Photorefractive Barium Titanate Research Articles

Generation of Multi Self-Pumped Phase-Conjugated Waves by Pulsed Laser

Two self-pumped phase-conjugated (SPPC) images produced from two signal laser beams transporting different types of information were observed using a photorefractive barium titanate (BaTiO3) crystal. We have studied the smallest numbers of incident pulses required to generate one and two phase-conjugated images for one and two types of information, respectively, in order to transport much information. The lowest energy required for the generation of one phase-conjugated image depended strongly on the product of the numbers of incident pulses and the power of the incident pulsed beam.

Generation of Two Self-Pumped Phase-Conjugated Waves at 532 nm by Nanosecond Pulsed Laser

Self-pumped phase-conjugated (SPPC) images produced by two SPPC beams generated from a single nanosecond-pulsed laser beam are successfully obtained at beam power ratios of up to 4:1. The two SPPC beams are incident on opposite faces [(100) and (100)] of a photorefractive barium titanate (BaTiO3) crystal, and the SPPC images are reproduced on the respective crystal faces with high quality due to the reduced interference between the two SPPC beams owing to the delay between the nanosecond pulses of each beam. The quality of the images and the maximum power ratio at which both images are observable are superior to the continuous-wave configuration.

Grating Decay in Reduced Barium Titanate Crystals

Under inhomogeneous illumination, a refractive index grating will form in a barium titanate crystal through the photorefractive effect. Due to its large electro-optic coefficients and self-pumped phase conjugation, a photorefractive barium titanate crystal is of a special interest for many applications such as optical signal processing, optical phase conjugation and optical information storage. In general, the grating stored in a barium titanate crystal will fade in the dark soon. That is a barrier for the applications of optical information storage. Here we made a series of reduced BaTiO3 crystals and investigated the optical absorption, the dark decay and the light-induced decay. We found that the decay time of the grating can be lengthened by the reduction treatment. The grating can be kept in the crystal reduced around compensation point for more than a month. At high intensity, the speed of writing/erasing a grating can be determined by the optical absorption.

Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

A phenomenological multi-wave mixing model has been used along with a more general solution of Kukhtarev's model to explain intensity-dependent beam-fanning in photorefractive (PR) BaTiO3 crystal. The variation of trap density owing to the photoexcitation of charge carriers has been considered to explain the intensity-dependent effect. Theoretical results predicted by the model show a confirmation with experimental results obtained from PR crystal, and also show some of the controlling mechanisms that enable use of beam-fanning for some potential applications.

-70 dB optical carrier suppression by two-beam coupling in photorefractive media

Suppression of an optical carrier from an RF modulated laser beam of wavelength 532 nm is performed using two-beam coupling in photorefractive barium titanate. A theoretical analysis reveals that perfect suppression can be achieved at a specific modulation strength, which depends on the gain and intensity ratio between the two beams. The experiments achieve a maximum of -72.9±2.4 dB carrier suppression at the theoretically ideal modulation strength, and -61.0±2.4 dB for small modulation strengths.

Photorefractive BaTiO_3 spheres and spherical disks

We have produced single-crystal spheres and spherical disks of photorefractive barium titanate (BaTiO(3)) . The spheres and disks allow direct fiber-to-fiber two-beam coupling without additional optical elements. We also investigate the spontaneous oscillation of triangles and other whispering-gallery modes in these spherical structures.

Self-pumped phase conjugation in cerium-doped barium titanate crystal using nanosecond pulses from a laser diode

Summary Self-pumped phase conjugation was performed in a photorefractive cerium-doped barium titanate crystal by nanosecond pulses from a laser diode. The reflectivity of the phase conjugator against the incident angle and the incident position was measured in two configurations. The steady-starte reflectivity of the pulsed phase-conjugate wave increases and the response time decreases as pulse width of incident pulses increases.

Three-dimensional rigid body motion measurement using speckle fanning in a photorefractive BaTiO 3 crystal

Summary A simple speckle technique, utilizing speckle fanning in a BaTiO 3 crystal, is demonstrated for the measurement of rigid body displacement in an arbitrary direction. Collimated laser beam illuminates a diffuser to generate speckles, which are then recorded inside a photorefractive barium titanate crystal thus producing speckle fan. Speckle correlation fringes are formed due to interference between the reconstructed speckle fans corresponding respectively to the initial- and final positions of the diffuser. Displacements occurring in a direction making small angles with the optical axis have been measured by profiling the fringes. The measurement of displacement in a direction making large angle with the optical axis is possible only by taking the Fourier transform of the fringes. This facilitates the separation of transverse and axial correlation peaks giving the transverse and axial components of the motion.

Out-of-plane displacement measurement by using speckle fanning in a photorefractive BaTiO 3 crystal

Summary We propose a method for the measurement of out-of-plane object displacement by a speckle technique utilizing the speckle fanning in a photorefractive barium titanate crystal. A portion of the converging incident beam has been used to illuminate the diffuser to generate speckles, while the remaining portion is allowed to fall directly on the crystal. Coupling of scattered beam with incident beam inside the crystal generates speckle fan of light after diffraction from the complex gratings formed in the volume of the crystal. Semi-circular speckle correlation fringes are formed due to the interference between reconstructed speckled fan beam recorded for an initial position of the diffuse surface, and directly transmitted speckled beam after introducing the out-of-plane displacement in the diffuser. Fringe diameter has been measured after ‘profiling’ operation on the fringes, for the calculation of out-of-plane displacement.

