We report the properties of a new polymer-guest photosensitive mixture based on two monomers with different
properties as basic components. The material has been characterized by recording holographic gratings using a low
power blue laser (405 nm) and a spectro-photometric technique to get the optical properties of the grating during and
after recording. The detected grating shows a very high sensitivity (of the order of 10<sup>3</sup> cm/J) and a surprising "antishrinkage" phenomenon (red-shift of the Bragg reflection grating wavelength).
In the last three decades several kinds of organic mixtures for holographic recording were developed in order to
achieve a new class of DVD-like optical memories for high-density optical data storage. The holographic materials
should satisfy the following requirements: high sensitivity to blue light, low losses, high spatial resolution and long term
stability. To this aim we developed new organic photosensitive mixtures based on only three components. We recorded
high spatial frequency reflection gratings up to 7400 lines/mm with blue laser light (405 nm) by using a conventional
holographic setup. We obtained a macro grating diffraction efficiency up to 67%, refractive index modulation over 0.01,
optical shrinkage < 2 % and overall losses ~5%. In order to characterize data-storage materials independently on the
experimental conditions, the sensitivity has been evaluated through the S parameter which takes into account the
diffraction efficiency, recording light intensity, exposure time and sample thickness. The amazing obtained values of <i>S</i>
>10<sup>5</sup> cm/J evidences a very fast recording process with a very low writing intensity (less than 20 mW/cm<sup>2</sup>) corresponding
to a recording energy density of few mJ/cm<sup>2</sup>. The performance of these materials have been also tested in the microholographic
Holographic techniques allow the recording of homogeneous, high resolution, large-area light intensity patterns in photo
sensitive organic materials. The choice of proper experimental conditions easily permits the fabrication of 1D, 2D and
3D periodic and quasi-periodic structures. In this work we present a simple way to fabricate planar complex photonic
structures characterized by high dielectric contrast values. To this purpose we used polymer dispersed liquid crystals as
photosensitive material for the holographic recording, followed by the removal of the liquid crystal from the recorded
structures via a specific solvent, thus obtaining large area regular polymer-air patterns. The obtained structures have been
simulated, recorded in the substrates and optically characterized in planar light guided configurations. The relevant
optical properties have been analyzed by means of a theoretical approach formally derived from the dynamical theory of
x-ray diffraction. The presented experimental technique allows easy fabrication of optical integrated devices to be used
either as high sensitivity sensors or in the field of optical telecommunications.
Holographic gratings recorded in polymer-dispersed liquid crystals (H-PDLC) are very interesting as systems for electrically switchable diffractive devices. From the early 1990s, experiments, Bragg and Raman-Nath gratings are recorded in PDLC. A very thorough review of these investigations is published recently. In this paper we report our first experiments for H-PDLC gratings, recorded in two different optical arrangements using total internal reflection (TIR): 1) Stetson's scheme - when low and high spatial frequencies gratings are simultaneously recorded in the PDLC's volume, and 2) Nassenstein's scheme - when low or high spatial frequency grating is recorded with evanescent waves in very thin layer of the PDLC. A polarization grating recorded by the Stetson's scheme is also reported. The realtime diffraction efficiency dependence during the recording and the polarization characteristics are investigated.
The aim of this work is to understand the effects of the shrinkage phenomenon in H-PDLC reflection gratings through the realtime analysis of their transmission spectra during the recording process. The realtime spectroscopic analysis of the samples showed a light intensity dependent free-radical polymerization indicated by a quick growth of the reflection peak whose corresponding diffraction efficiency, measured at normal incidence, is in the range of 40%-45% depending on the photopolymerization conditions. An optical shrinkage corresponding to a 4.3% displacement of the reflected wavelength from the expected value has been detected. This value of the shrinkage is in agreement to that mesaured in a similar system.
Reflection gratings have been recorded and investigated in H-PDLC materials by means of real-time spectroscopy during the polymerization process in view of possible application in optical data storage. High spatial frequency gratings (>6000lines/mm) with diffraction efficiency up to 45% and index modulation over 0.01 were obtained. The effects of the shrinkage of the reflection gratings has been detected showing a displacement of the reflected wavelength from the expected value of about 3.8%. Finally recording of microgratings has been carried out observing some optimal agreement between the microscopic and the macroscopic parameters.
We report on recent experiments performed on Polymer Dispersed Liquid Crystals (PDLCs) and dye doped nematic liquid crystals (DDLCs), showing that these materials are potentially applicable in the development of novel optical devices free from the limitations typical of the currently used liquid crystal spatial light modulators. Nano-sized PDLCs, standard PDLCs with well patterned liquid crystal droplets distribution and dye doped liquid crystals with huge nonlinear response will be taken into account and shown to be among the most promising class of materials for developing high quality and low cost spatial light modulators.
We show that light-induced modification of the anchoring conditions can lead to an extraordinarily large optical response in dye-doped nematic liquid crystals. The bulk reorientation due to the collective elastic behavior of the liquid crystal is the origin of the nonlinearity, which occurs without a direct optical torque on the molecular director in the bulk, We call this effect SINE (Surface Induced Nonlinear Effect). These results can also explain the origin of the supra-nonlinear behavior recently observed in the same composites.