Samoogniskowanie światła w strukturach fotonicznych z nematycznymi ciekłymi kryształami

Urszula Laudyn

Abstract

The interest in optical solitons has grown rapidly in recent years. The field has a considerable potential for technological applications and it presents many exciting research problems both from a fundamental and an applied point of view. The new optical devices are in various stages of development and at the same time the basic research in nonlinear optical phenomena maintains its vality. Optical solitons are light beams that do not broaden because of the balance between diffraction/dispersion and nonlinearity, i.e. their size and shape is unchanged during propagation. They can occur when the material has a nonlinear optical response that acts like a self-focusing or self-lensing: the self focusing exactly balances the diffraction. In recent years a lot of effort has been put on the manipulation of light in nematic liquid crystals. The nematic liquid crystals a possess large optical nonlinearities owing to their large refractive index anisotropy coupled with the collective molecular reorientation. Using the reorientational nonlinearity it is possible to generate spatial solitons called nematicons, at relatively low powers. Nematicons have been previously observed in different geometries and configurations, including planar, homeotropic and twisted structures. Their optical properties are sensitive to the presence of an external electric field hence the liquid crystals can be utilized as an electrically controllable birefringent media. Their applications are potentially attractive: all optical switching, light guided by light, parallel signal processing, etc. This dissertation provides a better understanding of the nonlinear effects that occur in chiral nematic liquid crystal and particularly the effects related to lateral propagation of light through the cell, when the incident light propagates perpendicular to the helical axis. It turns out that there are layers where molecules are arranged in the same way and nematicons can be created independently. In this thesis the experimental results on the nematicon propagation in different layers and possibility to create as many solitons as layers in the structure are presented. It is shown that the direction of propagation of such nematicons can be changed by applying an external electric field. Additionally, this effect can be modified by changing the input polarization of the light beam. The experimental results were obtained in four independent guiding layers created by a chiral nematic structure. The impact of an external electric field (voltage) as well as the input polarization on the soliton propagation and soliton steering was also considered. External fields and surface interactions can easily deform the initial helical configuration. Because of the complex molecular alignments, their reorientation by an applied field is complicated, although it is still governed by the basic physics principle that the director axis will tend to assume a new configuration, i.e. a tilt towards the cell normal. The idea of the experiment was to create a steerable structure by applying an external electric field (voltage). Due to the high birefringence of liquid crystals, the walk-off of nematicons has been also reported and discussed. Particularly interesting results were obtained with interaction of two beams. In a waveguide induced by the nematicon, other low-power light beam (signal beam) with different wavelength was trapped. Moreover, this dissertation also presents interaction between two beams with equal powers and the same wavelength. Depending on the initial geometry and the strength of the nonlinearity, the interaction of two beams can have different outputs, i.e. solitons can propagate parallel to each other, drag each other, pass through each other or merge in a single self-confined beam. Presented results are a good introduction to the investigation of light propagation in different layers of the analyzed structure and to investigate the discrete light propagation in x direction. Owing to the fact that the refractive index distribution changes along the x direction, i.e. perpendicular to the glass plates, the analyzed cell can be treated as a matrix of waveguide structure with periodically modified refraction index. In proposed geometry, it is possible to obtain conditions in witch conventional continuous diffraction is substituted by discrete one in a sense of discrete coupling between waveguides aside. The magnitude of discrete diffraction can be easily modified by changing geometrical dimensions of analyzed structure The combination of nonlinearity and periodicity in optical media gives rise to the new physical effects and unique opportunities for control and manipulation of light. The purpose was to exploit the advantages of each of them to demonstrate novel effects with potential application in active photonic devices. The work focuses on structure and materials which offer dynamic tenability of the optical properties and which allow for observation of nonlinear effects at moderate laser powers. This thesis presents the experimental studies on light-induced nonlinear transmission of one-dimensional periodic structure with a nematic liquid crystal. It was observed that in the presence of a periodic structure the self-action of light leads to a sharp powerdependent change in the transmission characteristic of the structure.In the next step, a novel experimental geometry based on liquid crystal filled photonic crystal fibers was proposed and implemented experimentally. This form of microstructure was analyzed as the hexagonal matrix of waveguides. The linear light propagation in such a photonic structure is based on the coupling between the neighboring optical channels, which can be also defined as a discrete diffraction. This system was used to demonstrate thermo-optically tunable discrete diffraction and thermal nonlinear self action leading to controllable beam defocusing. The use of fiber based structures in discrete nonlinear optics opens up a promising research areas and novel application in the domain of fiber optics.
Diploma typeDoctor of Philosophy
Author Urszula Laudyn (FP / OPD)
Urszula Laudyn,,
- Optics and Photonics Division
Title in PolishSamoogniskowanie światła w strukturach fotonicznych z nematycznymi ciekłymi kryształami
Languagepl polski
Certifying UnitFaculty of Physics (FP)
Disciplinephysics / (physical sciences domain) / (physical sciences)
Defense Date21-12-2010
Supervisor Miroslaw A. Karpierz (FP / OPD)
Miroslaw A. Karpierz,,
- Optics and Photonics Division

