Omar G. Morales-Saavedra

Optical absorption photoacoustic measurements for determination of molecular symmetries in a dichroic organic-film

A novel approach to specify symmetries and main optical axes in anisotropic polymeric films is proposed. This method is based on the analysis of the optical absorption via the pulsed laser photoacoustic (PLPA)- technique in a common polarizer film, while rotating the polarizer axis at normal incidence. Since the PLPA-signals are directly proportional to the optical absorption, it is shown that a symmetric and complementary Maluss law can be obtained over full root mean square (RMS)- and correlation (CA)-analysis of the PLPA-signals. Such data processing reveals the main material directions of the constituting film molecules defining the symmetry structure of the sample. PLPA-results were compared to the pure optical transmission experiments and show unambiguous information, allowing this technique to be used in nonstandard and opaque polymeric films, where the analysis of the optical measurements represents a difficult task, and in general, in anisotropic media.

Morphological, optical, and nonlinear optical properties of fluorine-indium-doped zinc oxide thin films

Chemically sprayed fluorine-indium-doped zinc oxide thin films (ZnO:F:In) were deposited on glass substrates. A mixture of zinc pentanedionate, indium sulfate, and fluoride acid was used in the starting solution. The influence of both the dopant concentration in the starting solution and the substrate temperature on the transport, morphological, linear, and nonlinear optical (NLO) properties were fully characterized with atomic force microscopy (AFM), scanning-electron microscopy (SEM), UV-VIS, and photoluminescence (PL) spectroscopies, and the second-harmonic generation (SHG) technique, respectively. A decrease in the resistivity was observed for increasing substrate temperatures, reaching a minimum value of 1.2 × 10−2 Ω cm for samples deposited at 500°C. The surface morphology was also dependent on the dopant concentration in the starting solution and on the substrate temperature. The X-ray diffraction (XRD) patterns revealed that the ZnO:F:In thin solid films are polycrystalline in nature fitting with a hexagonal wurtize type and showing (002) preferential growth for all of the studied samples. The optical transmittance of these films was found to be higher than 80%, from which the optical band gap of these samples was determined. Finally, a clear dependence on the quadratic NLO properties of the developed semiconducting ZnO:F:In thin films with the substrate temperatures was established, where huge x(2)-NLO coefficients on the order of x33(2) = 37 pm V−1 were measured for high substrate temperatures.

Synthesis of porphyrin-dendrimers with a pyrene in the periphery and their cubic nonlinear optical properties.

Dendrons of pyrene derivatives were attached to a porphyrin core. A marked effect in solution for the dendrimers was observed in the absorption spectra. All the compounds obtained were characterized by 1H-, 13C-NMR, FTIR, UV-vis, MALDI-TOF or FAB+ mass spectrometry and elemental analysis. The cubic nonlinear optical behavior of some the synthesized compounds was tested via Z-Scan measurements in spin-coated film samples.

Nonlinear optical performance of poled liquid crystalline azo-dyes confined in SiO2 sonogel films

The catalyst-free sonogel route was implemented to fabricate highly pure, optically active, hybrid azo-dye/SiO2-based spin-coated thin films deposited onto ITO-covered glass substrates. The implemented azo-dyes exhibit a push–pull structure; thus chromophore electrical poling was performed in order to explore their quadratic nonlinear optical (NLO) performance and the role of the SiO2 matrix for allowing molecular alignment within the sonogel host network. Morphological and optical characterizations were performed to the film samples according to atomic force microscopy (AFM), ultraviolet-visible (UV-Vis) spectroscopy and the Maker-fringe technique. Regardless of absence of a high glass transition temperature (T g) in the studied monomeric liquid crystalline azo-dyes, some hybrid films displayed stable NLO activity such as second harmonic generation (SHG). Results show that the chromophores were homogeneously embedded within the SiO2 sonogel network, where the guest–host molecular and mechanical interactions permitted a stable monomeric electrical alignment in this kind of environment.

SHG-Activity of Polar Nano-Structures of LC-RED-PEGM-7 Based Sono-Gel Hybrid Materials

The synthesis of sol-gel materials induced by ultrasonic action (sonolysis) is implemented as an alternative method for the fabrication of highly pure organic-inorganic composites with good monolithic and optical properties. A newly synthesized liquid crystalline mesogen (named here RED-PEGM-7) was introduced as dopant specie within the SiO2 matrix in order to create an active nonlinear optical hybrid material at room temperature. A comparative characterization of the structural properties between a pure reference sample and several doped composites was performed by X-ray diffraction (XRD) and atomic force microscopy (AFM); where, an optimal dissolution concentration of tetraethoxysilane (TEOS) and the liquid crystalline (LC) mesogen, to obtain good mechanical and nonlinear optical properties in thin film layers has been found.

