Marc Luennemann

Photorefractive properties of lithium niobate crystals doped with manganese

The photorefractive properties of lithium niobate crystals doped with manganese (Mn) have been investigated. It is found that the effect of dark decay due to electron tunneling, which is the limiting factor of the highest practical doping level, is less in LiNbO_3:Mn than in LiNbO_3:Fe , and higher doping levels can be used in LiNbO_3:Mn to achieve larger dynamic range and sensitivity for holographic applications. The highest practical doping level in LiNbO_3:Mn has been found to be ~0.5 wt.% MnCO_3, and refractive-index changes and sensitivities up to 1.5X10^-3 and 1.3 cm/J are measured for extraordinarily polarized light of the wavelength 458 nm. It has been found that, in terms of both dynamic range (or refractive-index change) and sensitivity, the optimal oxidation state is highly oxidized. The distribution coefficient of Mn has been determined to be ~1. Absorption measurements are used to obtain more information about charge-transport parameters. The material is excellently suited for holographic recording with blue light. The hologram quality is outstanding because holographic scattering is much weaker compared with that in, e.g., iron-doped lithium niobate. Thermal fixing has been successfully demonstrated in LiNbO_3:Mn crystals.

Temperature dependence of absorption in photorefractive iron-doped lithium niobate crystals

We present experimental data showing a significant dependence of light absorption on temperature in photorefractive LiNbO3:Fe crystals. The results are successfully explained by assuming that the widths of the Fe2+ absorption bands in the visible and in the infrared spectral region depend on temperature. The findings are of relevance for thermal fixing of holograms. Furthermore, a temperature-induced increase of the infrared absorption is promising for improved infrared recording.

Coupling effects for counterpropagating light beams in lithium niobate crystals studied by grating translation technique for extremely high external electric fields

The grating translation technique (GTT), which employs a momentary shift of light fringes during holographic recording, is a powerful tool for characterization of nonlinear materials. Earlier, it was used in the transmission geometry. We formulate the main relations of GTT for the reflection case. These allow us to determine in situ the real and imaginary parts of the coupling strength and to detect the formation of refractive-index and absorption gratings. We apply GTT to determine the photorefractive characteristics of two-wave coupling in z-cut iron-doped lithium niobate crystals for an extremely small grating spacing (Λ≃105 nm) and extremely large externally applied electric fields (up to 650 kV/cm). Stabilization of the light fringes, crucially important in the reflection geometry, has been achieved with the use of an electronic feedback loop. We prove that the feedback introduces a frequency detuning between the light beams and modifies strongly the nonlinear response.

Properties of photorefractive lithium niobate crystals for extremely large externally applied electric fields

Summary from only given. Application of external electrical fields of up to 70 kV/mm to LiNbO/sub 3/ is possible which enlarges strongly the applicability of this material for electro-optic and photorefractive purposes such as hologram recording.

A new wave-front sensor

Summary form only given. Wave-fronts are surfaces of constant phase. The local propagation directions are oriented perpendicularly to these surfaces. Ordinary cameras are not phase sensitive and thus only two-dimensional images can be captured. Although several approaches like, e.g., Shack-Hartmann sensors exist, wave-front imaging with high spatial resolution and large dynamic range is still an unresolved and demanding problem. Applications are, e.g., in the fields 3D imaging and optical inspection. We propose a new technique to measure the three-dimensional wave-front of light waves.