J. Lamela

Femtosecond laser and swift-ion damage in lithium niobate: A comparative analysis

Relevant damage features associated with femtosecond pulse laser and swift-ion irradiations on LiNbO3 crystals are comparatively discussed. Experiments described in this paper include irradiations with repetitive femtosecond-laser pulses (800 nm, 130 fs) and irradiation with O, F, Si, and Cl ions at energies in the range of 0.2–1 MeV/amu where electronic stopping power is dominant. Data are semiquantitatively discussed by using a two-step phenomenological scheme. The first step corresponds to massive electronic excitation either by photons (primarily three-photon absorption) or ions (via ion-electron collisions) leading to a dense electron-hole plasma. The second step involves the relaxation of the stored excitation energy causing bond breaking and defect generation. It is described at a phenomenological level within a unified thermal spike scheme previously developed to account for damage by swift ions. A key common feature for the two irradiation sources is a well-defined intrinsic threshold in the depo...

Optical and structural properties of Sb2S3/MgF2 multilayers for laser applications

Multilayers of MgF2 and Sb2S3 have been obtained by physical vapour deposition on glass substrates. Changes in the optical and structural properties have been studied as a function of annealing temperature and the number of layers. A drastic variation in optical transmission, microstrain and grain size is observed at a temperature near 225??C. A comparison of the material properties of multilayers and a monolayer is carried out.

field optical and micro-luminescence investigations of femtosecond laser micro-structured Nd:YAG crystals.

We report on the in-depth micro-modifications induced by femtosecond laser ablation in Nd:YAG crystals. Near Field Optical experiments have revealed that, below the ablated volume, a permanent change in the Nd:YAG reflectivity, extending parallel to the irradiation direction, has been induced. Micro-luminescence experiments have provided experimental evidence that these permanent changes in the surface reflectivity are accompanied by strong changes in the luminescence properties of Nd(3+) ions revealing relevant micro structural modifications in the Nd:YAG system.

Near-field imaging of femtosecond laser ablated sub-λ/4 holes in lithium niobate

We report on the direct femtosecond laser ablation of sub-λ/4 (80–250 nm) holes in LiNbO3 crystals and on its local near-field imaging. We show that the near-field transmission of holes can feature an attenuation of ∼75% at hole central position, and a ∼20% transmission enhancement at its sides. This high-contrast ring-shaped near-field distribution is found to be in agreement with simulations, suggesting the surface relief as the main contrast mechanism.

Analysis of artificial opals by scanning near field optical microscopy

Herein we present a detailed analysis of the optical response of artificial opal films realized employing a near-field scanning optical microscope in collection and transmission modes. Near-field patterns measured at the rear surface when a plane wave impinges on the front face are presented with the finding that optical intensity maps present a clear correlation with the periodic arrangement of the outer surface. Calculations based on the vector Korringa–Kohn–Rostoker method reproduce the different profiles experimentally observed as well as the response to the polarization of the incident field. These observations constitute the first experimental confirmation of the collective lattice resonances that give rise to the optical response of these three dimensional periodic structures in the high-energy range.

The effect of the ferroelectric domain walls in the scanning near field optical microscopy response of periodically poled Ba2NaNb5O15 and LiNbO3 crystals

A study of Ba(2)NaNb(5)O(15) and LiNbO(3) crystals with periodic ferroelectric domain structures using the scanning near field optical microscopy technique is reported. Optical contrast is observed in the regions of ferroelectric domain boundaries and it is analysed using beam propagation method modelling. This reveals that the optical contrast, a consequence of changes in the refractive index, is not due to variation of the waveguide-coupling efficiency, and supports the hypothesis that it is associated with the domain array, which is related to the size of the domain.

Near-field scanning optical microscopy to study nanometric structural details of LiNbO3 Zn-diffused channel waveguides

A near-field scanning optical microscope (NSOM) is used to perform structural and optical characterization of the surface layer after Zn diffusion in a channel waveguide fabricated on lithium niobate. A theoretical approach has been developed in order to extract refractive index contrast from NSOM optical transmission measurements (illumination configuration). As a result, different solid phases present on the sample surface can be identified, such as ZnO and ZnNb2O6. They appear like submicrometric crystallites aligned along the domain wall direction, whose origin can be ascribed to some strain relaxation mechanism during the annealing process after Zn diffusion.

Periodic Ferroelectric Domain Structures Characterization by Scanning Near Field Optical Microscopy

In this work, Scanning Near Field Microscopy (SNOM) observation and characterization of quasi-periodic ferroelectric domains in rare earth doped LiNbO 3 , Nd doped Ba 2 NaNb 5 O 15 , and LiTaO 3 crystals are presented. The formation of ferroelectric domains has been induced either during the crystal growth procedure (LiNbO 3 and Ba 2 NaNb 5 O 15 ) or by applying pulsed electric field (LiTaO3). SNOM reflection, transmission and collection modes have been used indistinctly to characterize the ferroelectric domain structure. In the three crystals under study, an unexpected optical contrast associated to the presence of ferroelectric domains has been observed. In particular, working in the SNOM collection mode the optical contrasts reached a value closed to ∼ 80% in Ba 2 NaNb 5 O 15 and LiTaO 3 crystals. These high optical contrast values are explained considering that the ferroelectric domains pattern operates as a periodic array of planar waveguides (optical super-lattices). The invoked light-pipe mechanism has been assessed by beam propagation method (BPM) simulations of the light propagation through the structure, from which the refractive index modulations have been obtained. These values compare favorably with data obtained by light diffraction experiments under normal and Bragg incidence.

Near Field Optical Microscopy in Periodically Poled LiNbO3 and LiTaO3 Superlattices

This paper presents a study of the behaviour as optical superlattices of periodically poled LiNbO3 and LiTaO3 crystals using the Scanning Near-field Optical Microscopy (SNOM) technique. The periodic ferroelectric domains were formed either by electric field poling technique or by the “off-centre Czochralski method” during crystal growth. SNOM patterns were recorded in both collective and reflective configuration, simultaneously with surface topography measurement by AFM (Atomic Force Microscopy). It has been found that regardless of the method used to create the domains, a modulation in the SNOM signal intensity is correlated with the ferroelectric domain structure. The maximum intensity of the collected or reflected SNOM light intensity is centred on the negative domains, whereas the minimum intensity is positioned on the positive domains. This result demonstrates that the modulation of the refractive index is related to the domains and not with the wall between them.