J. Villarroel

Understanding light intensity thresholds for catastrophic optical damage in LiNbO 3

The appearance of light intensity thresholds for catastrophic optical damage in LiNbO3 is satisfactorily explained by using a photorefractive model based on the Fe(2+)?Fe(3+) and NbLi(4+)?NbLi(5+) defect pairs. Model simulations of the photorefractive amplification gain as a function of the light intensity present sharp threshold behavior. A similar behavior is shown by the saturating refractive index change. In agreement with experiments, predicted thresholds appear shifted towards higher intensities (up to a 10(4) factor) when the Nb(Li) concentration is decreased or the temperature is increased. The model also explains very recent data on the threshold enhancement with the Fe(2+)/Fe(3+) ratio in optical waveguides.

Photovoltaic versus optical tweezers

The operation of photovoltaic (PV) tweezers, using the evanescent light-induced PV fields to trap and pattern nano- and micro-meter particles on a LiNbO(3) crystal surface, is discussed. The case of a periodic light pattern is addressed in detail, including the role of particle shape and the modulation index of the light pattern. The use of a single Gaussian light beam is also considered. Illustrative experiments for the two situations are presented. The performance of such PV tweezers in comparison to the best established case of optical tweezers, using optical forces, is considered. Differential features between the two trapping approaches are remarked.

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.

Light intensity dependence of holographic response and dark decays in α-phase PE:LiNbO3 waveguides

The influence of the light intensity on the recording/erasing of photorefractive gratings has been studied in α-phase proton exchanged LiNbO3 waveguides. The saturating refractive index modulation increases in a non-linear way as a function of the recording intensity and reaches a region of erratic values above a certain intensity threshold, which increases with the exchange time used for preparing the guides. Erratic values are attributed to photorefractive self-defocusing of the recording beams, while the thresholding effect is attributed to the sharp index modulation increase with the intensity associated to the intrinsic defect NbLi. It has also been confirmed that the proton exchange time affects the photorefractive response and that the dark conductivity can be disregarded with regard to the photoconductivity in the high intensity range near the intensity threshold.

Photovoltaic laser beam degradation in lithium niobate planar waveguides: two-center model approach

Photovoltaic laser beam degradation in lithium niobate (LiNbO3) has been investigated through a finite-differences beam propagation method for nonlinear media. The simulations use a two-center model (Fe2+↔Fe3+, NbLi4+↔NbLi5+) that has been recently proved to be necessary to successfully describe the photorefractive effect in nominally pure LiNbO3. Refractive index profiles and intensity and phase beam profiles have been calculated for a wide intensity range and several material distances. A good agreement is obtained on comparing simulations and experimental data. This includes self-defocusing for moderate intensities and the complex beam profile structures appearing for high intensities, providing a complete description of this nonlinear optical phenomenon.

Influence of the Geometrical Configuration on Optical Damage of LiNbO3 Planar Waveguides

Geometrical effects on the optical damage in proton exchanged LiNbO 3 waveguides are analyzed. Light intensity thresholds and beam profiles at different intensities have been measured for guides prepared on z- and x-cuts. z-cut α-phase proton exchanged guides show a threshold intensity of the same order of magnitude but slightly greater than x-cut. The more important differences between both guides cuts are a clear asymmetry of beam-profile in x-cut guides as opposite to z-cut and the existence of a chaotic evolution at high intensities only found in x-cut guides. The influence of the proton-exchange time on optical damage threshold has been also investigated in z-cut waveguides to compare them with recent reported data on x-cut guides.

Photorefractive Response and Optical Damage Control in Proton Exchanged LiNbO 3 Waveguides via Proton Exchange Time

Measurements in proton exchanged LiNbO3waveguides show that the duration of the exchange markedly affects the [Fe2+]/[Fe3+] ratio and, as a consequence, the waveguide photorefractive response. Optical damage resistance dramatically increases with the exchange time.