L. De Sio

POLICRYPS structures as switchable optical phase modulators.

We report on the electrically controlled optical phase modulator behavior of light sculptured periodic structures made of polymer slices alternated to films of well aligned Liquid Crystals (POLICRYPS). Arbitrarily polarized light normally incident on the structure experiences a birefringence that depends on the anisotropy of the composite liquid crystalline material and on the geometrical cell parameters. The sample behaves as a retardation plate in good agreement with the Jones matrices formalism. Birefringence tuning is obtained by applying a suitable voltage, while a negligible birefringence variation is detected by increasing the incidence power. This makes POLICRYPS structures suitable as switchable phase retarders for high power laser beams.

All-Optical and Thermal Tuning of a Bragg Grating Based on Photosensitive Composite Structures Containing Liquid Crystals

We report on the fabrication and characterization of a tunable filtering effect, observed in a waveguide grating made of alternated strips of photocurable polymer and a mixture of azo-dye doped liquid crystal. The grating is sandwiched between two borosilicate glasses, one of which includes an ion-exchanged channel waveguide, which confines the optical signal to be filtered. We analyze the effects of a low power visible, pump light beam that hits the sample and temperature effects. Both effects are investigated experimentally. The application of the pump light allowing the filtered wavelength to be tuned over a 6.6 nm range, while a temperature variation of 4°C, implies a tuning of 14 nm.

Characterization of the diffraction efficiency of polymer-liquid-crystal-polymer-slices gratings at all incidence angles.

Recently, a novel holographic diffraction grating made of polymer slices alternated to homogeneous films of nematic liquid crystal (POLICRYPS) was realized. We study the optical performance of the POLICRYPS gratings by both numerical simulations and experiments. Characterization of the grating at normal and conical reading mount are performed. The diffraction efficiency depends strongly on the angles of incidence. Besides, the characterization of the diffraction efficiency at Bragg angle incidence is studied. A uniform high diffraction efficiency is achieved when the incident wave satisfies the Bragg condition.

Optical interrogation system based on holographic soft matter filter

We present an optical interrogation system based on a holographic soft-matter filter. It provides a wavelength-dependent transmittance, with a linear relation between Bragg wavelength shift and variation in the filter output intensity. This is the demonstration of a simple and inexpensive technology to implement a, low cost, monitoring system based on a fiber Bragg grating sensor.

Integration and characterization of LC/Polymer gratings on glass and silicon platform

In this paper we present two practical technological approaches to demonstrate that liquid crystals and composite materials can be employed to make low cost integrated optic devices with the specifications of fibre optic systems. In a first approach active properties are conferred to passive glass waveguides, by means of LC-based composite holographic grating cladding: this grating presents a specific morphology called POLICRYPS (POlymer LIquid CRYstal Polymer Slices). It can be electrically addressed, so that a tunable integrated optical filter has been achieved. In this paper we present the electro-optical characterization. In a second approach SiO2/Si V-grooves are filled with composite materials to produce a high performance integrated optical filter with the same POLICRYPS morphology. A first passive prototype of such optical filter is presented hereafter. Both prototypes have been characterized in terms of spectral response.

Realization and Characterization of POLICRYPS-like Structures Including Metallic Subentities

A first characterization of newly realized micro periodic structures including metallic nanoparticles is reported. The original mixture, generally utilized for the realization of polymer-liquid-crystal-polymer-slices gratings (POLICRYPS), has been enriched with a small amount of silver nanoparticles. The obtained structure shows a spectral response that strongly depends on the polarization of the probing light. These first structures are oriented to the fabrication of devices with metamaterial properties.

Plasmon Resonance Tunability of Gold Nanoparticles Embedded in a Confined Cholesteric Liquid Crystal Host

Gold nanoparticles have been dissolved in a cholesteric liquid crystal and then infiltrated in a micro-periodic polymeric structure. This has been realized by combining a holographic step and a microfluidic etching process. The spectral behavior of the nano-composite soft-structure has been investigated in the UV-Vis range for two different polarization directions of an impinging probe light and in presence of external perturbations (electric field, temperature variation). Obtained results show a highly tunable plasmonic response of the material.

Phase Modulator Behavior of a Wedge-Shaped POLICRYPS Diffraction Grating

Experimental characterization of a switchable wedge-shaped holographic grating involved as a tunable phase retarder is reported. The structure is made of polymer slices alternated to films of well aligned Liquid Crystals (POLICRYPS). The sample shows a birefringence that depends on the anisotropy of the composite liquid crystalline material and on the geometrical cell parameters. The phase retardation introduced by the structure on the impinging probe light can be finely tuned by varying the amplitude of the externally applied electric field or by spatially shifting the probed area on the sample. The quarter wave plate condition for a He-Ne laser probe has been obtained confirming the possible application of the structure as an achromatic phase retarder.

Two-Wave Coupling during the Formation of POLICRYPS Diffraction Gratings: Experimental Results Theoretical Model

In this paper we report the investigation of a beam coupling effect that occurs during the formation (by UV curing) of POLICRYPS diffraction gratings. Along with observations, we present a complete theoretical model which accounts for the main experimental features of the effect. Numerical solutions confirm the observed absence of any energy transfer process for unit ratio of the impinging beam intensities. When this ratio is not unit, the transfer is instead present and tends to equalize the beam intensities during the curing process; the capability of performing this equalization is strongly related to the grating diffraction efficiency. Furthermore, in this case, numerical simulations enable also to visualize the final (distorted) morphology of the fabricated structure.

Low driving power integrated tuneable filter using composite holographic grating on glass waveguides

Experimental findings on a novel, compact, low cost tuneable and switchable guided wave optical filter using a composite holographic Bragg grating as the perturbation element of the guided optical field is summarized. The optical transmittance of the filter is measured by an optical spectrum analyzer. The tuning of the filter reflection response is also obtained.