Experimental results on determination of optical parameters of monomer-doped liquid crystalline materials are
presented. Refractive indices, as well as propagation losses, have been particularly determined as a function of the
monomer concentration. Materials characterized in this way can be applied for fabrication of waveguiding structures
with use of the photo-polymerization process. Several factors, such as composition of the LC-monomer mixture and UV
illumination conditions, are needed to be taken under consideration when fabricating structures of satisfactory quality.
Importantly, their optical properties may be additionally tuned after fabrication what is in a huge advantage when
compared to waveguiding structures manufactured in other materials.
In this paper, we present our studies on electrical and thermal tuning of light propagation in waveguide channels, made for the scope from a polydimethylsiloxane (PDMS) substrate infiltrated with nematic liquid crystal (LC). We demonstrated, via numerical simulations, the changes of the waveguide optical parameters when solicited by temperature changes or electric fields. Moreover, the paper goes through the fabrication process of a waveguide channel sample and its characterization, as well as some preliminary experimental trials of sputtering indium tin oxide (ITO) and chromium layers on PDMS substrate to obtain flat electrodes.
Liquid crystals over the last two decades have been successfully used to infiltrate fiber-optic and photonic structures initially including hollow-core fibers and recently micro-structured photonic crystal fibers (PCFs). As a result photonic liquid crystal fibers (PLCFs) have been created as a new type of micro-structured fibers that benefit from a merge of “passive” PCF host structures with “active” LC guest materials and are responsible for diversity of new and uncommon spectral, propagation, and polarization properties. This combination has simultaneously boosted research activities in both fields of Liquid Crystals Photonics and Fiber Optics by demonstrating that optical fibers can be more “special” than previously thought. Simultaneously, photonic liquid crystal fibers create a new class of fiber-optic devices that utilize unique properties of the photonic crystal fibers and tunable properties of LCs. Compared to „classical” photonic crystal fibers, PLCFs can demonstrate greatly improved control over their optical properties. The paper discusses the latest advances in this field comprising PLCFs that are based on nanoparticles-doped LCs. Doping of LCs with nanoparticles has recently become a common method of improving their optical, magnetic, electrical, and physical properties. Such a combination of nanoparticles-based liquid crystals and photonic crystal fibers can be considered as a next milestone in developing a new class of fiber-based optofluidic systems.
The aim of this work is to create the regions of different effective refractive index in typical liquid crystal cell thanks to the polymer-stabilization. For this purpose typical liquid crystalline material, namely E7, has been combined with a small amount of the mixture of RM257 monomer and UV-sensitive activator, with percentage weight less than 10%. Thanks to the photo-polymerization process it is possible to obtain polymer-stabilized molecular orientation inside LC cell. In particular, periodic changes in spatial distribution of effective refractive index in LC layer have been achieved thanks to selective irradiation with UV light. Determination of suitable dose of both the monomer and UV-activator to be added to LC material, as well as of irradiation intensity and time, is essential and highly required to provide repeatable and good-quality periodic waveguiding structures. Eventually, functionality of the liquid crystal cells with distinguished regions of different molecular orientation, and in particular with combination of the planar and homeotropic alignment, has been experimentally tested by launching the near-infrared light beams of orthogonal linear polarizations. Thanks to the molecular reorientation induced by external electric field and/or by electromagnetic wave, it is additionally possible to control character of light propagation by electric bias and optical power, respectively. Proposed polymer-stabilized periodic waveguiding structures in liquid crystalline materials may find potential applications as functional elements and devices for LC-based integrated optics.
In this communication we present our results on characterization of selected liquid crystalline materials in terms of their optical properties and their prospective applications as waveguiding layers in the integrated optic systems. Specifically, LCs refractive indices, with their dependence on temperature and within specific spectral range, have been measured and reported. The measurements were performed with use of the wedge-cell method. This simple goniometric technique is particularly useful when applied for liquid crystalline materials characterized by high refractive indices, for which refractometric methods are approaching their upper limits. It is important that the method proposed here requires relatively small amount of liquid crystalline material and gives reasonable results even if the light sources from the wide spectral range are applied. Experimental data allows for chromatic dispersion curves to be obtained by the numerical fitting with use of the Cauchy model.
