Plasmon-mediated discrete diffraction behaviour of an array of responsive waveguides.

We investigate the discrete diffraction phenomenon in a Polymer-Liquid Crystal-Polymer Slices (POLICRYPS) overlaying a random distribution of gold nanoparticles (AuNPs, plasmonic elements). We study the propagation of a CW green laser beam through the waveguide structure as a function of beam polarization, laser intensity and sample temperature. It turns out that the plasmonic field created at the interface between AuNPs and POLICRYPS waveguides enables and stabilizes the optical field propagation within the responsive nematic liquid crystal channels. The active role of the liquid crystal is pointed out by a polarization, temperature and beam divergence experimental analysis and evidenced by a peculiar trumpet-shaped discrete diffraction pattern. Theoretical simulations confirm that the observed optical behavior is governed by the interaction of the nematic liquid crystal with optical and plasmonic fields.

Antimicrobial Effects of Chemically Functionalized and/or Photo-Heated Nanoparticles.

Antibiotic resistance refers to when microorganisms survive and grow in the presence of specific antibiotics, a phenomenon mainly related to the indiscriminate widespread use and abuse of antibiotics. In this framework, thanks to the design and fabrication of original functional nanomaterials, nanotechnology offers a powerful weapon against several diseases such as cancer and pathogenic illness. Smart nanomaterials, such as metallic nanoparticles and semiconductor nanocrystals, enable the realization of novel drug-free medical therapies for fighting against antibiotic-resistant bacteria. In the light of the latest developments, we highlight the outstanding capabilities of several nanotechnology-inspired approaches to kill antibiotic-resistant bacteria. Chemically functionalized silver and titanium dioxide nanoparticles have been employed for their intrinsic toxicity, which enables them to exhibit an antimicrobial activity while, in a different approach, photo-thermal properties of metallic nanoparticles have been theoretically studied and experimentally tested against several temperature sensitive (mesophilic) bacteria. We also show that it is possible to combine a highly localized targeting with a plasmonic-based heating therapy by properly functionalizing nanoparticle surfaces with covalently linked antibodies. As a perspective, the utilization of properly engineered and chemically functionalized nanomaterials opens a new roads for realizing antibiotic free treatments against pathogens and related diseases.

Flexible thermo-plasmonics: an opto-mechanical control of the heat generated at the nanoscale.

The opto-mechanical control of the heat generated by an amorphous arrangement of homogenously distributed gold nanoparticles (AuNPs), excited by an external laser source, is investigated. Application of a macroscopic mechanical strain to the biocompatible elastomeric tape supporting the particles leads to a nanoscale modification of their mutual inter-distance. The resulting strong variation of the particles near-field coupling gives rise to a macroscopic variation of the photo-generated heat. A fine control of the amount of generated heat is thus possible by stretching the initially isotropic sample by only a few percent. Due to the anisotropy of the stretching procedure, the plasmon band shift and thus the heat generation becomes strongly polarization-dependent. A model of the system based on Mie theory is implemented by using a finite element method. Under optical excitation, two configurations of AuNPs, representing the same cluster of particles at rest and under stretching, show a relative increase of temperature that is in good quantitative agreement with experimental data, if normalized to the number of involved particles. This system realizes for the first time an opto-mechanical control of the temperature at the nanoscale which holds promise for the development of optically-active thermal patches, usable for biomedical applications, and flexible platforms for microfluidics and lab-on-a-chip devices.

Photo-Induced Heating in Plasmonic Nanoparticles Trapped in Thermo-Sensitive Liquid Crystals.

Thermo-sensitive liquid crystals may result, for some aspects, good host materials for plasmonic nanoparticles. In particular they are suitable to study and measure the temperature variations produced by photo-induced plasmonic joule effect in the metallic nanoparticles. Combining the properties of liquid crystals and metallic nanoparticles, allows to measure temperature variations in different ways by exploiting the optical properties of thermotropic liquid crystals: In a first attempt, by combining nematic liquid crystals and spherical metallic nanoparticles, we have predicted and measured temperature changes, under a suitable (resonant) optical illumination, by measuring the photo-thermal induced birefringence variation. In a different experiment, we have combined cholesteric liquid crystals and gold nanorods: Light-induced variations of structural colorations exhibited by cholesteric liquid crystals has been used as a new methodology to measure nanoscale heat variation with a very high sensitivity (0.03 K).

Conformal Silk-Azobenzene Composite for Optically Switchable Diffractive Structures.

