Luciano De Sio

POLICRYPS composite structures: realization, characterization and exploitation for electro-optical and all-optical applications

The acronym POLICRYPS indicates a nano/micro composite structure made of films of well aligned nematic liquid crystal (NLC) alternated to slices of almost pure polymer. Structures are fabricated by curing a homogeneous mixture of mesogenic material, monomer and a curing agent with UV radiation, under suitable physical and geometrical conditions. In particular, geometrical conditions determine (in the range 0.2 ÷ 15 μ m) the spatial periodicity of the realized sample, which can be utilized for transmitting, diffracting or reflecting an impinging light beam, with negligible scattering losses. The spatial modulation of the refractive index (from polymer to NLC) can be switched ON and OFF both by applying an electric field of few V/μ m or, in some cases, by irradiating the sample with a light beam of suitable wavelength. This electrical/optical tuneability is responsible for a series of distinctive characteristics of the optical effects produced by the structure, and determines the range of possible applications. In fact, in different geometries, the POLICRYPS can be exploited as a switchable holographic grating, a switchable optical phase modulator, a switchable beam splitter, a tuneable Bragg filter or it can be exploited as an electro-optical edge filter in an optical interrogation system.

Liquid Crystals as an Active Medium:Novel Possibilities in Plasmonics

Abstract The peculiar properties of Liquid Crystals (LCs) foster new possibilities in plasmonics. The combination of the intrinsic tunability of LCs with the plasmonic properties of metallic nanoparticles (NPs) provides novel and intriguing features of systems commonly identified as active plasmonics. Being LCs, one of the media whose refractive index can be controlled through the application of external stimuli, they represent a convenient host for enabling plasmonic tunability. On the other hand, the localized plasmonic resonance, typical of NPs, can strongly influence and control the behaviour of LCs. In this paper, we overview several systems of NPs combined with LCs arranged in different configurations. The properties of the resulting systems suggest novel, intriguing outcomes in both fundamental and applied research.

Fast Electro-Optical Device Based on Chiral Liquid Crystals Encapsulated in Periodic Polymer Channels

The chiral flexo-electro-optic effect produces a sub-millisecond, temperature independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback in the exploitation of this effect is that it relies on a texture, the Uniform Lying Helix (ULH), which is intrinsically unstable since neither planar nor homeotropic surface conditions are compatible with it. We present a method, based on the use of periodic polymeric micro-channels, to create highly ordered and stable ULH structures. We show that the periodic structure, which is created holographically, naturally aligns the cholesteric helical superstructure along the micro-channels, without requiring any elaborate ad-hoc procedure, even when the size Λ of the micro-channels is much larger than the pitch P (Λ > 20P). Electro-optic measurements performed on the test-device show a large contrast ratio between bright and dark states (better then 100:1), short switching time (200μs) and large optical rotation (>30°).

Digital polarization holography advancing 4G optics(Conference Presentation)

The fourth generation optics (4G optics) enables the realization of novel optical components (lenses, gratings, vector vortices, etc.) by patterning the optical axis orientation in the plane of an anisotropic film. Such components exhibit near 100% diffraction efficiency for wavelengths meeting half-wave retardation condition. In this framework, we have advanced a step-forward by realizing different diffractive waveplates (DWs) with arbitrary spatial patterns of the optical axis orientation by exploiting the capability of a Digital Spatial Light Polarization Converter (DSLPC). The DSLPC is based on a reflective, high resolution Spatial Light Modulator (SLM) combined with an “ad hoc” optical setup. The most attractive feature of the use of a DSLPC for photoalignment is that the orientation of the alignment layer, and therefore of the fabricated liquid crystal (LC) or liquid crystal polymer (LCP) DWs, can be specified on a pixel-by-pixel basis. By varying the optical magnification or de-magnification between the SLM and the alignment layer, the spatial resolution of the photoaligned layer can be adjusted to be optimal for each application. We show that with a simple “click” it is possible to record different high resolution optical components as well as arbitrary patterns ranging from lenses to invisible and even dual labels.

Manipulating Light and Images with 4G Optics: Digital Polarization Holography Using Liquid Crystals

Near 100% broadband efficiency of polarization holograms provides opportunities for shaping light beams, imaging, and displays. Thin, flexible and switchable between different optical states and functions, novel optical components and systems are smarter than ever.

CHAPTER 10:Lamellar and Circular Constructs Containing Self-Aligned Liquid Crystals

Holography and lithography allow realization of curved periodic microstructures comprising self-aligned liquid crystal (LC)s. The fabrication concept is based on phase separation of mixture of a photocurable polymer and a LC. Exploitation of an interference pattern allows realization of a periodic nano/microcomposite holographic grating made of slices of almost pure polymer alternating with well-aligned channels of LC. By means of a “Fresnel like” curing pattern, curved polymeric slices can impose radial alignment of nematic LCs. The distinctive features of the periodic structures enable high-quality and self-alignment of ordered fluids without the need for surface chemistry or functionalization.

Gold nanoparticles embedded in flexible materials: new frontiers in Plasmonics

Fabrication of samples showing plasmonic properties is fundamental for the realization of devices that can exhibit peculiar electromagnetic properties. Here we illustrate results of experiments performed on systems with Au nanoparticles embedded in flexible substrates.