External reviewers Andrzej Kołodziejczyk
Andrzej Kołodziejczyk,,
-

Ewa Weinert-Rączka - [Faculty of Electrical Engineering (WE) [Zachodniopomorski Uniwersytet Technologiczny w Szczecinie (ZUT)]]
Ewa Weinert-Rączka,,
-
- Wydział Elektryczny
Pages161
Keywords in Englishxxx
Abstract in EnglishThe interest in optical solitons has grown rapidly in recent years. The field has a considerable potential for technological applications and it presents many exciting research problems both from a fundamental and an applied point of view. The new optical devices are in various stages of development and at the same time the basic research in nonlinear optical phenomena maintains its vality. Optical solitons are light beams that do not broaden because of the balance between diffraction/dispersion and nonlinearity, i.e. their size and shape is unchanged during propagation. They can occur when the material has a nonlinear optical response that acts like a self-focusing or self-lensing: the self focusing exactly balances the diffraction. In recent years a lot of effort has been put on the manipulation of light in nematic liquid crystals. The nematic liquid crystals a possess large optical nonlinearities owing to their large refractive index anisotropy coupled with the collective molecular reorientation. Using the reorientational nonlinearity it is possible to generate spatial solitons called nematicons, at relatively low powers. Nematicons have been previously observed in different geometries and configurations, including planar, homeotropic and twisted structures. Their optical properties are sensitive to the presence of an external electric field hence the liquid crystals can be utilized as an electrically controllable birefringent media. Their applications are potentially attractive: all optical switching, light guided by light, parallel signal processing, etc. This dissertation provides a better understanding of the nonlinear effects that occur in chiral nematic liquid crystal and particularly the effects related to lateral propagation of light through the cell, when the incident light propagates perpendicular to the helical axis. It turns out that there are layers where molecules are arranged in the same way and nematicons can be created independently. In this thesis the experimental results on the nematicon propagation in different layers and possibility to create as many solitons as layers in the structure are presented. It is shown that the direction of propagation of such nematicons can be changed by applying an external electric field. Additionally, this effect can be modified by changing the input polarization of the light beam. The experimental results were obtained in four independent guiding layers created by a chiral nematic structure. The impact of an external electric field (voltage) as well as the input polarization on the soliton propagation and soliton steering was also considered. External fields and surface interactions can easily deform the initial helical configuration. Because of the complex molecular alignments, their reorientation by an applied field is complicated, although it is still governed by the basic physics principle that the director axis will tend to assume a new configuration, i.e. a tilt towards the cell normal. The idea of the experiment was to create a steerable structure by applying an external electric field (voltage). Due to the high birefringence of liquid crystals, the walk-off of nematicons has been also reported and discussed. Particularly interesting results were obtained with interaction of two beams. In a waveguide induced by the nematicon, other low-power light beam (signal beam) with different wavelength was trapped. Moreover, this dissertation also presents interaction between two beams with equal powers and the same wavelength. Depending on the initial geometry and the strength of the nonlinearity, the interaction of two beams can have different outputs, i.e. solitons can propagate parallel to each other, drag each other, pass through each other or merge in a single self-confined beam. Presented results are a good introduction to the investigation of light propagation in different layers of the analyzed structure and to investigate the discrete light propagation in x direction. Owing to the fact that the refractive index distribution changes along the x direction, i.e. perpendicular to the glass plates, the analyzed cell can be treated as a matrix of waveguide structure with periodically modified refraction index. In proposed geometry, it is possible to obtain conditions in witch conventional continuous diffraction is substituted by discrete one in a sense of discrete coupling between waveguides aside. The magnitude of discrete diffraction can be easily modified by changing geometrical dimensions of analyzed structure The combination of nonlinearity and periodicity in optical media gives rise to the new physical effects and unique opportunities for control and manipulation of light. The purpose was to exploit the advantages of each of them to demonstrate novel effects with potential application in active photonic devices. The work focuses on structure and materials which offer dynamic tenability of the optical properties and which allow for observation of nonlinear effects at moderate laser powers. This thesis presents the experimental studies on light-induced nonlinear transmission of one-dimensional periodic structure with a nematic liquid crystal. It was observed that in the presence of a periodic structure the self-action of light leads to a sharp powerdependent change in the transmission characteristic of the structure.In the next step, a novel experimental geometry based on liquid crystal filled photonic crystal fibers was proposed and implemented experimentally. This form of microstructure was analyzed as the hexagonal matrix of waveguides. The linear light propagation in such a photonic structure is based on the coupling between the neighboring optical channels, which can be also defined as a discrete diffraction. This system was used to demonstrate thermo-optically tunable discrete diffraction and thermal nonlinear self action leading to controllable beam defocusing. The use of fiber based structures in discrete nonlinear optics opens up a promising research areas and novel application in the domain of fiber optics.
Thesis file
Laudyn.pdf 3.63 MB
Citation count*5 (2020-09-26)

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