J. Aguilar, W. Bilnik, L. Bogacz, T. Bulik, A. Christov, D. della Volpe, M. Dyrda, A. Frankowski, M. Grudzińska, J. Grygorczuk, M. Heller, B. Idźkowski, M. Janiak, M. Jamrozy, M. Karczewski, J. Kasperek, E. Lyard, A. Marszalek, J. Michalowski, M. Rameez, R. Moderski, T. Montaruli, A. Neronov, J. Nicolau-Kukliński, J. Niemiec, M. Ostrowski, P. Paśko, Ł. Płatos, E. Prandini, J. Rafalski, P. Rajda, M. Rataj, M. Rupiński, K. Rutkowskai, K. Seweryn, M. Sidz, Ł. Stawarz, M. Stodulska, M. Stodulski, M. Tokarz, S. Toscano, I. Troyano Pujadas, R. Walter, P. Wawer, R. Wawrzaszek, L. Wiśniewski, K. Winiarski, K. Ziętara, P. Ziółkowski, P. Źychowski
The Cherenkov Telescope Array (CTA), the next generation very high energy gamma-ray observatory, will consist of three types of telescopes: large (LST), medium (MST) and small (SST) size telescopes. The small size telescopes are dedicated to the observation of gamma-rays with energy between a few TeV and few hundreds of TeV. The single-mirror small size telescope (SST-1M) is one of several SST designs. It will be equipped with a 4 m-diameter segmented mirror dish and a fully digital camera based on Geiger-mode avalanche photodiodes. Currently, the first prototype of the mechanical structure is under assembly in Poland. In 2014 it will be equipped with 18 mirror facets and a prototype of the camera.
In this report, the results of theoretical analyses on the light guidance in the microstructured fibers infiltrated with liquid crystalline materials are presented. More precisely, the analyzed photonic structure is considered as 2D optical lattice (i.e., matrix of the mutually parallel waveguide channels) allowing thus the light to be switched (tunneled) between adjacent channels, as predicted by the series of the numerical simulations performed. The latter are based on the finite difference beam propagation method with the Crank-Nicholson scheme applied. It has been demonstrated that different scenarios for discrete light propagation can be obtained, depending on the internal and external factors governing geometrical and optical properties of both the light beam and the fiber. Our findings pave the way for all-optical switching to be successfully developed in the future practical photonic devices.
In this report, numerical analysis on light propagation in photonic crystal fibers infiltrated with liquid crystalline
materials is presented. In particular, influence of the optical power is shown, as obtained from numerical simulations
based on the finite difference beam propagation method.
Liquid Crystal Photonic Crystal Fibers (LC-PCFs) known also as Photonic Liquid Crystal Fibers (PLCFs) are advanced
specialty fibers that benefit from a combination of "passive" photonic crystal fiber host microstructures infiltrated with
"active" liquid crystal guest materials and are responsible for a diversity of new and uncommon spectral, propagation,
and polarization properties. This combination has simultaneously reinvigorated research in both fields of Liquid Crystals
Photonics and Fiber Optics by demonstrating that optical fibers can be more "special" than previously thought.
Simultaneously, photonic liquid crystal fibers create a new class of optical waveguides that utilizes unique guiding
properties of the micro-structured photonic crystal fibers and attractive tunable properties of liquid crystals. Comparing
to the conventional photonic crystal fibers, the photonic liquid crystal fibers can demonstrate greatly improved control
over their optical properties.
The paper describes basic physics including guiding mechanisms, spectral properties, polarization phenomena, thermal,
electrical and optical controlling effects as well as innovative emerging technology behind these developments. Some
examples of novel LC-PCFs highly tunable photonic devices as: attenuators, broadband filters, polarizers, waveplates,
and phase shifters recently demonstrated at the Warsaw University of Technology are also presented. Current research
progress in the field indicates that a new class of emerging liquid crystals tunable photonics devices could be expected.
