A. García-Cabañes

Generation of high-confinement step-like optical waveguides in LiNbO3 by swift heavy ion-beam irradiation

We demonstrate a swift ion-beam irradiation procedure based on electronic (not nuclear) excitation to generate a large index jump step-like optical waveguide (Δn0≈0.2,Δne≈0.1) in LiNbO3. The method uses medium-mass ions with a kinetic energy high enough to assure that their electronic stopping power Se(z) reaches a maximum value close to the amorphous (latent) track threshold inside the crystal. Fluorine ions of 20 and 22MeV and fluences in the range (1–30)×1014 are used for this work. A buried amorphous layer having a low refractive index (2.10 at a wavelength of 633nm) is then generated at a controlled depth in LiNbO3, whose thickness is also tuned by irradiation fluence. The layer left at the surface remains crystalline and constitutes the core of the optical waveguide which, moreover, is several microns far from the end of the ion range. The waveguides show, after annealing at 300°C, low propagation losses (≈1dB∕cm) and a high second-harmonic generation coefficient (50%–80% of that for bulk unirradiat...

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.

Thick optical waveguides in lithium niobate induced by swift heavy ions (~10 MeV/amu) at ultralow fluences

Heavy mass ions, Kr and Xe, having energies in the approximately 10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (lambda = 633 nm). They propagate ordinary and extraordinary modes with low losses keeping a high nonlinear optical response (SHG) that makes them useful for many applications. Complementary RBS/C data provide consistent values for the partial amorphization and refractive index change at the surface. The proposed method is based on ion-induced damage caused by electronic excitation and essentially differs from the usual implantation technique using light ions (H and He) of MeV energies. It implies the generation of a buried low-index layer (acting as optical barrier), made up of amorphous nanotracks embedded into the crystalline lithium niobate crystal. An effective dielectric medium approach is developed to describe the index profiles of the waveguides. This first test demonstration could be extended to other crystalline materials and could be of great usefulness for mid-infrared applications.

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.

LiNbO3: A photovoltaic substrate for massive parallel manipulation and patterning of nano-objects

The application of evanescent photovoltaic (PV) fields, generated by visible illumination of Fe:LiNbO3 substrates, for parallel massive trapping and manipulation of micro- and nano-objects is critically reviewed. The technique has been often referred to as photovoltaic or photorefractive tweezers. The main advantage of the new method is that the involved electrophoretic and/or dielectrophoretic forces do not require any electrodes and large scale manipulation of nano-objects can be easily achieved using the patterning capabilities of light. The paper describes the experimental techniques for particle trapping and the main reported experimental results obtained with a variety of micro- and nano-particles (dielectric and conductive) and different illumination configurations (single beam, holographic geometry, and spatial light modulator projection). The report also pays attention to the physical basis of the method, namely, the coupling of the evanescent photorefractive fields to the dielectric response of the nano-particles. The role of a number of physical parameters such as the contrast and spatial periodicities of the illumination pattern or the particle deposition method is discussed. Moreover, the main properties of the obtained particle patterns in relation to potential applications are summarized, and first demonstrations reviewed. Finally, the PV method is discussed in comparison to other patterning strategies, such as those based on the pyroelectric response and the electric fields associated to domain poling of ferroelectric materials.

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.

THG from copper phthalocyanines in a sol—gel host

Abstract Copper phthalocyanine molecules have been incorporated into silica xerogels at several levels of concentration. The gelation process has been monitored by optical and leachability tests. The third-harmonic (THG) susceptibilities χ (3) have been measured at λ = 1.064 μm for the xerogel composites and at λ = 1.064 μm and λ = 1.904 μm after thermal treatment at 200 °C. Values up to 2.6 times higher than those of the xerogel host have been obtained for the treated samples with 10 −4 mol/l concentration of copper phthalocyanine in the sol.

Photorefractive charge compensation in α-phase proton-exchanged LiNbO 3 waveguides

Photorefractive recording and light and dark erasures have been measured in unannealed α-phase proton-exchanged LiNbO3 waveguides. The saturation index change, Δns≅9×10-6, is independent of the light intensity within the studied range, 0.3–50 W/cm2. The time dependencies are well represented by the sum of two exponential components. After complete optical erasure, diffraction efficiency η increases in the dark (i.e., dark developing) up to ∼17% of the saturation value ηs≅0.12 and then decays to zero in ∼4 h. All experimental results are reasonably well simulated by a model in which the Fe2+/Fe3+ light-induced charge distribution is compensated for by a light-insensitive species (ionic charges or holes) that is mobile at room temperature.

Optoelectronic tweezers under arbitrary illumination patterns: theoretical simulations and comparison to experiment

Photovoltaic tweezers are a promising tool to place and move particles on the surface of a photovoltaic material in a controlled way. To exploit this new technique it is necessary to accurately know the electric field created by a specific illumination on the surface of the crystal and above it. This paper describes a numerical algorithm to obtain this electric field generated by several relevant light patterns, and uses them to calculate the dielectrophoretic potential acting over neutral, polarizable particles in the proximity of the crystal. The results are compared to experiments carried out in LiNbO₃with good overall agreement.