D. E. Lucchetta

Phase-only modulation by nanosized polymer-dispersed liquid crystals

We report the preparation methods and electro-optical characterization of polymer-dispersed liquid crystals with nanosized liquid crystal domains obtained by using a multifunctional monomer in a prepolymer mixture that is suitable for a fast photopolymerization process. The electric-field-induced optical phase shift has been measured using a Mach–Zender interferometer, getting values up to π/2 with an applied voltage of about 20 V/μm with no dependence on the light polarization. The high transmittance of the samples make them suitable for photonic devices and telecom applications.

Light amplification by dye-doped holographic polymer dispersed liquid crystals

In this letter we report a “one-step” fabrication technique of an optical active organic photonic band gap (PBG) structure allowing the onset of the amplification spontaneous emission effect and narrowing of the corresponding band. This result has been achieved through the addition of the dye rhodamine 6G to a standard holographic polymer dispersed liquid crystal pre-polymer mixture. The effects of the dye addition on the optical properties of the polymerized PBG structure have been analyzed. The spectra emitted by this optically active periodic structure have been studied as a function of the excitation energy.

Optical characterization of polymer dispersed liquid crystals for holographic recording

In this study we present an extended investigation made on holographic polymer dispersed liquid crystals. A spectroscopic study of the photosensitive mixtures during the polymerization and phase separation process was performed, and an optical characterization of the recorded phase gratings was carried out using two alternative methods. The behavior of the diffraction efficiency of the transmission phase gratings versus time, angle of incidence, and wavelength was experimentally determined, and the results were successfully analyzed using the available theories for anisotropic phase gratings. Finally, the electro-optical properties concerning the beam deflection and response times were measured.

Wavelength flipping in laser emission driven by a switchable holographic grating

We report lasing from a simple wafer structure made of a thin layer of rhodamine 6G as active material and of a switchable holographic mirror, based on a reflection grating made of polymer dispersed liquid crystals. It is shown that switching-off the mirror reflectivity by application of a suitable voltage allows wavelength flipping of the laser emission by 10 nm.

Superior-Performance Polymeric Composite Materials for High-Density Optical Data Storage†

Holographic data-storage (HDS) technologies have the potential to meet the ‘‘information age’’ requirements for fast data transfer and high storage capacity. There are a variety of different optical materials that can be used to fabricate these kinds of optical memories. In particular, photosensitive polymers and inorganic photorefractive crystals can exhibit significant changes in their optical properties (in the entire sample volume in which holograms are written) when exposed to laser irradiation. Such a property is necessary to achieve high storage capacities (up to one Terabyte) and to outperform the present optical supports: compact discs (CDs), digital versatile discs (DVDs), high-definition DVDs (HD-DVDs), blue rays (BR). At present, the implementation of HDS technology is limited by the availability of suitable photosensitive materials. The main HDS requirements, in the blue-violet wavelength range (typically 405 nm), are the following: high recording sensitivity (>10 cm J ), high spatial resolution (>7000 lines/mm), easy processability, high optical quality over large areas or volumes, low shrinkage (<1%), high transparency (>80%), and long temporal stability. Photorefractive crystals are interesting materials for rewritable devices, but are characterized by low sensitivity and low storage capacity. On the contrary, photopolymers have been extensively studied because of their capability of satisfying most of the fundamental requirements of HDS systems. Moreover, they are cheap and easily processable, e.g., they can be spin-coated to form a uniform thick disk layer. However, a polymeric material that fulfils all HDS prerequisites still does not exist. For instance, polymers undergoing cationic ring-opening polymerization are usually characterized by low shrinkage, but exhibit reduced sensitivity due to their characteristic low rate of photopolymerization. On the other hand, free-radical polymerization reactions are usually faster, but characterized by much higher shrinkage values. In this work, we propose a new approach to overcome these limitations, based on the use of a new composite polymeric mixture. It consists of a combination of a multifunctional acrylate monomer (di-pentaerythritol-hydroxypenta/hexa-acrylate, DPHP/HA) and an epoxy-aromatic resin (tri-phenyl-o-methane-tri-glycidil-ether, TPMTGE). The latter, solid at room temperature, is known to be a low molecular weight glass-forming liquid. The final mixture is a highly viscous fluid that can be spin-coated over a substrate or introduced into a glass cell by capillarity, as explained in the Experimental section below. The high viscosity of the mixture and the absence of any solvent are useful features for data-storage applications, since thick polymeric films, hence high storage capacities, can be easily obtained. The DPHP/HA-TPMTGE syrup is photosensitized to blueviolet radiation (405 nm), by adding bis(2,4,6-trimethylbenzoyl)phenyl-phosphine-oxide (Irgacure 819), a very efficient photoinitiator, yielding carbon-centered and phosphorous-centered free radicals and a very high acrylate photo-polymerization rate. On the contrary, Irgacure 819 is not able to activate TPMTGE polymerization in our experimental conditions. The chemical formulae of all used compounds are in Figure 1. In this work, we also report a full characterization of the above-mentioned mixture, and discuss the physical-chemical mechanism at the basis of its optical properties. We recorded high spatial resolution ( 7400 lines/mm) reflection gratings using a standard two-beam holographic set-up operating at l1⁄4 405 nm (I1⁄4 18mW cm 2 per beam). The grating growth was monitored in real time, by observing the appearance of a notch in the transmission spectra of the sample, in correspondence of the Bragg reflection wavelength of the periodic structure. This technique allows the measurement of the full set of HDS optical parameters. Figure 2a shows the transmission spectra detected during and after irradiation for a sample containing DPHP/HA and TPMTGE in a 70:30 weight ratio. It clearly evidences a first important feature: the mixture transparency exceeds 80% at 405 nm (over 90%, if glass reflection losses are taken into account). In fact, due to the low Irgacure 819 concentration (1%), our material exhibits negligible scattering and absorption losses in the visible range, before and after polymerization, as shown in the inset of Figure 1. The concentration used is sufficient to activate a very fast photo-polymerization reaction as a result of the high photoinitiator efficiency. As a consequence, we observe the formation of a reflection grating just after an irradiation of 0.1 s (Fig. 2a). For a longer exposure, the refractive index modulation Dn and the corresponding grating diffraction efficiency h, i.e., the notch depth, increase (Fig. 2b). In this way a diffraction efficiency as high as 60% was obtained, corresponding to a refractive-index modulation Dn1⁄4 1.4 10 . This value is very significant when considering the very high spatial frequency of our reflection gratings. It was calculated using the standard expression for