All-optical communications and data processing exemplifies an important alternative to overcome the speed and
bandwidth limitations imposed by electronics. Specifically, practical implementation of analog operations, including
optical temporal differentiation, is fundamental for future ultrafast signal processing and computing networks. In
addition, the development of fully integrated systems that allow on-single-chip operations is of significant interest. In
this work we report the design, fabrication tolerances and first experimental demonstration of an integrated, ultrafast
differentiator based on π-phase-shifted Bragg gratings. By using deeply-sidewall-etched Silicon-on-Insulator (SOI)
ridged waveguides, first-order optical differentiation has been achieved on sub-millimeters length scales, reaching THz
processing speeds. The proposed device has numerous potential applications, including all-optical, analog solving of
differential equations (important for virtual modeling of scientific phenomena)1, data processing and analysis2, as well as
for the generation of Hermite-Gaussian waveforms (used for arbitrary optical coding and decoding)3.
We present the possibility of light beam propagation control in a Kerr nonlinear magneto-optic medium through the
efficient management of linear and circular birefringences. We show numerically that the joint birefringences, achieved
through the combined use of the Cotton-Mouton and Faraday effects, can effectively accelerate, postpone or even arrest
the nonlinear collapse (for a fixed value of the optical power). We also present the experimental observation of collapse
tuning in a bulk Yttrium Iron Garnet (YIG) crystal placed in an external magnetic field. The obtained results offer new
possibilities towards the use of magneto-optic effects for controlling various nonlinear phenomena.
In this work we present the experimental results of measurements of spatial solitons (nematicons) in chiral nematic liquid
crystalline film. We measured the propagation of light beam at the distance of few millimeters and the nonlinear selffocusing
was observed for a light power of order of few tenths of milliwats. The experimental results are in a good
agreement with theoretical predictions.
A theoretical study of photonic liquid crystal fibers (PLCFs) is presented. Applied numerical approach is based on fully-vectorial plane wave method for the calculation of modal structure of photonic crystal fiber. Used method allows for including material anisotropy that is essential in numerical simulations of the light propagation in photonic liquid crystal fibers. Influence of such parameters as molecules ordering and orientation on the photonic band gaps location and on the propagation constant values of guided modes are studied. It is shown that the guiding mechanism can be easily switched between index guiding and bandgap guiding simply by changing temperature. Moreover, the hybrid guiding for different polarizations can be achieved by applying external electric field perpendicular to the propagation direction.
In this paper the analyses of the linear and nonlinear light propagation in the photonic crystal fiber infiltrated with nematic liquid crystal is presented. Our theoretical investigations, carried out by using the finite difference beam propagation method, show the extreme importance of the discrete space representation aspect. The main aim of this work is to reveal that application of the triangular grid for the discretization of the analyzed structure of the photonic crystal fiber with the hexagonal symmetry gives more reasonable results than in the case of the standard square mesh utilization.
The paper analyzes a modal structure of the guided modes in photonic liquid crystal fibers (PLCFs) by using numerical methods and presents experimental verification of the theoretical results. The theoretical analyses is based on numerical methods applied to find solution of the Helmholtz equation that describes possible forms of modal field propagating inside photonic crystal fiber (PCF) of defined geometry, as well as optical parameters. Numerical calculations based on the Finite Difference method were preformed for two different kinds of digitization of the PLCF structure (i.e. for using square and triangular lattice). Several boundary conditions applied in the case of analyzed lattices such as: constant, Dirichlet and semi-transparent condition were discussed. Input parameters of preformed simulations can be divided into two groups: physical and purely numerical. Physical parameters are: wavelength, number of capillaries, and spatial distribution of refractive index (in cross section of fiber surface). Numerical parameters are: type of used lattice, resolution of the lattice and the boundary condition. Numerical output is the value of effective refractive index and additionally shape of propagating mode field. In experimental part the measurements of numerical aperture (NA) were done in the way to find effective refractive indices of modes propagating in PCFs and PLCFs.
