction efficiency of 344 ms at Tg + 4.6 °C. Comparing the photorefractive grating response time of the 3,3′‐dicarbazole‐containing polymer composite with that of the N,N′‐di‐toly‐N,N′‐diphenylbiphenyldiamine‐based composite shows that the former is more than three times faster.

Photovoltaic dependence of photorefractive grating on the externally applied dc electric field

Photovoltaic dependence of photorefractive grating (i.e., space-charge field and phase-shift of the index grating) on the externally applied dc electric field in photovoltaic–photorefractive materials has been investigated. The influence of photovoltaic field (EPhN), diffusion field and carrier concentration ratio r (donor/acceptor impurity concentration ratio) on the space-charge field (SCF) and phase-shift of the index grating in the presence and absence of the externally applied dc electric field have also been studied in details. Our results show that, for a given value of EPhN and r, the magnitude of the SCF and phase-shift of the index grating can be enhanced significantly by employing the lower dc electric field (EON<10) across the photovoltaic–photorefractive crystal and higher value of diffusion field (EDN>40). Such an enhancement in the magnitude of the SCF and phase-shift of the index grating are responsible for the strongest beam coupling in photovoltaic–photorefractive materials. This sufficiently strong beam coupling increases the two-beam coupling gain that may be exceed the absorption and reflection losses of the photovoltaic–photorefractive sample, and optical amplification can occur. The higher value of optical amplification in photovoltaic–photorefractive sample is required for the every applications of photorefractive effect so that technology based on the photorefractive effect such as holographic storage devices, optical information processing, acousto-optic tunable filters, gyro-sensors, optical modulators, optical switches, photorefractive–photovoltaic solitons, biomedical applications, and frequency converters could be improved.

Formation speed and formation mechanism of self-written surface wave-based waveguides in photorefractive polymers

The formation speed of a self-written waveguide structure formed owing to the propagation of surface waves in a photorefractive polymer composite is measured. The formation speed linearly increases with the power of a laser beam. From the measurements of the dynamics of photorefractive grating and the photocurrent output of the polymer, it is revealed that the waveguide structure is formed by a single photorefractive grating, which is identical for the different power levels of the injected pump beam.

Perturbative approach to space-charge field dynamics in photorefractive semiconductors

A general perturbative approach to solving photorefractive material equations for a two-wave mixing geometry is developed. The method allows the study of both stationary and transient states of the space-charge field. As an example, solutions were given for GaAs–AlGaAs photorefractive multiple quantum well structures working in the Franz–Keldysh geometry. Approximate explicit expressions describing the recording and erasure dynamics of photorefractive gratings have been derived. The results are compared with numerical solutions of a complete set of material equations.

Talbot effect by a photorefractive volume phase grating

In our proposal a light intensity distribution generated by an incoherently illuminated planar amplitude grating is projected into a photorefractive crystal. This 3D distribution is mapped as an index refractive perturbation via the photorefractive effect thereby generating a volume phase grating. The self-imaging phenomenon in the Fresnel field of this volume phase grating coherently illuminated is theoretically and experimentally analyzed. A model to simulate this volume grating that considers the 3D light intensity distribution formed in the crystal combined with the photorefractive grating formation theory is proposed. A path-integral approach to calculate the self-image patterns which account for the inhomogeneous propagation through the photorefractive grating is employed. The experimental and theoretical results show that the self-images location coincides with that of the self-images generated by planar phase grating of the same period. Moreover, the self-images visibility depends on three parameters: the exit pupil diameter of the incoherent recording optical system, the external electric field applied on the crystal, and the crystal thickness. To study the visibility behavior, a phase parameter which includes the three mentioned parameters is proposed. The self-images visibility shows the typical sinusoidal dependence found in planar phase grating. A good agreement between theoretical and experimental results is observed.

Optical surface wave in a crystal with diffusion photorefractive nonlinearity

We consider a steady-state nonlinear photorefractive surface wave (PR SW) with TE or TM polarisation when the refractive index of the photorefractive crystal (PRC) depends on the strength of the diffusion crystal electric field emerging upon the wave propagation. We have determined the phase trajectory and transverse structure of the PR SW intensity distribution for different values of the diffusion photorefractive nonlinearity. We have investigated a photorefractive diffraction grating, which arises inthe surface PRC layer during propagation of the nonlinear PR SW.

Organic Photorefractive Materials and Applications

AbstractThis review describes recent advances and applications in the field of organic photorefractive materials, an interesting area in the field of organic electronics and promising candidate for various aspects of photonic applications. We describe the current state of knowledge about the processes involved in the formation of photorefractive gratings in organic materials and focus on the chemical and photo‐physical aspects of the material structures employed in low glass‐transition temperature amorphous composites and organic photorefractive glasses. State‐of‐the art materials are highlighted and recent demonstrations of photonic applications relying on the reversible holographic nature of the photorefractive materials are discussed.

Photorefractive gratings in C60-doped nematic liquid crystals under non-biased sinusoidal alternating electric fields

We report on the effect of the frequency of a non-biased sinusoidal alternating (AC) electric field on the optical non-linear response of 60 μm-thick homeotropic C60-doped nematic liquid crystals by using a two-beam coupling experiment. Experimental results indicate that the liquid crystal sample has two intrinsic frequencies of ∼0.05 Hz and ∼10 Hz. Below 0.05 Hz the energy transference is unstable, the first-order diffraction flashes in phase with the external AC electric field and its minimal intensity is almost zero. When the frequency is larger than ∼10 Hz, stable asymmetric energy transference, as that achieved under an external DC electric field, is obtained. In the regions between the above two frequencies, a stable asymmetric energy transference exists; at the same time, the transmitted intensities of the two writing beams are modulated by the external AC field and follow its variations.

Feedback-controlled recording and fixing of photorefractive holograms in reflection geometry on lithium niobate crystals

A digital feedback control loop system is used to produce high-efficiency photorefractive gratings in reflection geometry for volume narrow band reflection filters. The system allowed hologram recording up to saturation. Several strategies are investigated to obtain higher fixed efficiency. A double controlled recording/fixing process with stationary fringes produced holograms with fixed efficiency higher than 90%. This method is interesting for producing reproducible high-reflectivity narrow band reflection filters of customized wavelength in the visible and near-infrared regions.

Electric-field-controlled two-dimensional Raman–Nath diffraction from paraelectric potassium lithium tantalate niobate

Electric-field-controlled two-dimensional Raman–Nath diffraction has been realized using a photorefractive diffraction grating. The grating was produced by two-wave coupling (at a wavelength of 632.8 nm) at small incidence angles using a potassium lithium tantalate niobate single crystal. Results for the Raman–Nath diffraction from the g 44 grating are presented, in which the externally applied field is perpendicular to both the grating vector and the wave vector of the incident beam. Two pairs of coherent beams were used to record the grating for two-dimensional Raman–Nath diffraction. The wave vector and the polarization of one pair lay in the ( x, z) plane, and those of the other pair lay in the ( y, z) plane. The influence of the applied electric field was studied, and the results show that the intensity of the Raman–Nath diffraction could be controlled by the direction and intensity of the applied field.

Formal demonstration of the independence on the optical activity of the maximum gain position in a two-wave coupling experiment

The two-beam coupling method has been a useful procedure to estimate the screening Debye length in a photorefractive crystal. The position and value of the maximum gain value as a function of the photorefractive spacing grating is used to obtain the Debye length value. This position and not the maximum value of the gain, appears to be evidently independent of the optical activity of the material. However, a formal treatment to this effect is not found in the specialized literature. In this paper, we present such formal treatment. Full text: PDF