Hidden reservoir of photoactive electrons in LiNbO3 crystals.

We show that a continuous-wave (cw) pump beam at a wavelength of 532 nm produces substantial light-induced (LI) absorption in the visible range in initially transparent undoped LiNbO3 crystals. The LI absorption coefficient stays linear in the pump intensity I(p) up to I(p)(max)=48 kW/cm2. Together with other features including long-term stretched-exponential relaxation of the LI absorption, it indicates that the present concept of LI electron processes in this important optical material must be revised: the amount of photoactive electrons increases already within the cw intensity range. A quantitative model is proposed that explains the experimental data and employs two-step excitations from filled localized states near the valence band via intermediate deep centers into the conduction band. The introduced localized states serve as a hidden reservoir of electrons.

Analysis of photorefractive optical damage in lithium niobate: application to planar waveguides.

Photorefractive optical damage of single beams in LiNbO(3) crystals is analyzed within a framework of two photoactive centres (Fe(2+)/Fe(3+) and Nb(Li) (4+)/Nb(Li) (5+)). It compares model simulations and significant experimental measurements in LiNbO(3) waveguides. A good agreement is found in the performed comparisons: photovoltaic currents, refractive index changes and, especially relevant, in degraded beam-profiles. The progress of the degraded wavefront has been simulated by implementing a finite-difference beam-propagating method which includes the model equations. These results, together with previous ones on grating recording, provide a comprehensive, satisfactory explanation of most important questions on photorefractive optical damage.

Correlation between photorefractive index changes and optical damage thresholds in z-cut proton-exchanged-LiNbO(3) waveguides.

An interferometric Mach-Zehnder technique very recently developed has been applied to measure photorefractive index changes in different types of z-cut proton-exchanged planar waveguides in LiNbO(3). These measurements are complemented by determining the intensitythreshold for the onset of optical damage with a standard single-beam setup. In the intensity region just below the threshold-intensity obtained in the single-beam experiment the refractive index change is found to saturate at values around1x10(-4). Furthermore, we measure the dark conductivities of proton-exchanged waveguides by monitoring the decay of the light-induced index changes. Via the time constant of the decay we obtain dark conductivities of the order of about 5x10(-16) Omega (-1) cm (-1), that are negligible compared with the photoconductivity within the light intensity range used. The results of the measurements compare well with the predictions of a recent work, that uses a two-center model to explain the optical damage.