The use of biomaterials as optical components has recently attracted attention because of their ease of functionalization and fabrication, along with their potential use when integrated with biological materials. We present here an observation of the optical properties of a silk-azobenzene material (Azosilk) and demonstrate the operation of an Azosilk/PDMS composite structure that serves as a conformable and switchable optical diffractive structure. Characterization of thermal and isomeric properties of the device, along with its overall performance, is presented in terms of diffractive characteristics and response times. The ease of manufacturing and functionalization opens a promising avenue for rapid device prototyping and interfaces of expanded utility.

Photo-thermal effects in gold nanoparticles dispersed in thermotropic nematic liquid crystals.

The last few years have seen a growing interest in the ability of metallic nanoparticles (MNPs) to control temperature at the nanoscale. Under a suitable optical radiation, MNPs feature an enhanced light absorption/scattering, thus turning into an ideal nano-source of heat, remotely controllable by means of light. In this framework, we report our recent efforts on modeling and characterizing the photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in thermotropic Liquid Crystals (LCs). Photo-induced temperature variations in GNPs dispersed in Nematic LCs (NLCs) have been studied by implementing an ad hoc theoretical model based on the thermal heating equation applied to an anisotropic medium. Theoretical predictions have been verified by performing photo-heating experiments on a sample containing a small percentage of GNPs dispersed in NLCs. Both theory and experiments represent an important achievement in understanding the physics of heat transfer at the nanoscale, with applications ranging from photonics to nanomedicine.

From Cartesian to polar: a new POLICRYPS geometry for realizing circular optical diffraction gratings.

We report on the realization of a liquid crystal (LC)-based optical diffraction grating showing a polar symmetry of the director alignment. This has been obtained as a natural evolution of the POLICRYPS technique, which enables the realization of highly efficient, switchable, planar diffraction gratings. Performances exhibited in the Cartesian geometry are extended to the polar one by exploiting the spherical aberration produced by simple optical elements. This enables producing the required highly stable polar pattern that allows fabricating a circular optical diffraction grating. Results are promising for their possible application in fields in which a rotational structure of the optical beam is needed.

Directed organization of DNA filaments in a soft matter template.

We have developed a noninvasive, all-optical, holographic technique for permanently aligning liquid crystalline DNA filaments in a microperiodic template realized in soft-composite (polymeric) materials. By combining optical intensity holography with a selective microfluidic etching process, a channelled microstructure has been realized which enables self-assembly of DNA. The striking chemicophysical properties of the structure immobilize the DNA filaments within the microchannels without the need of any kind of surface chemistry or functionalization. Polarized optical, confocal, and electronic microscopies have been used for characterizing the DNA geometry inside the microchannels in terms of birefringence, fluorescence, and nanoscale organization properties. In particular, observation of a far-field diffraction pattern confirms a periodic organization of the DNA filaments inside the polymeric template.

Soft periodic microstructures containing liquid crystals.

An empty polymeric structure has been realized by combining a high precision level optical holographic setup and a selective microfluidic etching process. The distinctive features of the realized periodic microstructure enabled aligning several kinds of liquid crystal (LC) compounds, without the need of any kind of surface chemistry or functionalization. In particular, it has been possible to exploit light sensitive LCs for the fabrication of all-optical devices, cholesteric and ferroelectric LCs for ultrafast electro-optical switches, and a common LC for a two-dimensional periodic structure with high anisotropy. All-optical and electro-optical experiments, performed for investigating the samples in terms of switching voltages and response times, confirm good performances of the realized devices.

Double active control of the plasmonic resonance of a gold nanoparticle array.

A two-fold active control of the plasmonic resonance of randomly distributed gold nanoparticles (GNPs) has been achieved. GNPs have been immobilized on an Indium Tin Oxide (ITO) coated glass substrate and then covered with a liquid crystalline compound. The system has been investigated by means of atomic force and scanning electron microscopy, revealing the presence of isolated and well distributed GNPs. The application of an external electric field to the sample has a two-fold consequence: the re-orientation of the hybrid-aligned liquid crystal layer and the formation of a carrier accumulation layer in the proximity of the ITO substrate. The refractive indices of both liquid crystal and accumulation layers are influenced by the applied field in a competitive way and produce a "dancing behavior" of the GNP's plasmonic resonance spectral position.

Electro-switchable polydimethylsiloxane-based optofluidics.

We report on the fabrication and characterization of a new generation of electro-switchable optofluidic devices based on flexible substrates, combined with the extraordinary properties of reconfigurable soft-materials. A conductive polydimethylsiloxane microstructure has been first sputtered with an Indium Tin Oxide (ITO) layer and then functionalized with an amorphous film of SiO(x). Then, the "layer" by "layer" microstructure has been infiltrated with an anisotropic and reconfigurable fluid (Nematic Liquid Crystal, NLC). The sample has been characterized in terms of morphological, optical and electro-optical properties: the soft-conductive microstructure exhibits a uniform and regular morphology, even after testing with mechanical stretching and deformations. Combination of the conductive ITO with the functionalization film (which has been employed for inducing in-plane alignment of NLC molecules) enables us to carry out a series of optical and electro-optical experiments; these confirm excellent properties in terms of a reconfigurable device and a diffractive element as well.

