Dynamical behaviour of holographic gratings with a nematic film--Polymer slice sequence structure

Plasmon-mediated discrete diffraction behaviour of an array of responsive waveguides

We investigate the discrete diffraction phenomenon in a Polymer-Liquid Crystal-Polymer Slices (POLICRYPS) overlaying a random distribution of gold nanoparticles (AuNPs, plasmonic elements). We study the propagation of a CW green laser beam through the waveguide structure as a function of beam polarization, laser intensity and sample temperature. It turns out that the plasmonic field created at the interface between AuNPs and POLICRYPS waveguides enables and stabilizes the optical field propagation within the responsive nematic liquid crystal channels. The active role of the liquid crystal is pointed out by a polarization, temperature and beam divergence experimental analysis and evidenced by a peculiar trumpet-shaped discrete diffraction pattern. Theoretical simulations confirm that the observed optical behavior is governed by the interaction of the nematic liquid crystal with optical and plasmonic fields.

Optically isotropic, electrically tunable liquid crystal droplet arrays formed by photopolymerization-induced phase separation

Phase separation has been an interesting and important topic in liquid crystal (LC)-polymer composites. We investigated the photopolymerization-induced phase separation in an LC-polymer composite through a maskless lithographic system based on an amplitude-modulated spatial light modulator. By optimizing both exposure conditions and materials, we achieved a two-dimensional (2D) liquid crystal droplet array (LCDA) in the LC-polymer composite. Further investigations revealed that such 2D LCDAs, working as microlens arrays, demonstrated polarization-independent, electrically tunable focusing properties under a certain voltage. With advantages in cost-effectiveness, fast fabrication, and polarization-independent, electrically tunable focusing, such phase-separated microlens arrays in the LC-polymer composite could find many potential optical applications.

Visible and near-infrared characterization and modeling of nanosized holographic-polymer-dispersed liquid crystal gratings

We have studied the electro-optical and angular behavior of holographic-polymer-dispersed liquid crystal gratings at different wavelengths, in the visible and in the near-infrared range. As usual in these kinds of materials, a strong polarization dependent behavior was observed. Our samples showed very high diffraction efficiency for p-polarized radiation at 1.55 microm, which is very interesting for many possible applications in the telecom field. However, we also observed a very unusual behavior for visible p-polarized light and we try to suggest some explanation for that. By analyzing the angular dependence of the diffraction efficiency, we could measure the components of the permittivity modulation tensor and infer important information about the main parameters involved in the grating structure: the degree of phase separation and the anisotropy in the liquid crystal droplet distribution. In our opinion, this simple and nondestructive methodology can be very useful for studying these kinds of materials and getting information on their morphology, in view of optimizing their properties. Finally, we discuss the role of the refractive index optical dispersion in order to describe the behavior of these materials at different wavelengths. These remarks are especially important when properties in the infrared range are extrapolated from measurements in the visible.

Spectroscopic ellipsometry study of liquid crystal and polymeric thin films in visible and near infrared

In this work, we propose spectroscopic ellipsometry as a suitable method for measuring optical properties in soft materials, polymers and liquid crystals, specially selected for use in photonics applications. We show the results of our measurements on some multilayered samples, in the range from visible to the near-IR region, of interest for telecom applications. We point out potentialities and limits of the technique and compare the obtained results with another experimental method, the m-lines spectroscopy, and/or with existing data in the literature. The results about the optical parameters for the analysed materials (the nematic liquid crystal 5CB, one commercial and one lab made optical polymer, and an Indium Tin Oxide film) are useful and interesting by themselves. In fact, as a paradigmatic example, we briefly discuss how an incomplete knowledge of this kind of data can lead to a wrong design of a Bragg grating device. However, more than in the provided data, we put the interest of the present analysis in the warnings about spectroscopic ellipsometry utilization and eventually the necessity of getting complementary information.