Roberto Caputo

Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material

We present a new kind of UV-cured holographic grating that consists of polymer slices alternated with pure nematic films. By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increases diffraction efficiency.

Model for the photoinduced formation of diffraction gratings in liquid-crystalline composite materials

We modelize the photoinduced formation of switchable diffraction gratings in polymer-based liquid-crystalline composite materials. The model assumes that redistribution of molecules is due to mass diffusion and incorporates a realistic kinetic description of polymerization processes. Numerical simulations predict two different kinds of structure that have already been experimentally observed. Two parameters governing the phenomenon are pointed out, which determine the structure that will be formed after the curing process has been completed.

Characterization of the diffraction efficiency of new holographic gratings with a nematic film-polymer-slice sequence structure

We have performed a first characterization of the diffraction efficiency of gratings written in liquid-crystalline composite materials by the interference pattern of two curing beams. The grating fringes consist of polymer slices separated by films of continuous nematic phase. The dependence of the diffraction efficiency on temperature reveals a nonmonotonic behavior, with several maxima and minima. The shapes of curves are dependent on slight changes in the initial concentration of the nematic component of the mixture; the number of extrema increases with an increase of this concentration. The dependence of the diffraction efficiency on an applied external voltage also appears to be nonmonotonic: The shape depends on the sample’s temperature. Both switch-on and switch-off responses have been observed. The behavior of our gratings can be explained in the framework of the conventional Kogelnik theory for the diffraction efficiency of Bragg gratings.

Electro-optic properties of switchable gratings made of polymer and nematic liquid-crystal slices

We report the diffraction properties at wavelengths of 632.8 and 1550 nm for volume transmission gratings made of a sequence of continuously aligned nematic liquid-crystal layers separated by isotropic polymer slices. The gratings are generated by holographically curing a solution of liquid crystal diluted in an isotropic prepolymer by means of a laser beam at a wavelength of 352 nm with a total intensity of approximately 10 mW/cm2. A diffraction efficiency of 98% was measured, and an electric field as low as 5 V/microm switches off the phase grating. Measured angular spectra are fitted by use of the modified coupled-mode theory including the effects of grating birefringence.

In situ optical control and stabilization of the curing process of holographic gratings with a nematic film-polymer-slice sequence structure

We report on the realization of what we believe to be a new holographic setup for the fabrication of polymer liquid-crystal polymer-slice diffraction gratings, which utilizes an optical-feedback-driven nanopositioning technique. We have increased the stability of the interference pattern by means of a simple piezomirror used in a feedback configuration to keep constant the phase of the interferometer. The feedback system is driven by a proportional, integral, derivative control software, and the stability degree is controlled by the reference signal coming from a standard test grating. A preliminary experimental characterization indicates that good control and stabilization of parasitic fluctuations of the interference pattern are obtained.

Growing gold nanoparticles on a flexible substrate to enable simple mechanical control of their plasmonic coupling

A simple method is presented to control and trigger the coupling between plasmonic particles using both a growing process of gold nanoparticles (GNPs) and a mechanical strain applied to the elastomeric template where these GNPs are anchored. The large scale samples are prepared by first depositing and then further growing gold nanoparticles on a flexible PDMS tape. Upon stretching the tape the particles move further apart in the direction of the stretching and closer together in the direction perpendicular to it. The synergy between the controlled growth of GNPs and the mechanical strain, leads to a drastic shift of the plasmon band and a color change of the sample. Furthermore, the stretching by only a few percent of the amorphous and initially isotropic sample results in a strong polarization-dependent plasmon shift. At smaller gap sizes between neighboring particles, induced by stretching the PDMS tape, the plasmon shift strongly deviates from the behaviour expected considering the plasmon ruler equation. This shows that multipolar coupling effects significantly contribute to the observed shift. Overall, these results indicate that a macroscopic mechanical strain allows one to control the coupling and therefore the electromagnetic field at the nanoscale.

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.

Periodic and aperiodic liquid crystal-polymer composite structures realized via spatial light modulator direct holography

In this work we present the first realization and characterization of two-dimensional periodic and aperiodic POLICRYPS (Polymer Liquid Crystal Polymer Slices) structures, obtained by means of a single-beam holographic technique exploiting a high resolution spatial light modulator (SLM). A first investigation shows that the gratings, operating in the Raman Nath regime, exhibit a morphology and a electro-optical behavior that are typical of the POLICRYPS gratings realized by two-beam interference holography.

Kogelnik-like model for the diffraction efficiency of POLICRYPS gratings

We report on diffraction gratings consisting of continuous and well-aligned nematic films separated by polymer slices, which are originated in nematic-containing polymer composites when cured by a UV interference pattern. The diffraction efficiency of these gratings is experimentally investigated in detail; its temperature dependence reveals a rather complex, nonmonotonic shape, with a next-to-unit maximum at high temperatures and a next-to-zero minimum at lower ones. When the influence of an external electric field is investigated, the dependence of the diffraction efficiency on the applied voltage appears to be nonmonotonic too, the particular shape depending on the actual value of the sample temperature; in general, both switch-on and switch-off of the diffraction mechanism can be observed as an effect of the external field. For this kind of grating, a Kogelnik-like model has been implemented that makes use of only real values of some physical quantities, without the necessity of any fitting parameter. Numerical solutions account for the temperature dependence of the diffraction efficiency with good accuracy.

Broad band tuning of the plasmonic resonance of gold nanoparticles hosted in self-organized soft materials

The extraordinary properties of reconfigurable soft materials are used to drive the resonance properties of noble metal nanoparticles, using an approach that puts a bridge between soft matter and plasmonics. Gold nanoparticles have been dissolved in a cholesteric liquid crystal and then infiltrated in a micro-periodic polymeric structure, 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 the impinging probe light. Correlation between the optical response and external perturbations (electric field, temperature variation) gives an outstanding example of broadband tuning of an “active” plasmon resonance.