POlymer LIquid CRYstal Polymer Slices Research Articles

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.

The POLICRYPS liquid-crystalline structure for optical applications

Abstract We present a review of polymer-liquid crystal-based devices for optical applications. Starting from a particular fabrication technique, which enables to obtain the POLICRYPS (POlymer LIquid CRYstal Polymer Slices) structure, we illustrate different realizations, along with their working principle and main features and performances. The name POLICRYPS indicates a structure made of parallel slices of pure polymeric material alternated to films of well-aligned nematic liquid crystal (NLC), with a spatial periodicity that can be settled in the range 0.2÷15 μm. Suitably designed samples can be utilized as optical devices with a high efficiency, which can be switched on and off both by applying an electric field of a few V/μm or by irradiating samples with a suitable light beam. In different geometries, POLICRYPS can be specialized to operate as switchable diffraction grating, switchable optical phase modulator, switchable beam splitter, or tunable Bragg filter. The POLICRYPS framework can be also used as a soft matter template for aligning different types of LCs or to create an array of tunable microlasers. Finally, we present a POLICRYPS structure with a polar symmetry of the director alignment, which enables local shaping of light polarization, allowing to convert circularly polarized beams into cylindrical vector beams.

Characterisation of POLICRYPS structures assembled through a two-step process

We describe optical diffraction properties of polymer liquid-crystal polymer slices (POLICRYPS) assembled through a two-step procedure. At first, a scaffold of periodic polymer slices is fabricated by a direct laser writing method based on two-photon polymerisation. Then the channels between the slices are filled with liquid crystal, and the assembly is incorporated between two ITO-coated glass plates. In contrast to the ‘classical’ POLICRYPS that are fabricated on the basis of a phase separation process and that exhibit spontaneous liquid crystal alignment in the direction perpendicular to the polymer slices (homeotropic alignment), the structures fabricated by the two-step method exhibit spontaneous liquid crystal alignment in a direction parallel to the polymer slices (planar alignment). Consequently, their optical diffraction properties as a function of the polarisation direction of the optical radiation exhibit orthogonal behaviour with respect to the classical structures. The threshold external field for electro-optic switching is in the range of 0.5 V/μm. The associated electro-optic switching times are in the range of milliseconds.

POLYCRYPS visible curing for spatial light modulator based holography

Realization of highly efficient holographic structures can be obtained by combining the polymer liquid crystal polymer slices (POLICRYPS) technique and the use of a spatial light modulator. To achieve this result, a new prepolymer mixture is necessary that is sensitive to visible light and fulfills all requirements of the POLICRYPS technique. In this paper, we report on our efforts to realize this new mixture and on the first attempts made for fabricating one-dimensional POLICRYPS gratings. Newly obtained diffractive structures have been compared with standard POLICRYPS showing negligible differences.

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.

Soft-matter structures: From switchable diffraction gratings to active plasmonics

We present a review of a series of nano/micro, soft-matter–based, periodic, composite structures, along with a theoretical model, which explains their main physical and optical features as determined by a suitable choice of the values of given physical and geometrical parameters. The historical development is illustrated by starting from the fabrication technique, which enables obtaining a structure made of films of well-aligned Liquid Crystal alternated to polymer slices. Realised samples, which exhibit a spatial periodicity in the range 0.2–15 μm, are given the generic name POLICRYPS (as an acronym of POlymer LIquid CRYstal Polymer Slices) and are obtained by curing a homogeneous mixture of mesogenic material, monomer and curing agent under suitable conditions and by means of a UV radiation. A number of interesting applications are determined by the possibility of tuning, or even switching on/off, the spatial modulation (from polymer to LC) of the refractive index of these structures; this effect of tuneability can be obtained by applying an electric field of few V/μm or, in some cases, by irradiating the sample with a light beam of suitable wavelength. We show that, depending on the used geometry, these particular structures can be exploited as switchable diffraction gratings, tuneable beam splitters, switchable optical phase modulators, tuneable Bragg filters, soft matter templates for aligning different kinds of LCs, arrays of tuneable microlaser, or can enable realising “active plasmonics” devices.

