Vicente Torres-Zúñiga

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.

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.

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.

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.

Photoacoustic effect applied to sound speed measurement.

We present a simple experiment to show the photoacoustic effect, a well established but not widely known effect which has many applications. The photoacoustic effect consists of the generation of acoustic waves by pulsed radiation incident on a sample. In our case, we used a homemade Nitrogen laser as a source of pulsed light for many samples in order to measure the speed of sound. The Nitrogen laser is easy to build by undergraduate students, it is a transversal discharge laser at atmospheric pressure (TEA), excited by a Blumlein circuit, emitting nanosecond pulses in the ultraviolet region of the spectrum ((lambda) equals 337.1 nm) and has been previously reported. The acoustic waves generated on the surface of the samples travel through the material and are detected with a piezoelectric sensor. The transducer is also easy to build using the piezoelectric of cigar lighters. The electric signals are registered by a 100 Mhz oscilloscope triggered by the light produced at the laser discharge. Knowing the thickness of the sample and the arrival time of the acoustic wave we can precisely measure the speed of sound.

Thermomechanical and Photophysical Properties of Crystal-Violet-Dye/H2O Based Dissolutions via the Pulsed Laser Photoacoustic Technique

Different thermoelastic parameters, for example, the acoustic attenuation and the speed of sound, are fundamental for instrumental calibration and quantitative characterization of organic-based dissolutions. In this work, these parameters as functions of the concentration of an organic dye (crystal-violet: CV) in distillated water (H2O) based dissolutions are investigated. The speed of sound was measured by the pulsed-laser photoacoustic technique (PLPA), which consists in the generation of acoustic-waves by the optical absorption of pulsed light in a given material (in this case a liquid sample). The thermally generated sound-waves traveling through a fluid are detected with two piezoelectric sensors separated by a known distance. An appropriate processing of the photoacoustic signals allows an adequate data analysis of the generated waves within the system, providing an accurate determination of the speed of sound as function of the dye-concentration. The acoustic attenuation was calculated based on the distance of the two PZT-microphones to an acoustic-source point and performing linear-fitting of the experimental data (RMS-amplitudes) as function of the dye-concentration. An important advantage of the PLPA-method is that it can be implemented with poor or null optical transmitting materials permitting the characterization of the mechanical and concentration/aggregate properties of dissolved organic compounds.