Liquid crystal cells with LiNbO<sub>3</sub>:Fe crystals as substrates, are described. The photovoltaic field generated by the substrates is able to reorient the liquid crystal director thus giving rise to a phase shift on the light propagating through the cell, as in liquid crystal light valves. The process does not require the application of an external electric field, thus being potentially useful for applications requiring a high degree of compactness. A detailed characterization of several cells based on lithium niobate crystals with different iron concentration has been carried out. The correlation between the LiNbO<sub>3</sub>:Fe characteristics and the liquid crystal reorientation is also discussed.
In this paper we outline that light-induced effects in liquid crystals are still able to provide scientific and technological novelty in spite of a long time investigation started more than thirty years ago. Here we review some recent achievements related to new phenomena that have been studied in the past few years. In the first part of our report we discuss optical trapping of nematic colloids whose origin relies on the elastic properties of liquid crystals rather than on the field gradient that is on the basis of conventional optical tweezing. In the second part we present some recent results obtained in studying the self-phase modulation in bent core nematic liquid crystals, pointing out a peculiar two regimes behavior.
We report the realization and characterization of porous nanostructures where a periodic refractive index modulation is
achieved by stacking layers with different nano-architectures. One multilayer photonic crystal has been fabricated
starting from colloidal dispersion of silicon dioxide and zirconium dioxide using spin coating technique. Improved
efficiency of Bragg reflectivity (up to 85%) has been obtained by a new bottom-up fabrication technique of photonic
hierarchical nanostructures based on self-assembly from the gas-phase at low temperature whit a very thin (≈ 1 μm)
photonic crystal devices. Due to the high porosity, these systems can be infiltrated with nematic liquid crystals leading to
tuning of the Bragg reflection band by applying low voltages to the structure.
We report the experimental demonstration that both the extra-ordinarily large nonlinear response and the light-induced permanent reorientation in liquid crystals doped by the azo-dye Methyl-Red originates from the modification of the charge density on the irradiated surface. By recording the sample response by applying dc or ac voltage, it is shown that in the latter case no permanent anchoring is possible. It is also demonstrated the limited role of photo-isomerization that gives a contribution to the nonlinear reorientation process only in the high dose regime. The effects on light-induced tuning of the Freedericksz transition are also reported.
Optical trapping and manipulation of micrometric silica particles dispersed in a nematic liquid crystal is reported.
Several kind of samples are considered: homeotropic and planar undoped cells and homeotropic and planar cells doped
by a small amount of the azo-dye Methyl-Red. The incident light intensity is over the threshold for optical reorientation
of the molecular director. The refractive index of the dispersed particles is lower than the ones of the liquid crystal
therefore the usual conditions for laser trapping and manipulation are not fulfilled. Nevertheless optical trapping is
possible and is closely related to the optical nonlinearity of the hosting liquid crystal<sup>1</sup>. Trapping in doped and undoped
cells are compared and it is shown that in the first case intensity lower by more than one order of magnitude is required
as compared to the one needed in undoped samples. The effect is faster and the structural forces are of longer range. The
formation of bubble-gum like defects in doped samples under certain experimental conditions is also reported and
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).
We present a novel optical sensor able to measure the distance between the tip of an endoscopic probe and the anatomical object under examination. In medical endoscopy, knowledge of the real distance from the endoscope to the anatomical wall provides the actual dimensions and areas of the anatomical objects. Currently, endoscopic examination is limited to a direct and qualitative observation of anatomical cavities. The major obstacle to quantitative imaging is the inability to calibrate the acquired images because of the magnification system. However, the possibility of monitoring the actual size of anatomical objects is a powerful tool both in research and in clinical investigation. To solve this problem in a satisfactory way we study and realize an absolute distance sensor based on fiber optic low-coherence interferometry (FOLCI). Until now the sensor has been tested on pig trachea, simulating the real humidity and temperature (37°C) conditions. It showed high sensitivity, providing correct and repeatable distance measurements on biological samples even in case of very low reflected power (down to 2 to 3 nW), with an error lower than 0.1 mm.
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.
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.
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.
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.
A review of the recent results of our group in the field of light-induced anchoring and reorientation effects in dye-doped liquid crystals (LCs) is presented. In particular, the phenomena of photoinduced anchoring and permanent reorientation over a polymeric boundary surface of a dye-doped LC cell is reported, both in the isotropic phase and in the orientationally ordered nematic phase. The results have been interpreted microscopically in terms of adsorption and desorption of the dichroic azo-dye (methyl-red) molecules onto the illuminated surface during light irradiation. The model proposed is in agreement with recent results on the dynamic and stable grating formation in methyl-red doped LCs .
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.
In this paper an experimental study of thermo-optical properties of polymer dispersed liquid crystals (PDLC), prepared by PIPS in bulk and in confined cylindrical geometry, is presented. The transmissivity of PDLC In bulk as a function of temperature proves the existence of temperature optical switching. We have also demonstrated the existence of optical bistability, which could be interesting in develop of logical optical devices as optical memory elements. A bistable al fiber optic sensors based on PDLC is also presented. In this device PDLC permits at the same time the optomechanical interconnection of tow fibers and the modulation of the light crossing the device. As the modulation can be controlled by external temperature, the device has been proved to be suitable for the realization of a heat flow sensor. Without any optimization of the device we have obtained an ON-OFF contrast of 8 dB and a response time comparable with other conventional device using nematic LCs. This sensor is compact, rugged and is cheap, because it does not require a complex fabrication and alignment technology. It presents the typical advantages of both the fiber optic sensor and the liquid crystal technology. We note that its main advantage is a small thermal capacity, which is comparable with electronic device as thermistors, and it represents a significant improvement for the sensor based on liquid crystals. Further theoretical studies are necessary in order to understand in depth it thermo-optical characteristics.
We analyzed the self-transparency effect of a laser beam traveling in a sample of an unsaturated polyester. Polymer Dispersed Liquid Crystal. This effect is of thermal nature and occurs when a change of the refractive index of the considered composite material is induced by variations of the local temperature due to the incident light power. We studied the mechanism governing this phenomenon and report a detailed 3D map showing how the transmitted beam profile changes as a function of both incident power and time. It is discussed how light intensity and temperature can be used as control parameters for the nonlinear part of the refractive index. Our experimental results indicates the possibility of employing this material to design thermal sensors as devices working as optical switch.
We present a new type of effective photoregulation of the LC orientation on an aligning surface through light action upon the liquid crystal bulk. The illumination of an azo- containing LC bulk with polarized light is found to induce both stable high-resolution alignment onto the rigid surface and dynamic sliding of the director over the surface. Typical times of the director sliding over the aligning surface are of the order of 1 ms. Both effects are characterized by extremely small light intensities and seem to be very promising for the development of optical devices for information processing and high density information storage. Light-induced surface memory effects are also observed in these system as a consequence of induced anchoring of the molecular director. This technique has been applied to record both stationary and dynamic holograms with a spatial resolution of 7 micrometers . The surface memory effects have been applied to record both binary and gray scale images. As compared to other methods used in these materials the highest sensitivity and spatial resolution can be achieved: energy densities as low as 10<SUP>-1</SUP>J/cm<SUP>2</SUP> were sufficient to write gratings with a resolution better than 100 lines/mm.