M. Carrascosa

Theoretical modeling of the fixing and developing of holographic gratings in LiNbO 3

A mathematical model is developed to account for the fixing and developing behavior of holographic gratings in photorefractive materials such as LiNbO3. All transport processes for the carriers (electrons and protons) are considered. Moreover, unlike in previous studies, the thermal excitation of carriers is taken into account. Two possible experimental procedures that involve fixing during or after writing are theoretically described. The model is applied to simulate the kinetics, overall efficiency, and temperature dependence of the fixing process for LiNbO3:Fe, for which the most experimental information is available.

Monte Carlo simulation of the performance of PMMA luminescent solar collectors.

A Monte Carlo method has been developed to simulate the performance of luminescent solar collectors (LSC) consisting of a PMMA plate with an attached film (or multiple-film stack) of dye-activated PMMA. Rhodamine 6G and Fluorol 555 have been considered as dopant dyes. Direct and diffuse solar spectra have been simulated in order to compare extreme insolation conditions. Efficiency factors have been determined as a function of the main geometrical and optical parameters of the LSC.

Nonlinear optical waveguides generated in lithium niobate by swift-ion irradiation at ultralow fluences

A novel method to produce optical waveguides is demonstrated for lithium niobate (LiNbO(3)). It is based on electronic excitation damage by swift ions, i.e., with energies at approximately 1 MeV/amu or above. The new technique uses high-energy medium-mass ions, such as Cl, with electronic stopping powers above the threshold value for amorphization (5-6 keV/nm), reaching the maximum value a few micrometers inside the crystal. At the ultralow fluence regime (10(12)-10(13) cm(-2)) an effective nanostructured medium is obtained that behaves as an optical waveguide where light propagates transversally to the amorphous nanotracks created by every single impact. The method implies a reduction of 4 orders of magnitude with respect to He implantation. The optical waveguides present reasonable losses (~10 dB/cm) and significant second-harmonic generation (SHG) and electro-optic (EO) responses (>50% bulk) for the lowest fluences.

Determination of H concentration in LiNbO3 by photorefractive fixing

A new method to determine the proton concentration in LiNbO3 is presented. The method is based on the measurement of the diffraction efficiency of a photorefractive grating in two situations. It is first measured after recording at room temperature, and second after saturation of the fixing process at a given temperature (about 150 °C). From only these two experimental data, the value obtained for the proton concentration in our sample is H0=(4.4±0.7)×1018 cm−3. This value agrees, within the experimental error, with that obtained from the infrared absorption arising from the OH− stretching bond.

Recording and erasure kinetics in photorefractive materials at large modulation depths

A numerical approach has been developed for simulating the recording and erasure kinetics in photorefractive materials for high modulation of the light-intensity pattern. The recording kinetics for the fundamental grating strongly deviates from exponential behavior, and it is much slower than the kinetics inferred from a linearized solution. The harmonics of the grating show lower growth rates for higher Fourier orders and exhibit an initial delay time. On the other hand, in the optical erasure experiment (starting from the steady-state solution), the decay rate of the fundamental harmonic is exponential and closely matches the predictions of the linear approximation. Moreover, the decay rate of the harmonics are in good agreement with the value corresponding to a fundamental grating with the same k vector. This uncoupling of the Fourier components is no longer intact when erasure is started from a nonsaturated grating. The effect of the grating vector of the light pattern and an externally applied field on the growth and the erasure kinetics have also been investigated. The applied field modifies the kinetic behavior and introduces some differential features with regard to the linear solution.

Buried amorphous layers by electronic excitation in ion-beam irradiated lithium niobate: Structure and kinetics

The formation of buried heavily damaged and amorphous layers by a variety of swift-ion irradiations (F at 22MeV, O at 20MeV, and Mg at 28MeV) on congruent LiNbO3 has been investigated. These irradiations assure that the electronic stopping power Se(z) is dominant over the nuclear stopping Sn(z) and reaches a maximum value inside the crystal. The structural profile of the irradiated layers has been characterized in detail by a variety of spectroscopic techniques including dark-mode propagation, micro-Raman scattering, second-harmonic generation, and Rutherford backscattering spectroscopy∕channeling. The growth of the damage on increasing irradiation fluence presents two differentiated stages with an abrupt structural transition between them. The heavily damaged layer reached as a final stage is optically isotropic (refractive index n=2.10, independent of bombarding ion) and has an amorphous structure. Moreover, it has sharp profiles and its thickness progressively increases with irradiation fluence. The dy...

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.

Study of developing thermal fixed holograms in lithium niobate

Experimental results on the developing kinetics and final diffraction efficiency of fixed holograms in iron-doped lithium niobate are presented. Samples with two different oxidation states are studied. The developing kinetics of well-oxidized samples show oscillations superposed to a saturation dependence, whereas they are not present for the less oxidized sample. The final developed ratio is found to depend on the grating spacing and the oxidation state of the samples. All these features are well explained with the charge-transport theory and are found to be dependent on the photovoltaic properties of the samples, doping, and oxidation state. From the analysis of the experimental data, the photovoltaic field amplitude of the samples is obtained.

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.

Optimization of selective erasure in photorefractive memories

A theoretical analysis for the optimization of the selective-erasure process in holographic photorefractive memories is presented. It is based on the solution of the standard material equations under a linear approximation (low modulation depths). Specific expressions for the optimum phase shifts and erasure rates are obtained. The approach includes all transport processes and so applies to photovoltaic materials such as LiNbO3. The different behavior with regard to nonphotovoltaic materials is discussed. Some additional strategies to improve the overall erasure process are proposed.