C. Medrano

PHOTOREFRACTIVE EFFECT IN LINBO3 CRYSTALS ENHANCED BY STOICHIOMETRY CONTROL

LiNbO3 single crystals with a composition close to stoichiometry are grown by using a double crucible Czochralski method equipped with an automatic powder supply system. The photorefractive properties are compared with those of LiNbO3 crystals grown from the congruent composition melt. Two beam coupling experiments show that both nondoped and Fe-doped stoichiometric crystals exhibit stronger photorefractive effect than the Fe-doped congruent crystals.

Photorefractive properties of iron-doped stoichiometric lithium niobate

The photorefractive properties of stoichiometric LiNbO(3) crystals with a small number of defect densities grown by the double-crucible Czochralski method are investigated and compared with the defect densities of commercially available congruent Fe-doped LiNbO(3) crystals. Two-wave-mixing experiments show that novel stoichiometric crystals exhibit larger photorefractive gain and considerably faster response times than congruent ones. The results indicate that the nonstoichiometry defect control of photorefractive crystals is of key importance for the improvement of their properties.

PHOTOREFRACTIVE EFFECT IN CU- AND NI-DOPED KNBO3 IN THE VISIBLE AND NEAR INFRARED

Nickel‐ and copper‐doped KNbO3 crystals were grown in order to investigate the photorefractive response in the visible and near infrared. Optical absorption, photoconductivity, and two‐beam coupling experiments between 515 and 1064 nm are reported. A two‐beam coupling constant at 860 nm of 1.4 and 3.2 cm−1 was measured in the nickel‐ and copper‐doped crystals, respectively.

High photorefractive sensitivity at 860 nm in reduced rhodium-doped KNbO(3).

We show that high-temperature reduction in a CO-CO(2) atmosphere increases the photorefractive sensitivity of KNbO(3):Rh at 860nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spacing of 0.15 mu;m is accelerated by a factor of 400. The grating buildup time at a grating spacing of 0.7 microm and an intensity of 1Wcm(-2) is 0.5 s, a value comparable with that of as-grown KNbO(3):Fe at visible wavelengths. The optical and photorefractive parameters of Rh-doped KNbO(3) subjected to reduction treatment are characterized for wavelengths of 0.48-1.064 microm .

Self-pumped and incoherent phase conjugation in Fe-doped KNbO 3

Self-pumped phase-conjugated reflectivities in KNbO3 crystals doped with Fe have been optimized in specially cut crystals. Without using external cavities, we have obtained reflectivities of as high as 60%. With the pump beams derived from the self-pumped phase-conjugation process, degenerate four-wave mixing buildup time is 6 s, with a reflectivity of 30%. For the first time, the double incoherent phase conjugation in KNbO3 is also reported with reflectivities of as high as 65% and buildup times of 10 s. We present what are, to our knowledge, the first experiments on quantitative investigations of the fidelity of double phase-conjugate mirrors by interferometric methods that showed the high quality of the phase-aberration correction achieved in Fe:KNbO3.

Self-pumped optical phase conjugation and light oscillation in Fe doped KNbO3

Abstract We report different experiments on self-pumped phase conjugation in iron doped KNbO 3 crystals at room temperature. Self-pumped phase conjugate reflectivities of a linear cavity, an external ring mirror and a configuration where no external optical elements are required have been measured. Using the passive ring resonator a reflectivity of 30% of a self-pumped phase conjugate mirror has been measured at room temperature. In the configuration requiring no external optical elements besides the KNbO 3 crystal a reflectivity of 12% has been measured. In degenerate four-wave mixing phase conjugate reflectivities of up to 270% have been observed in the diffusion recording mode.

Optimization of the photorefractive properties of KNbO3 crystals

Abstract Photorefractive effects of pure and iron doped KNbO3 crystals with concentrations between 150 ppm and 300 ppm have been investigated. In order to optimize the photorefractive recording times and the amplitude of the refractive index changes the crystals were thermally treated with simultaneous photorefractive testing. The photoconductivity data and the photorefractive beam coupling gain dependent on the grating spacing show that the relative influence of electron and hole contribution is altered depending on the reduction treatment. The experimental results are discussed in terms of a two carrier model.

Infrared photorefractive effect in doped KNbO 3 crystals

The photorefractive sensitivity of potassium niobate crystals doped with Ce, Co, Cu, Fe, Mn, Ni, and Rh and double-doped with Mn and Rh is investigated over an extended spectral range. We present experimental evidence on extrinsic properties important for the photorefractive effect, such as absorption and effective trap density. Photorefractive gratings are investigated with two-wave mixing experiments. Results on exponential gain, response time, and photorefractive sensitivity at near-infrared wavelengths are reported. The best photorefractive sensitivities at 860 and 1064 nm were obtained in crystals doped with Rh, Fe, Mn, and Mn–Rh. This makes them suitable for applications at laser-diode wavelengths; at 1064 nm, however, Rh:KNbO3 shows a better photorefractive sensitivity than the others.

The Photorefractive Effect in Inorganic and Organic Materials

Light-induced changes in optical properties such as refractive index and absorption coefficients are always related to material excitation, i.e. deviation from (thermal) equilibrium [4.1]. Examples of material excitations are an increase in the number of conducting free electrons, the phonon density or the number of electrons occupying excited states. As a result of spatially inhomogeneous material excitation, the optical properties become spatially modulated in the interference region of two intense coherent light waves and create gratings that may be permanent or dynamic, depending on the mechanism and the material. Permanent gratings hold for longer times, while dynamic or transient gratings disappear after the inducing laser source is switched off. The latter have been produced in a large variety of materials and are usually detected by diffraction of a probe beam or by self-diffraction of the light waves inducing the grating.

KNbO 3 crystals for photorefractive applications

Characterization and optimization of KNbO 3 crystals for photorefractive applications over an extended spectral range is described. Intrinsic properties are used to describe the refractive index response to space-charge field gratings. Extrinsic properties that are important for the photorefractive effect, such as absorption, photoconductivity, charge transport, and trapping, are discussed. Some results on the extended IR response of crystals doped with Fe, Ni, Cu, Ce, Mn, and Co are presented. Post growth annealing at temperatures of 400 to 900°C in a controlled atmosphere is shown to modify the charge-transport parameters, therefore also modifying the response time of photoinduced refractive index changes.