Molecular Orientation of E7 Liquid Crystal in POLICRYPS Holographic Gratings: A Micro-Raman Spectroscopic Analysis

Micro-Raman Spectroscopy was used to determine the orientation of the E7 nematic liquid crystal molecules inside the POLICRYPS (POlymer LIquid CRYstal Polymer Slices) holographic gratings. The POLICRYPS grating was analyzed after defining the Raman features of each single component of the system: i.e the random aligned nematic liquid crystal E7 and Norland Optical Adhesive (NOA-61). Among the Raman bands of the liquid crystal molecules, a specific one was selected, which exhibits strong anisotropy of the Raman tensor. The modulation of its intensity, depending on the sample orientation with respect to the laser polarization, provided the experimental evidence about the orientation of the E7 molecules, which occurred to be perpendicular to the polymeric slices.

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.

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.

POLICRYPS: a liquid crystal composed nano/microstructure with a wide range of optical and electro-optical applications

POLICRYPS (an acronym of polymer liquid crystal polymer slices) is a nano/microcomposite structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystal (NLC). The structure is obtained by irradiating a homogeneous syrup of NLC, monomer and curing agent molecules with an interference pattern of UV/visible light under suitable experimental and geometrical conditions; the spatial periodicity can be easily varied from an almost nanometric (200 nm) to a micrometric (15 µm) scale. Where the effect on an impinging reading light beam is concerned, the POLICRYPS can be utilized either in a transmission or a reflection configuration (depending on the geometry and substrate used) with negligible scattering losses, while the effect of spatial modulation of the refractive index (from polymer to NLC values) can be switched on and off by applying an external electric field of the order of few V µm−1. In this paper, we start by reviewing the general features of the POLICRYPS structure, that is the 'recipe' to fabricate it, along with a chemical–diffusive model that indicates the right physical and chemical conditions to make samples exhibiting good morphological, optical and electro-optical properties. We then show some possible utilizations of POLICRYPS with a light beam impinging almost perpendicularly to the structure: a switchable diffraction grating and a switchable optical phase modulator. Furthermore, we put into evidence that POLICRYPS channels can become an array of mirrorless optical micro-resonators for lasing effects. Finally, we report about utilization of POLICRYPS with a light beam impinging parallel to the structure and perpendicular to the channels, demonstrating that, in this case, the structure becomes a tuneable Bragg filter. Performances exhibited in all above applications put the POLICRYPS structure at the top level of the state of art of application oriented research in optics of liquid crystalline composite materials.

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.

A Switchable Optical Add-Drop Multiplexer using Ion-Exchange Waveguides and a POLICRYPS Grating Overlayer

We propose a novel add-drop multiplexer for dense wavelength division multiplexing optical communications (DWDM) operating in C band (1530–1565 nm). The device has been designed for channel in-diffused glass waveguides. The device is switchable because a POLICRYPS (POlymer LIquid CRYstal Polymer Slices) based grating is deposited on top of the wider bimodal waveguide. At our knowledge this is the first example of switchable add-drop multiplexer.

Observation of two-wave coupling during the formation of POLICRYPS diffraction gratings

We report on the experimental observation of two-wave mixing that occurs inside a sample between the beams used for the fabrication of polymer-liquid-crystal-polymer slices (POLICRYPS) diffraction gratings. The effect depends on the phase shift between the curing interference pattern and the grating being cured. This shift can be mechanically induced by accidental vibrations of the experimental setup; thus a high setup stability is needed. We devised a mechanism that enables us to control setup vibrations in situ and used it to monitor the experiment. When the mechanically induced shift was significantly small, wave mixing was observed only if the initial intensities of the two curing beams were different from each other.