Observation of tunable optical filtering in photosensitive composite structures containing liquid crystals.

We report on the investigation and characterization of an optically 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. Exposure to a low power visible light beam modifies the azo-dye molecular configuration, thus allowing the filtered wavelength to be tuned over a 6.6 nm range. Simulations of the filtering response are well described with our experimental findings.

In situ polarized micro-Raman investigation of periodic structures realized in liquid-crystalline composite materials.

In situ polarized micro-Raman Spectroscopy has been utilized to determine the liquid crystal configuration inside a periodic liquid crystalline composite structure made of polymer slices alternated to films of liquid crystal. Liquid crystal, Norland Optical Adhesive (NOA-61) monomer and its polymerized form have been investigated separately. The main Raman features, used as markers for the molecular orientation estimation, have been identified. In situ polarized Raman spectra indicate that the orientation of the liquid crystal director inside the structure is perpendicular to its polymeric slices. Results show the usefulness of in situ polarized micro-Raman spectroscopy to investigate liquid crystalline composite structures.

Dual-mode control of light by two-dimensional periodic structures realized in liquid-crystalline composite materials.

We report on the realization of a 2D refractive structure consisting of a polymer-liquid-crystal polymer slice grid. Nematic liquid crystal microdomains are confined inside well-sculptured elliptical cavities; experimental investigation shows that the liquid crystal director lies in the plane of the structure and its orientation follows a preferred direction. The sample exhibits both an electro-optical and an all-optical response owing to a small percentage of photosensitive azo dye included in the structure. A double external control of the two-dimensional grating efficiency can indeed be operated either by an optical pump beam or by the standard technique of applying a suitable external voltage to the sample.

Jones matrix analysis of dichroic phase retarders realized in soft matter composite materials.

Materials showing birefringence and polarization selective absorption (dichroism) affect the polarization state of incoming light in a peculiar way, quite different from the one exhibited by phase retarders like waveplates. In this paper, we report on the characterization of a Polymer LIquid CRYstal Polymer Slices (POLICRYPS) diffraction grating used as a dichroic phase retarder; the dichroic behaviour of the grating is due to the polarization-dependent diffraction efficiency. Experimental data are validated with a theoretical model based on the Jones matrix formalism, while the grating behavior is modeled by means of the dichroic matrix. In this way, the birefringence of the analyzed structure is easily obtained. For comparison purposes, also two systems different from POLICRYPS have been fabricated and tested.

Observation of hysteresis effects in POLICRYPS holographic gratings.

We report on the observation of hysteresis effects that take place in the electro-optical response of a polymer-liquid-crystal-polymer slice grating named POLICRYPS. Investigation of transmittance as a function of an applied external voltage reveals an hysteresis area that is strongly correlated to the delay between consecutive applications of square wave voltages of different amplitude. A suitable software has been implemented to control this delay: when it is long, results show a large hysteresis area, which can be reduced by diminishing the delay. In addition, correlation between this area and the frequency of the applied voltage is analyzed, taking into account that the effect of impurity ions migration is not negligible in the case of low frequencies.

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.

Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating.

We report the fabrication and the optical characterization of a hybrid tunable integrated optical filter. It consists of a diffused ion-exchanged channel waveguide on a borosilicate glass substrate with a cover of the same glass to form a gap filled with a holographic grating. The grating morphology, called POLICRYPS (POlymer LIquid CRYstal Polymer Slices), is made of alternating stripes of polymer and liquid crystal acting as overlayer for the underneath waveguide. The filter structure includes aluminum coplanar electrodes to electrically control the grating properties, allowing the tunability of the filter. The electric driving power required to tune the filter obtained was in the range of submilliwatts due to the efficient liquid crystal electro-optic effect.

Characterization of an active control system for holographic setup stabilization.

We report on the realization and characterization of an active control system of the optical holographic setup used for fabrication of holographic gratings in liquid-crystalline composite materials. The system exploits a reference diffraction grating and a piezomirror in closed-loop feedback. The piezoelectric mirror exhibits a hysteresis that depends not only on the applied voltage, but also on the history of the mirror motion. In an open-loop configuration, the hysteresis can be reduced by adjusting the delay time between the application of two different control voltage values; in a closed-loop operation, it is possible to eliminate the residual hysteresis. By testing the system in different conditions, it has been shown that residual fluctuations are comparable to the resolution of the piezomirror operation.