Geminiano Martinez-Ponce

Investigations on photoinduced processes in a series of azobenzene-containing side-chain polymers

We present a spectrophotometric investigation of the processes in three azobenzene polymers induced by a linearly polarized Ar beam (488 nm). The polymers differ only in the length of the spacers attaching the chromophores to the main chain. The experimental results show that this difference determines not only the polymer structure; it also substantially influences the value of the photoinduced dichroism and its time behaviour after the illumination. We explain this on the basis of the different efficiencies of the two main photoinduced processes in the azobenzenes—the selective trans–cis isomerization and the reorientation of the trans-azobenzenes. The results from a real-time measurement of the photoinduced anisotropic changes in the refractive indices of the three polymers support this understanding.

Optical and Liquid Crystalline Properties of New Alkyl-Substituted Azopolymers

In the present work, we report the synthesis, the mesomorphic behavior and the laser-induced optical characteristics of side-chain liquid crystalline azopolymers without a spacer, but substituted with a hexyloxy group. The azopolymers were studied by DSC, POM and XRD. Results have revealed that the monomer exhibits a nematic phase, whereas polymers develop a double layered SmC phase. Thin films of azopolymers exhibited good response to a laser beam irradiation. Surface relief gratings were recorded with the interference pattern produced by a 413 nm circularly polarized laser beam.

Polarization gratings with surface relief in dyed gelatin and their postdevelopment diffraction.

Some characteristics of dyed gelatin films when polarization gratings are recorded are reported. Two dyes, Malachite Green and Methylene Blue, are used in our experiment. In particular, no dichromate was added to Methylene Blue plates before or after exposure. It is shown that, in spite of its having high viscosity, a relief grating is formed when orthogonal linearly polarized beams are superimposed upon the plates. In contrast with previous observations in another photoanisotropic medium, the frequency of the gelatin relief grating does not depend on the directions of the electric field of the orthogonal recording beams. Besides, when orthogonal circularly polarized beams are used in the formation of polarization gratings, no relief grating is produced. Finally, we study the phase modulation obtained after dehydration of the gelatin plates in which the polarization gratings were formed. Postdevelopment phase gratings do not preserve the polarization information of the interference pattern. Also, the grating frequency is increased by a factor of 2 for linear polarization, whereas for circular polarization there is no phase modulation.

Photocrosslinking using linear polyols in xanthene dye-doped polyvinyl alcohol plates

A simple and highly efficient system for holographic recording based on a polyvinyl alcohol matrix is presented. A linear polyol is added to the solution made with the polymer and a xanthene dye in order to increase the photoinduced crosslinking rate. The optimization of the film fabrication process is determined experimentally. It is found that the length of the assistant unit, the position and the number of the attached hydroxyl groups is related with the saturation level and the shelf lifetime of the holographic planar grating. Also, host matrix and photoinitiator attributes are significant in the photosensitive medium response.

All-optical switching using supramolecular chiral structures in azopolymers

The self-rotation on the polarization azimuth and the formation of a supramolecular chiral structure when an elliptically polarized light beam propagates through an amorphous azopolymer film are used to implement a two-wavelength chiroptical switch. The macrostructures generated by each excitation beam have different pitches, which produce different rotation angles of the polarization ellipse when an arbitrary probe beam is used. In the proposed device, one of the excitation beams is taken as a probe. The value and the sign of the rotation angle of the polarization ellipse of the probe beam when the second beam is illuminating the sample mainly depend on the relationship between their input states of polarization.

Dyed-polyvinyl alcohol films: molecular weight and hydrolysis degree influence on optical recording

An analysis of different polyvinyl alcohol films dyed with Malachite Green is presented. Absorbance and diffraction efficiency of holographic gratings are compared, taking as a parameter the molecular weight and hydrolysis degree of the polymer. It is observed that, using the same dye concentration, the absorption coefficient of the films increases as the molecular weight increases. The absorbance of these plates can be modified when exposed to UV light. In addition, it is found that for holographic recording there is an optimal dye-polymer system film.

Polarization holographic gratings with surface relief in azobenzene-containing polymers

Polarization holographic gratings in azopolymers are due to three photoinduced effects: linear anisotropy, circular anisotropy, and surface relief. In our polymers the surface relief grating is with doubled frequency with respect to the volume anisotropic grating. A theoretical analysis is presented and compared with experimental data.

Induced and form birefringence in high-frequency polarization gratings

High-frequency phase polarization gratings are fabricated holographically in dichromated gelatin dyed with malachite green. It is observed that the intensity of the -1 diffracted beam is a sinusoidal function of the incident polarization angle. In addition, we analyze the dependence of the diffracted order polarization on grating frequency. It is evident from our results that form birefringence becomes significant when the grating period is smaller than the illumination wavelength, thus modifying the optically induced birefringence. Then, in polarization hologram reconstruction, it is not possible to obtain the polarization distribution at the recording step for high-frequency objects.

Polarization holographic element using an azobenzene polymer

A holographic lens is recorded superimposing two beams with orthogonal linear polarizations on an azobenzene polymer film. The polarization pattern on the interference plane induces two modulations in the media: the volume and the surface modulation. The spatial frequency of the surface relief is twice the one for the volumetric modulation resulting in a holographic lens with two different focal lengths. Additionally, because of the modulated anisotropy induced in the medium, the polarization at the longer focus distance is orthogonal to the polarization at the shorter one. We propose this polarization element to send or detect information in two planes simultaneously or separately by using an analyzer behind the holographic lens.

Mueller imaging polarimetry of holographic polarization gratings inscribed in azopolymer films

Three types of polarization gratings have been recorded in azopolymer films by the symmetrical superposition of different orthogonal pairs of polarized beams. The inscribed holographic elements have been analyzed microscopically in a Mueller polarimeter in order to image the optical anisotropies photoinduced in the film. In the most of cases, the spatial modulation of diattenuation, birefringence, and optical rotation reproduced quite well previous results reported in the literature. Nevertheless, in the particular case of coherent superposition of p- and s-polarized beams, the spatial frequency for optical rotation (related to the Stokes parameter V) was different from the one observed in linear anisotropy (related to the Stokes parameter U). It is shown by theory and experiment that, in the polarized field used to record this polarization grating, the fourth-Stokes parameter changes sign, which implies a change in circular polarization handedness, practically once between two adjacent maxima.