Sine: Surface Induced Nonlinear Effects

Pump-probe and TIR experiments have been performed on the dye-doped nematic liquid crystal 5CB, in order to study the basic mechanisms of the extraordinarily large nonlinear response recently reported. Experimental data show that light-induced modifications of the anchoring conditions can be the origin of the observed effect. The bulk reorientation due to the collective elastic behaviour of the liquid crystal gives then rise to the nonlinearity, which occurs without a direct optical torque on the molecular director in the bulk. We have called this effect SINE (Surface Induced Nonlinear Effect).

Large-area photonic structures in freestanding films

The authors report the fabrication of freestanding two-dimensional photonic crystals in organic materials. Large-area patterned structures, with submicrometric spatial resolution, have been recorded in spin-coated polymer-dispersed liquid crystals via holographic photolithography. After the removal of the liquid crystal, the polymeric film is peeled off from the substrate. Scanning electron microscope images show the high homogeneity of the structure in any spatial direction. These films can be conveniently used in a wide range of applications, from templates for high-refractive index photonic crystals to flexible photonic elements.

Distributed feedback all-organic microlaser based on holographic polymer dispersed liquid crystals

An optically pumped vertically emitting all-organic laser, based on a holographic polymer dispersed liquid crystal reflection grating operating in a distributed feedback configuration, is presented. The used experimental geometry overcomes the main drawbacks characterizing similar systems, allowing virtually infinite operations and absence of damages associated with the high energy of the optical pumping.

Nanocomposite polymeric materials for high density optical storage

We report the results of an extended investigation performed on composite polymeric materials with the aim of obtaining compounds suitable for holographic recording. In order to investigate the material properties a characterization of holographic reflection gratings at different writing wavelength (514.5, 457 and 405 nm) has been performed. The volume grating presents high diffraction efficiency (>60%), high sensitivity (>103 cm J−1) and refractive index modulation Δn≈0.01 even for writing wavelength in the blue range. We show that following a strategy of two basic components leading to phase separation during the photopolymerization process, most of the requirements for holographic data storage are achieved. The one that needs further improvement concerns long term mechanical stability.

Determination of small anisotropy of holographic phase gratings.

We show the possibility of detecting small anisotropies in holographic polymer dispersed liquid-crystal samples, using a simple experimental setup that allows us to determine the behavior of the diffraction efficiency versus incident angle for two reading polarizations. This analysis is extremely sensitive to small changes in the parameters that define the grating anisotropy, giving us a way to determine with great accuracy the components of the modulated part of the dielectric tensor.