Physics-based multistep beam propagation in inhomogeneous birefringent media

Poy, Guilhem; Žumer, S.

doi:10.1364/oe.400984

Cited by 16 publications

(16 citation statements)

References 29 publications

(56 reference statements)

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“…The numerical simulation of the transmission for crossed polarizers was made by the open-source software Nemaktis that was based on a generalized beam propagation method. [11] The incident light was linearly polarized, and after transmission through the LC layer the light passed through a second polarizer.…”

Section: Methodsmentioning

confidence: 99%

Nematic Liquid Crystal Disclination Lines Driven by A Photoaligned Defect Grid

Nys

Berteloot

Beeckman

et al. 2021

Advanced Optical Materials

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behavior in the neighborhood of defects and their dynamics. [4] Disclination lines provide a topological stability that can be instrumental for new switching modes, to realize bistable switching, etc. Studying the formation, interaction, and responsiveness of LC disclinations is therefore not only interesting from a theoretical point of view but also opens up new possibilities for technological applications.Disclination lines can connect defects defined at the surface or can form closed loops in the bulk of a NLC, for example when encircling colloidal inclusions, [5] or when rapid but continuous variations in the alignment direction occur independently at opposing substrates of a device. [1d,e,h,i,3c] To realize patterned surface anchoring, scribing, microrubbing, surface topography, pillars, and photoalignment have been used. [1,2] In the case of strong anchoring, a defect in the alignment pattern with charge s generates 2|s| disclination lines in the bulk of the LC. In the case of weak anchoring and gliding, defects can become mobile and disclinations lines may connect two defects of the same charge on opposing substrates. [6] Thanks to photoalignment, stable defects can now easily be realized experimentally, as has been demonstrated in a few papers. [1d,e,3] The flexibility of photoalignment patterning in combination with the stimuli-responsiveness (e.g., electrical tunability) of LCs allows to design disclination networks with preprogrammed functionality. [2b,3c] Enhanced understanding of the LC behavior in cells with 2D patterns of topological defects is however still necessary to successfully engineer functional LC devices such as multistable photonic components, programmable origami or devices with directed self-assembly of colloidal particles. To study the interaction of NLC disclination lines and their behavior under applied voltages, we defined a square grid pattern of ½ defects on the bottom substrate in combination with a uniformly planar aligned top substrate. Different types of interconnections occur between the +1/2 and −1/2 surface defects, and the three most common types of bulk disclinations are analyzed in detail by comparing experiments with numerical simulations (with and without applied voltage). In the discussion section also a theoretical analysis is provided, explaining the curvature of the different disclination lines and including the effect of unequal elastic constants for splay, twist, and bend. To reduce the splay energy, a tilt of the director is induced close to the +1/2 surface defect. We managed to make this visible in the microscope by increasing the tilt under influence of an electric field.Photoalignment for nematic liquid crystals makes it possible to design complex alignment patterns with point defects, that can act as anchoring points for disclination lines. This feature may be used to realize novel electro-optical devices with bistability or enhanced scattering and is promising for material applications such as stimuli-responsive actuators, active matter, and assemb...

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Section: Methodsmentioning

confidence: 99%

Nematic Liquid Crystal Disclination Lines Driven by A Photoaligned Defect Grid

Nys

Berteloot

Beeckman

et al. 2021

Advanced Optical Materials

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“…The director configuration is simulated on a tetrahedral volume mesh and the results are interpolated on a regular grid for BPM calculations. The optical simulations rely on the open-source software Nemaktis (https://github.com/warthan07/Nemaktis) which implements part of the generalized BPM framework recently introduced by Poy et al for general birefringent media [45]. In short, Maxwell equations for birefringent media are recast in a simpler Schrödinger-like propagation equation for the transverse field.…”

Section: Methodsmentioning

confidence: 99%

“…The equilibrium director configuration is found by minimizing the Landau-De-Gennes (LDG) free energy with the help of finite element (FE) Q-tensor simulations [39][40][41][42][43][44]. Based on the director configuration, optical micrographs are simulated with a beam propagation method (BPM) [45]. Comparison between the simulated micrographs and optical microscopy measurements validates the accuracy of the simulation results.…”

Section: Introductionmentioning

confidence: 99%

Surface Stabilized Topological Solitons in Nematic Liquid Crystals

2020

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Photo-alignment is a versatile tool to pattern the alignment at the confining substrates in a liquid crystal (LC) cell. Arbitrary alignment patterns can be created by using projection with a spatial light modulator (SLM) for the illumination. We demonstrate that a careful design of the alignment patterns allows the stabilization of topological solitons in nematic liquid crystal (NLC) cells, without the need for chirality or strong confinement. The created LC configurations are stabilized by the anchoring conditions imposed at the substrates. The photo-aligned background at both substrates is uniformly planar aligned, and ring-shaped regions with a 180° azimuthal rotation are patterned with an opposite sense of rotation at the top and bottom substrate. A disclination-free structure containing a closed ring of vertically oriented directors is formed when the patterned rings at the top and bottom substrate overlap. Thanks to the topological stability, a vertical director orientation in the bulk is observed even when the centra of both patterned rings are shifted over relatively large distances. The combination of numerical simulations with experimental measurements allows identification of the 3D director configuration in the bulk. A finite element (FE) Q-tensor simulation model is applied to find the equilibrium director configuration and optical simulations are used to confirm the correspondence with experimental microscopy measurements. The created LC configurations offer opportunities in the field of optical devices, light guiding and switching, particle trapping and studies of topological LC structures.

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“…In our simulations, we coupled a wide-angle beampropagation method [22] with a standard minimization scheme of the CLC free energy assuming a constant scalar order parameter [23,24], thus allowing to simultaneously calculate the optical and director fields in the CLC layer and simulate POM images. The nonlinear optical response results from the reorientation of CLC molecules-initially aligned with x-due to the electric torque imposed by the optical fields (represented schematically as E i on Fig.…”

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confidence: 99%

“…To first order in the reorientation angle, the permittivity tensor in the nonlinear regime is written as ϵ ¼ ϵ ð0Þ þ δϵ with δϵ ≡ ðn 2 e − n 2 o Þ × ðn 0 ⊗ n ⊥ þ n ⊥ ⊗ n 0 Þ and n ⊥ ¼ f0; n y ; n z g. Based on the formalism of Refs. [22,37], neglecting optical activity and keeping only nonlinear terms of the lowest order, we then find that the director and optical fields can be calculated from the following system of differential equations:…”

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confidence: 99%

Chirality-Enhanced Periodic Self-Focusing of Light in Soft Birefringent Media

et al. 2020

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We experimentally and numerically show that chirality can play a major role in the nonlinear optical response of soft birefringent materials, by studying the nonlinear propagation of laser beams in frustrated cholesteric liquid crystal samples. Such beams exhibit a periodic nonlinear response associated with a bouncing pattern for the optical fields, as well as a self-focusing effect enhanced by the chirality of the birefringent material. Our results open new possible designs of nonlinear optical devices with low power consumption and tunable interactions with localized topological solitons.

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Physics-based multistep beam propagation in inhomogeneous birefringent media

Cited by 16 publications

References 29 publications

Nematic Liquid Crystal Disclination Lines Driven by A Photoaligned Defect Grid

Nematic Liquid Crystal Disclination Lines Driven by A Photoaligned Defect Grid

Surface Stabilized Topological Solitons in Nematic Liquid Crystals

Chirality-Enhanced Periodic Self-Focusing of Light in Soft Birefringent Media

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