Photorefractive KNbO3 Research Articles

Manipulation of optical patterns by frequency detuning of the pump beams

We show that manipulation and selection of optical patterns is possible by detuning the frequency of one of the two waves in counterpropagating photorefractive two-wave mixing with a feedback mirror. An analytical expression for the onset of modulational instability is derived for the general case of an arbitrary complex coupling constant, including a possible frequency shift of the generated sidebands. The influence of frequency detuning of the pump beams on the spatial scale of the resulting pattern and on the threshold value of the photorefractive coupling strength for modulational instability is investigated. The spatial scale and the frequency detuning of the generated sidebands were measured in an experiment using a photorefractive KNbO3 crystal. Comparison of theory and experiment show agreement for positive frequency detuning of the pump beams. For negative detuning we observe clamping of the spatial scale and no frequency shift of the sidebands. Possible explanations of this deviation between analytical results and experiment are discussed.

Long dark decay in highly sensitive Ce doped photorefractive KNbO3crystals

We show that cerium doped KNbO3 crystals exhibit high photorefractive sensitivities of S1 = 25×10-11 m3 J-1 and S2 = 78×10-9 m2 J-1 at 488 nm. The grating build-up time at a grating spacing of 1.5 µm and an intensity of 1 W cm-2 is 40 ms and the exponential gain is Γ = 6 cm-1. The decay time of the photorefractive grating in the dark was measured and found to be 31 h. The sample was completely characterized by additional photoconductivity measurements. The ratio between the photoconductivity at 1 W cm-2 and the conductivity in the dark is as high as 107.

Hexagonal optical structures in photorefractive crystals with a feedback mirror

A nonlinear theory is presented for the formation of hexagonal optical structures in a photorefractive medium equipped with a feedback mirror. Oppositely directed beams in photorefractive crystals are unstable against the excitation of sideband waves. It is shown here that as this instability evolves to its nonlinear stage, the three-wave interaction between weak sideband beams does not stabilize it, but rather leads to explosive growth of the amplitudes of beams whose transverse wave vectors form angles that are multiples of π/3. As a result, sideband beams at these angles are found to be correlated. A range of parameters is found in which four-wave interactions saturate the explosive instability, which explains the appearance of stable hexagons in the experiment. Outside this region, nonlinearities of higher order saturate the explosive instability, and the process of hexagon generation must be studied numerically. Matrix elements are obtained for the three-and four-wave interactions as functions of the distance to the feedback mirror, and an equation for the time evolution of the sideband wave amplitudes is derived that describes the hexagon generation. A comparison is made with experimental results for the photorefractive crystals KNbO3 and BaTiO3.

Self-trapping of one-dimensional and two-dimensional optical beams and induced waveguides in photorefractive KNbO_3

We demonstrate self-trapping of one-dimensional and two-dimensional optical beams in a photorefractive KNbO(3) crystal. We study the waveguides induced by the self-trapped beams for prospective applications of tunable nonlinear frequency conversion in soliton-induced waveguides.

Photoinduced electric current in Fe-doped KNbO_3

Photorefractive KNbO3:Fe is characterized by monitoring of the electric currents induced in the crystal that are due to applied illumination. Important photorefractive parameter values, such as the Maxwell relaxation time, carrier-diffusion length, carrier-screening length, and the magnitude of the photogalvanic current are thereby estimated.

Improvement of the Two-Beam Coupling Response Time in Photorefractive Double-Doping KNbO3

We report experimental studies on the photorefractive response of KNbO3 doubly doped with manganese and rubidium, iron and rubidium, respectively. Two-beam coupling response time in KNbO3:Mn:Rb is 3 to 10 times shorter while the two-beam coupling gain coefficient remains almost the same when compare with that in KNbO3:Mn. On the other hand, under similar conditions, two-beam coupling response time in KNbO3:Fe:Rb is 6 times faster than that in KNbO3:Fe.

Phase conjugation, edge detection and image broadcasting using two-beam coupling configurations in photorefractive KNbO 3

Two-beam coupling configurations are employed to obtain the phase conjugate image and edge extraction of an object using photorefractive KNbO 3: Fe. Also, the self-organization of a beam in the material into a hexagonal spot array is utilized to broadcast an input object to the location of each of the spots.

Simulational and experimental studies on anomalous reflectivity of phase conjugate wave

Abstract The condition of the appearance of multivalued conjugate reflectivities was investigated in a real crystal by computer simulation. For this aim, we performed first the experiment to determine the intrinsic coupling strength and the phase shift of the photorefractive crystal KNbO3:Fe using both two-wave and four-wave mixing techniques. Reflectivities obtained by the experiment were compared with the theory and quantitative agreement was obtained. Using the values of the coupling strength and phase shift obtained from the experiment, the simulation study was performed. To obtain large coupling strengths and phase shift near 0°, the application of an external electric field was assumed in the simulation. In operation of moving grating technique we found the transition from singlevalued regions to multivalued regions in (K 2, K 4) space, where K 2 and K 4 are the intensity ratios of counter-pump beam to pump beam and probe beam to pump beam, respectively.

Flow and controlled rotation of the spontaneous optical hexagon in KNbO_3

The flow and rotation of the optical hexagonal pattern spontaneously formed in a photorefractive KNbO(3) crystal is studied. It is demonstrated that the direction and speed of rotation can be controlled with an additional erase laser beam.