In this work nonlinear light propagation in a photonic crystal fiber (PCF) infiltrated with a nematic liquid crystal
(NLC) is presented. Such a photonic structure, called the photonic liquid crystal fiber (PLCF), combines the passive PCF
and the active NLC guest mixture. The analyzed configuration with a periodic modulation of spatial refractive index
distribution corresponds to the matrix of waveguides. This kind of structure can be controlled by optical power and
additionally by temperature and it allows for studying variety of discrete optical phenomena. For properly chosen
parameters of the analyzed fiber, discrete diffraction in the linear case and generation of the discrete spatial soliton in
nonlinear regime can be obtained. In this paper a possibility of the transverse light localization and delocalization due to
both focusing and defocusing Kerr-type nonlinearity was analyzed. In the case of the positive nonlinearity the refractive
index increases as a function of light intensity in such a way that the stronger guiding of the light within NLC cores is
obtained. Light modifies the refractive index distribution inducing a defect in the periodic structure. That can lead to the
situation in which light becomes self-localized and its diffractive broadening is eliminated. Eventually the discrete
soliton can be created. In the case of negative nonlinearity, the difference between NLC waveguides and glass refractive
indices decreases and the beam guidance becomes weaker for higher light intensities. In such a case the generation of the
bright soliton is possible only in the regime of negative discrete diffraction. However, in the case of defocusing
nonlinearity a decrease of refractive index with the optical power can lead to the bandgap shifting. The incident beam
with a frequency initially within a bandgap is then turned outside the bandgap resulting in changing of the propagation
mechanism to the modified total internal reflection.
In this paper the tunable properties of the photonic liquid crystal fibers are analyzed. Both numerical and experimental
results on the linear light propagation in the photonic crystal fiber filled with the glycerin-water solution and with
6CHBT nematics are presented. Investigated fiber called as photonic liquid crystal fiber combines the passive photonic
crystal fiber host structure and an active nematic liquid crystal material. Such a photonic structure, with a periodic
modulation of refractive index, which could be additionally controlled by the temperature and by the optical power,
allows for the study of discrete optical phenomena.
In our work we investigate the hexagonal matrix of cylindrical step-index waveguides placed inside a silica fiber. The linear and nonlinear parameters of the analyzed structure are optimized to observe discrete diffraction and creation of discrete spatial solitons. Numerical simulations were done for both focusing and defocusing Kerr-type nonlinearities.
In this work we investigate light beam propagation in twisted nematic liquid crystalline film. In the linear case (for low power of the light beam) the diffraction and walk-off of the light beam is observed. Due to the optical reorientation nonlinearity light beam is self-focusing and finally spatial solitary wave is created. The direction of light beam propagation is also changing with increasing the nonlinear effect. The samples were filled with 6CHBT nematic liquid crystals and we measured the propagation of light beam at the distance of few millimeters. Nonlinear self-focusing was observed for a light power of order of few tenths of milliwats. The experimental results are in a good agreement with theoretical predictions and numerical simulation. The proposed configuration of our cell can be applied to switching of the light beam in low power all-optical systems.
We discuss Bloch oscillations in waveguide arrays created in a nematic liquid crystalline layer. Bloch oscillations can originate from the specific distribution of refractive index, where a linear gradient is added to the transverse periodicity. Light can oscillate periodically in the transverse direction as it propagates, returning to its initial spatial position and profile after each full cycle. To introduce a spatially periodic refractive index modulation in nematic liquid crystalline waveguides a set of comb-shaped transparent ITO electrodes is placed on one of the glass surfaces. The applied bias allows tuning the structure from light confinement in one dimension, i.e. planar waveguiding, to bidimensional confinement. In the proposed geometry, the thickness of the liquid crystal layer changes linearly as a function of the transverse coordinate. In this way, both linear and nonlinear effective index changes are introduced in each waveguide.
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