Temperature distribution and thermal-induced defects in photorefractive barium titanate crystals illuminated with intense cw or pulsed laser irradiations

The temperature distribution in a parallelepipedic crystal irradiated with a Gaussian repartition of light is calculated by solving the three-dimensional heat equation and taking into account the losses through the sides of the samples. The theoretical and experimental maps of temperature are satisfactorily compared. For barium titanate crystals, the resistance to intense cw laser (25.7 kW/mm2 at 514.5 nm) and to Nd:YAG pulsed laser (peak power of 9.2 MW at 532 nm) is investigated. The defects induced by the irradiation are analysed. Under pulsed illumination, the damage threshold is found to be 0.54 GW/cm2 in a nominally undoped BaTiO3 and 0.44 GW/cm2 in a rhodium-cobalt doped BaTiO3 crystal.

Dependence of the refractive index grating in photorefractive barium titanate on intensity

The interference pattern of light waves creates a refractive index modulation in photorefractive media. This process is relatively well described by the theory (Kukhtarev equations) but deviations are found. Therefore, experimental methods are used in order to characterise the processes. The influence of the absolute intensity as well as the intensity ratio of the interfering waves on the refractive index modulation are studied in a two-wave mixing arrangement. Especially for the dependence on the absolute intensity the interesting relation Δn= f( I abs)∼ I abs α was found that is not predicted by the theory. Nevertheless, this experimental approximation can be used in calculations in order to minimise the error.

Self-enhanced diffraction properties of a nominally undoped barium titanate crystal with a thermally fixed grating

We describe recent studies on diffraction properties of a photorefractive barium titanate crystal having a thermally fixed grating. Buildup dynamics of diffracted signals are measured to analyze the contributions of ionic and re-created photocarrier (hole) gratings to the temperature-dependent diffraction efficiency in self-enhanced readout. An activation energy of mobile ions as nonphotoactive secondary carriers for fixing volume holograms is determined holographically. This value is compared with that of hydrogen ions estimated from polarization-dependent infrared absorption spectra in order to indentify the fixing species.

Threshold Behavior in Formation of Optical Hexagons and First Order Optical Phase Transition

The dynamics of optical hexagon excitation is studied experimentally in photorefractive barium titanate. The hard mode of the oscillation onset is discovered as well as a pronounced hysteresis loop in the dependence of the hexagon intensity on the crystal coupling strength. The analogy of excitation threshold of optical hexagon and first order optical phase transition is confirmed.

Light-induced charge transport in photorefractive BaTiO3: Fe and Ba0.77Ca0.23TiO3: Fe

Photorefractive barium titanate and barium—calcium titanate crystals with the congruently melting composition (Ba0.77Ca0.23TiO3) doped with iron are annealed in a hydrogen atmosphere at various temperatures. The samples are studied with electron paramagnetic resonance (EPR) and optical absorption both performed separately as well as simultaneously before and after illumination. The samples are electron conductive and the charge transport at room temperature is governed by only one level identified by photo-EPR as Fe2+/3+. The optical absorption of Fe2+ in BaTiO3: Fe is found to be at 2.1 eV and in (Ba0.77Ca0.23TiO3): Fe at 1.9 eV.

Mathematical simulation of a dynamic storage process

The possibility of realizing an optical memory with photo-refractive crystals using dynamic refreshment via phase conjugation is discussed. The experimental realization is based on photorefractive barium titanate crystals. Series of refresh cycles, dependent on several parameters, are simulated numerically. It is shown that an extension of the storage time can be reached.

A method for single-pass distortion correction using holography in rhodium-doped barium titanate

We present a holographic method of one-way image transmission with ‘real-time’ correction of distortion through an optical system. The technique is demonstrated using a crystal of photorefractive rhodium-doped barium titanate as the holographic medium and corrects for phase distortions imposed upon a US Air Force test chart. This technique is suitable for applications in both cw and pulsed regimes, and for either fixed or changing distortions.

Physical origin of laser frequency scanning induced by photorefractive phase-conjugate feedback

We present numerical simulations of the complex grating structure that is generated when several longitudinal modes from a laser induce a self-pumped phase conjugator in a photorefractive barium titanate crystal. The results of the numerical analysis clearly show that the detuning curve of the generated grating structure is asymmetric with respect to the center wavelength of the laser that induced it. The asymmetric feedback to the laser, which is generated by diffraction in the gratings of the structure, initiates the frequency scanning of the laser. It is found that the material frequency dispersion of the barium titanate crystal causes the asymmetry and is the origin that initiates the scanning process. The theoretical predictions are in agreement with the reported experimental observations.

Photorefractive coherence-gated interferometry

We have demonstrated a novel optical interferometric technique for a highly scattering environment, using photorefractive coherence gating. A two-wave mixing process in a photorefractive barium titanate crystal effectively reduces scattering noise, and we have successfully detected interferometric signals through scattering media of up to 17 mfp, with a high visibility. We have also demonstrated a two-dimensional thickness measurement of an optically scattering thin film using our new interferometry.

Light-Induced Absorption in Photorefractive Barium Titanate Doped with Rhodium

The light-induced absorption of barium titanate doped with rhodium (BaTiO3:Rh) is characterized in near-infrared spectral region. It decreases with the wavelength of the probe beam from 700 to 840 nm, and is significantly dependent on the polarization of the probe beam and insensitive to the polarization of the pump beam. The light-induced absorption as high as 0.32 cm-1 is obtained for ordinary-polarized probe beam at λ = 730 nm. The observations are interpreted by the photorefractive two-center or one center with three valence states model.