Photorefractive two-wave mixing with focused Gaussian beams

We report on numerical simulations of two-wave mixing of Gaussian beams in photorefractive materials. By using a model that takes the lateral intensity distribution of the two beams into account we demonstrate that the gain and the time response of Gaussian beams differ considerably from plane wave theory. We show that the gain and its time response can be interpreted in terms of an apparent interaction length and a spatially varying time constant. As an example, we apply numerical simulations in the framework of a two-dimensional model to analyse experimental results on two-wave mixing of focused Gaussian beams in photorefractive KNbO 3 planar waveguides.

Self-organization of scattering in photorefractive KNbO_3 into a reconfigurable hexagonal spot array

The scattering cone (~2 degrees ) from an Ar laser beam (514 nm) in photorefractive KNbO(3):Fe has been observed to transform into a reconfigurable hexagonal spot array. This phenomenon can be explained through the creation of laser-induced transmission and reflection grating arrays.

Reflection-grating self-pumped phase conjugation with optical feedback free of input conditions

A reflection-grating self-pumped phase-conjugate mirror is demonstrated with a photorefractive KNbO(3) crystal in a novel ring configuration. The feedback beam in the ring configuration is made to track the input probe beam automatically so that the phase conjugation is sustained without readjustment of the optical feedback loop when the angle or the position of the input probe beam is changed. The tracking feedback is realized by means of a simple three-lens erect imaging system. The angular and the positional acceptances for a tightly focused input beam of 50-µm diameter are Δθ(x). = 55 mrad, Δθ(y), = 67 mrad, and Δ(x) = 2.1 mm, Δ(y) = 4.2 mm, respectively.

Compact reflection-grating-based phase conjugator in KNbO_3:Fe crystals

We present the design and the results of experimental investigations of a self-pumped phase conjugator through a reflection grating in a special-cut photorefractive KNbO(3):Fe crystal. High reflectivity, good phase-conjugate fidelity, and fast response time are obtained.

Noise-free, high-gain signal beam amplification using beam fanning in a photorefractive KNbO 3:Fe crystal

A new scheme for high-gain, noise-free amplification by two-beam coupling in a photorefractive KNbO 3:Fe crystal, using the beam fanning effect, is described, in which the signal beam passes through the crystal in the direction of the positive C-axis. Large values (∼ 900) of the signal beam gain are achieved.

Characterization of photorefractive KNbO 3: Fey non-steady-state photocurrent techniques

Non-steady-state photocurrent measurements are carried out with a n-type photorefractive KNbO 3:Fe crystal at the light wavelength 488 nm. Spatial frequency dependent measurements yield the effective trap density. From the dielectric relaxation time for different crystal orientations we deduce the mobility ratios for charge carrier migration parallel to the a- , b- and c- axis.

Emission Characteristics of Internally Self-Pumped Phase-Conjugate Wave in Photorefractive KNbO3:Fe and BaTiO3 Crystals

The reflectivity and the response time of the self-pumped phase conjugation in unreduced KNbO3:Fe and BaTiO3 were measured and compared as functions of incident beam intensity, incidence angle and incidence position of the crystal. The self-pumped phase conjugation of KNbO3:Fe was generated in narrow regions of the incidence angle and position, in comparison with that of BaTiO3. It was also observed that the response time of KNbO3:Fe was longer than that of BaTiO3 under the same experimental conditions.

Thermal gratings and pyroelectrically produced charge redistribution in BaTiO_3 and KNbO_3

The influence of a pyroelectric charge-driving force on photorefractive currents is demonstrated experimentally for the first time to my knowledge. Illumination of photorefractive BaTiO3 and KNbO3 crystals by two intersecting coherent light pulses (light wavelength λl = 532 nm) induces thermal gratings. A spatially modulated pyroelectric field arises. Photoexcited carriers move in this field and tend to compensate for it. The pyroelectric field vanishes after termination of the pulse with a characteristic decay time, and redistributed charge remains. Measurements demonstrate that this pyroelectric charge-transport driving force can be much stronger than diffusion for fringe spacings greater than 2 μm. From this new charge-transport mechanism I derive a promising proposal for hologram writing and reading at the same wavelength without erasure.

Hexagonal pattern formation due to counterpropagation in KNbO_3

We report the transverse instability of laser beams due to counterpropagation in photorefractive KNbO3. Hexagonal unstable emission emerges around two counterpropagating pump beams. This emission is in the same linear polarization with the pump beams and is observed in configurations where reflection gratings are dominant.

Anisotropic grating recording in photorefractive KNbO 3:Fe

The first observation of an anisotropically recorded grating (with two orthogonally polarized beams) in photorefractive KNbO 3:Fe is reported. We attribute the effect to influences of the nondiagonal elements β 113 and β 223 of the photovoltaic tensor. With the help of anisotropic diffraction from the grating we estimate the lower limit of these tensor elements. The diffraction efficiency strongly depends on the recording angle.

All-optical associative memory based on a nonresonant cavity with image-bearing beams

A new implementation of an all-optical associative memory is presented. It is realized in a nonresonant cavity containing a photorefractive and a nematic liquid crystal. The images, which are stored on slides, modulate the intensities of the signal beams circulating in the cavity. In a photorefractive KNbO3 crystal the signal images are compared with an arbitrary input image on the pump beam by multiple two-beam coupling. In the nematic liquid-crystal cell the signal images are thresholded by a feedback-induced optical bistability. Associative readout, error correction, and image completion are demonstrated for two and three stored black-and-white images. The performance of the new scheme is estimated to be comparable with that of nonlinear holographic associative memories based on thin holograms.