Impact of dyes on the nonlinear optical response of liquid crystals implementing the Z-scan technique

The study of the nonlinear refractive index response γ of several organic dyes and their impact on the nonlinear optical (NLO) properties of nematic liquid crystals (LC) was performed via Z-scan measurements. For his purpose, a low power CW He-Ne laser system (λ ≈ 633 nm) was implemented. Studies were carried out at the low absorption spectroscopic region of the implemented samples (dyes, liquid crystals and mixtures at different ratios of these materials). Samples were prepared at 1% weight of the used solvent (THF) and were sandwiched in glass cells with a gap thickness of ~100 μm. The implemented dyes have shown the largest optical nonlinearities and represent the main contributors to the cubic NLO-properties of the LC:Dye mixtures. In our particular studies, 5CB liquid crystal doped with DR1 azo-dye, resulted in the simultaneous positive and negative exhibition of nonlinear refractive indexes γ, depending on the polarization state of the excitation laser beam. Experimental conditions and results are described in detail.

Morphology, Linear and Nonlinear Optical Response of Octopolar Chromophores Embedded in a Silica Sonogel Matrix

In this work, a liquid crystalline octopolar molecule 1,3,5-tris(ethynylphenyl)benzene-(1) was successfully embedded as dopant chromophore within a SiO2 based sonogel network in order to prepare stable solid state hybrid materials with intense second harmonic generation (SHG). This recently synthesized octopolar compound exhibit a centrosymmetric C 3 -symmetry structure, which is constituted by an electron-rich core and electron-poor periphery groups. The sonogel method, induced by energetic ultrasonic (US) waves applied at the TEOS/H2O reactant mixture interface has proven to be a suitable route in the manufacture of monolithic hybrid systems suitable for optical characterizations. The octopolar-based hybrid samples were comprehensively studied in their morphology, spectroscopic, linear and second-order nonlinear optical (NLO) properties, according to several characterization techniques: epimicroscopy, AFM, UV-Vis- and fluorescence (PL) spectroscopies, and the NLO-SHG-technique. Epimicroscopy, photoluminiscent and absorption spectra evidence an optimal and homogeneous inclusion of the octopolar compounds within the glassy sonogel network. Without necessity of molecular poling processes, the effective quadratic-NLO susceptibility of the hybrid was estimated to 13.6 × 10−3 pm/V at 1064 nm by SHG-transmission experiments recorded far-off of the resonance regime.

Chlorine-Substituted Bent-Core LC-Based Sono-Gel Hybrid Materials: Synthesis and Optical Properties

We implemented the catalyst-free sonogel method to disperse and encapsulate a chlorine substituted thermotropic bent shaped liquid crystalline compound. By this means, advanced organic-inorganic hybrid materials with high optical and mechanical quality, suitable for different kind of optical applications, were obtained. Bent-core molecules have shown interesting optical properties, which have not yet been investigated in the solid-state. Therefore, we implemented the sonogel route to fabricate highly pure SiO2 porous glassy networks which allowed the inclusion of this kind of mesogens in the colloidal sol-state. In this work, we present the preparation and optical performance of these amorphous “banana”-based hybrids via absorption and fluorescence spectroscopies, electro-optical and cubic third-harmonic generation nonlinear optical measurements.

Bis(4-nitroanilines) in interactions through a π-conjugated bridge: conformational effects and potential molecular switches

Two molecules, 2,2′-(1,4-phenylenebis(ethyne-2,1-diyl))bis(4-nitroaniline) (PNA) and 2,2′-(1,4-phenylene-bis(ethyne-2,1-diyl))bis(N,N-diethyl-4-nitroaniline) (PDNA), built up from two p-nitroaniline “push–pull” nonlinear optical (NLO) chromophores slightly conjugated through a 1,4-phenylenebis(ethyne-2,1-diyl) bridge have been synthesized and fully characterized. The crystal structure of PNA reveals that the ground (off) state conformation corresponds to the centrosymmetric anti-conformer in which the molecular hyperpolarizability (β) is equal to zero. A computational investigation reveals that, once submitted to an intense electric field (E), the two p-nitroaniline subunits are gradually aligned in the direction of the field with the appearance of a metastable (on) state (β ≠ 0, when E = 0), stabilized by conjugation. A loop of hysteresis is observed during the on off cycles leading to a form of molecular bistability. The NLO response of PNA has been investigated at different field intensities by the electric-field induced second-harmonic (EFISH) technique to provide experimental evidence for this behavior.

Fabrication and optical testing of hybrid SiO2: azo-polymer based planar waveguides for NLO/SHG laser emission

Predesigned push-pull azo-dye polymers were homogeneously dispersed within a SiO2 sol-gel matrix synthesized via the sonogel (SG) route. High-quality spin-coated films were obtained with these hybrid structures in the liquid sol-phase. The spectroscopic UV- Vis analyses reveal the appropriate insertion of these organic compounds within the highly pure SG-environment whereas the thermal (DSC) analysis and photoacoustic measurements evidence the thermomechanical stability of the amorphous hybrid layers. As the optical attenuation, refractive index and film thickness values of the obtained films are adequate for opto-electronic applications; these hybrid films were implemented to fabricate optical waveguiding prototypes. In this sense, functional planar waveguides were fabricated for nonlinear optical (NLO) applications after performing a molecular ordering via a corona DC-poling procedure in order to achieve a macroscopic polar order (ferroelectric and noncentrosymmetric arrangement of the organic chromophores). The poled films were then able to exhibit stable NLO-waveguiding effects as excited with a Nd:YAG laser system in order to generate second harmonic waves travelling within the planar layer.