Silicon oxides as alignment surfaces for vertically-aligned nematics in photonic devices

A comparative study on alignment performance and microstructure of inorganic layers used for liquid crystal cell conditioning has been carried out. The study has focused on two specific materials, SiOx and SiO2, deposited under different conditions. The purpose was to establish a relationship between layer microstructure and liquid crystal alignment. The surface morphology has been studied by FESEM and AFM. An analysis on liquid crystal alignment, pretilt angle, response time, contrast ratio and the conditions to develop backflow effect (significant rise time increase due to pure homeotropic alignment) on vertically-aligned nematic cells has been carried out. A technique to overcome the presence of backflow has been identified. The full comparative study of SiOx and SiO2 layer properties and their influence over liquid crystal alignment and electrooptic response is presented.

Scalable UWB photonic generator based on the combination of doublet pulses

We propose and experimentally demonstrate a scalable and reconfigurable optical scheme to generate high order UWB pulses. Firstly, various ultra wideband doublets are created through a process of phase-tointensity conversion by means of a phase modulation and a dispersive media. In a second stage, doublets are combined in an optical processing unit that allows the reconfiguration of UWB high order pulses. Experimental results both in time and frequency domains are presented showing good performance related to the fractional bandwidth and spectral efficiency parameters.

1D and 2D neutral particle patterning by dielectrophoretic forces on z-cut Fe:LiNbO3 crystals

1D and 2D patterning of uncharged micro- and nanoparticles via dielectrophoretic forces on photovoltaic z-cut Fe:LiNbO3 have been investigated for the first time. The technique has been successfully applied with dielectric micro-particles of CaCO3 (diameter d = 1-3 μm) and metal nanoparticles of Al (d = 70 nm). At difference with previous experiments in x- and y-cut, the obtained patterns locally reproduce the light distribution with high fidelity. A simple model is provided to analyse the trapping process. The results show the remarkably good capabilities of this geometry for high quality 2D light-induced dielectrophoretic patterning overcoming the important limitations presented by previous configurations.

Thermally tunable polarization by nanoparticle plasmonic resonance in photonic crystal fibers

A photonic crystal fiber selectively filled with silver nanoparticles dispersed in polydimethylsiloxane has been numerically studied via finite elements analysis. These nanoparticles possess a localized surface plasmon resonance in the visible region which depends on the refractive index of the surrounding medium. The refractive index of polydimethylsiloxane can be thermally tuned leading to the design of polarization tunable filters. Filters found with this setup show anisotropic attenuation of the x-polarization fundamental mode around ?x = 1200dB/cm remarkably higher than the y-polarization mode. Moreover, high fiber birefringence and birefringence reversal is observed in the spectral region of the plasmon.

Scalable high-order UWB pulse generation employing an FBG-based photonic superstructure

In this letter, we propose and experimentally demonstrate a compact, flexible, and scalable ultrawideband (UWB) generator based on the merge of phase-to-intensity conversion and pulse shaping employing an fiber Bragg Grating-based superstructure. Our approach offers the capacity for generating high-order UWB pulses by means of the combination of various low-order derivatives. Moreover, the scheme permits the implementation of binary and multilevel modulation formats. Experimental measurements of the generated UWB pulses, in both time and frequency domain, are presented revealing efficiency and a proper fit in terms of Federal Communications Commission settled standards.

Design and demonstration of an electronically scanned reflectarray antenna at 100 GHz using multiresonant cells based on liquid crystals

The design, fabrication and measured results are presented for a reconfigurable reflectarray antenna based on liquid crystals (LC) which operates above 100 GHz. The antenna has been designed to provide beam scanning capabilities over a wide angular range, a large bandwidth and reduced Side-Lobe Level. Measured radiation patterns are in good agreement with simulations, and show that the antenna generates an electronically steerable beam in one plane over an angular range of 55º in the frequency band from 96 to 104 GHz. The Side Lobes Level is lower than -13 dB for all the scan angles and -18 dB is obtained over 16% of the scan range. The measured performance is significantly better than previously published results for this class of electronically tunable antenna, and moreover verifies the accuracy of the proposed procedure for LC modeling and antenna design.

Manufacturing arbitrarily-shaped active waveguides with liquid crystal cladding

Many photonic devices are based on waveguides (WG) whose optical properties can be externally modified. These active WGs are usually obtained with electrooptic materials in either the propagating film (core) or the substrate (cladding). In the second case, the WG tunability is based on the interaction of the active material with the evanescent field of the propagating beam.Liquid crystals (LCs) are an excellent choice as electrooptic active materials since they feature high birefringence, low switching voltage, and relatively simple manufacturing. In this work, we have explored alternative ways to prepare WGs of arbitrary shapes avoiding photolithographic steps. To do this, we have employed a UV laser unit (Spectra Physics)attached to an xyzCNC system mounted on an optical bench. The laser power is 300mW, the spot size can be reduced slightly below 1 µm, and the electromechanicalpositioning is well below that number.Different photoresinshave been evaluated for curing time and uniformity; the results have been compared to equivalent WGs realized by standard photolithographic procedures. Best results have been obtained with several kinds of NOA adhesives (Norland Products Inc.) and SU8 (Microchem). NOA81 optical adhesive has been employed by several groups for the preparation ofmicrochannels [1] and microfluidic systems[2]. In our case, several NOAs having different refractive indices have been tested in order to optimize light coupling and guiding. The adhesive is spinnedonto a substrate, and a number of segmented WGs are written with the laser system. The laser power is attenuated 20 dB. Then the laser spot is swept a number of times (from 1 to 900) on every segment. It has been found that, for example, the optimum number of sweeps for NOA81 is 30-70 times (center of the figure) under these conditions. The WG dimensions obtained with this procedure are about 7 µm high and 12 µm wide.