Research

16 pages, 2440 KiB

Open AccessArticle

Optical Kossel Lines and Fluorescence in Photonic Liquid Crystals

by Vladimir A. Belyakov

Crystals 2020, 10(6), 541; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst10060541 - 24 Jun 2020

Cited by 3 | Viewed by 1992

We propose a general analytical way to describe the fluorescence peculiarities in photonic liquid crystals (revealing themselves as an optical analog of the X-ray Kossel lines in conventional crystals) based at the localized optical edge modes existing in perfect photonic liquid crystal layers. [...] Read more.

We propose a general analytical way to describe the fluorescence peculiarities in photonic liquid crystals (revealing themselves as an optical analog of the X-ray Kossel lines in conventional crystals) based at the localized optical edge modes existing in perfect photonic liquid crystal layers. The proposed approach allows us to predict theoretically the properties of optical Kossel lines in photonic liquid crystal (fluorescence polarization, spectral and angular fluorescence distribution, influence of the light absorption in liquid crystal, and, in particular, existing the optical Borrmann effect if the absorption in liquid crystal is locally anisotropic). Comparison of the theoretical results and the known experimental data shows that the theory reproduces sufficiently well the observation results on the fluorescence in photonic liquid crystals. For confirming a direct connection of the optical Kossel lines to the localized optical edge modes in perfect photonic liquid crystal, we propose the application of time-delayed techniques in studying the optical Kossel lines. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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16 pages, 1890 KiB

Open AccessArticle

Localized Conical Edge Modes in Optics of Spiral Media (First Diffraction Order)

by V. A. Belyakov

Crystals 2019, 9(12), 674; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9120674 - 16 Dec 2019

Cited by 4 | Viewed by 1793

Abstract

In cholesteric liquid crystals (CLC) problems related to the localized optical modes for a non-collinear geometry are studied here in the two wave dynamic diffraction theory approximation. This approximation, which insures the results accuracy order of δ (where δ is the CLC dielectric [...] Read more.

In cholesteric liquid crystals (CLC) problems related to the localized optical modes for a non-collinear geometry are studied here in the two wave dynamic diffraction theory approximation. This approximation, which insures the results accuracy order of δ (where δ is the CLC dielectric anisotropy), is applied because for a non-collinear geometry there is no exact analytic solution of the Maxwell equations and a theoretical description of the experimental data becomes more complicated. The dispersion equation for non-collinear localized edge modes (called conical modes (CEM)) is found and analytically solved for the case of thick layers and for this case the lasing threshold and the conditions of the anomalously strong absorption effect are found. It is shown that qualitatively CEMs are very similar to the localized edge modes (EM) in CLCs related to a collinear geometry, i.e., for the case of light propagation along the spiral axis however the CEMs differ by their polarization properties (the CEM eigen polarizations are elliptical ones depending on the degree of CEM deviation from the collinear geometry in contrast to the circular eigen polarizations in the EM case). What is concerned of the CEM quantitative values of the parameters they are “worth” (the photonic effects are not so pronounced) than for the corresponding ones for EM. The CEM lasing threshold is higher than the one for EM, etc. Performed theoretical studies of possible conversion of EMs into CEMs showed that it can be due to the EM reflection at dielectric boundaries at the conditions of a high pumping wave focusing. Known experimental results on the CEM are discussed and optimal conditions for CEM observations are formulated. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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10 pages, 1367 KiB

Open AccessArticle

Thermo-Optical Generation of Particle-Like Structures in Frustrated Chiral Nematic Film

by Sergey Shvetsov, Tetiana Orlova, Alexander V. Emelyanenko and Alexander Zolot’ko

Crystals 2019, 9(11), 574; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9110574 - 31 Oct 2019

Cited by 6 | Viewed by 2444

Abstract

The creation of metastable particle-like structures in frustrated (unwound) chiral nematic film containing light-absorbing additive is studied. It is shown that such localized structures can be generated by the thermo-optical action of a focused laser beam or arise spontaneously at a phase transition [...] Read more.

The creation of metastable particle-like structures in frustrated (unwound) chiral nematic film containing light-absorbing additive is studied. It is shown that such localized structures can be generated by the thermo-optical action of a focused laser beam or arise spontaneously at a phase transition from an isotropic to a liquid crystal state. Observed axisymmetric patterns resemble cholesteric spherulites with toroidal double-twisted director-field configuration. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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Graphical abstract

13 pages, 2248 KiB

Open AccessArticle

Localized Conical Edge Modes of Higher Orders in Photonic Liquid Crystals

by Vladimir A. Belyakov and Sergei V. Semenov

Crystals 2019, 9(10), 542; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9100542 - 20 Oct 2019

Cited by 5 | Viewed by 2019

Abstract

Most studies of the localized edge (EM) and defect (DM) modes in cholesteric liquid crystals (CLC) are related to the localized modes in a collinear geometry, i.e., for the case of light propagation along the spiral axis. It is due to the fact [...] Read more.

Most studies of the localized edge (EM) and defect (DM) modes in cholesteric liquid crystals (CLC) are related to the localized modes in a collinear geometry, i.e., for the case of light propagation along the spiral axis. It is due to the fact that all photonic effects in CLC are most pronounced just for a collinear geometry, and also partially due to the fact that a simple exact analytic solution of the Maxwell equations is known for a collinear geometry, whereas for a non-collinear geometry, there is no exact analytic solution of the Maxwell equations and a theoretical description of the experimental data becomes more complicated. It is why in papers related to the localized modes in CLC for a non-collinear geometry and observing phenomena similar to the case of a collinear geometry, their interpretation is not so clear. Recently, an analytical theory of the conical modes (CEM) related to a first order of light diffraction was developed in the framework of the two-wave dynamic diffraction theory approximation ensuring the results accuracy of order of δ, the CLC dielectric anisotropy. The corresponding experimental results are reasonably well described by this theory, however, some numerical problems related to the CEM polarization properties remain. In the present paper, an analytical theory of a second order diffraction CEM is presented with results that are qualitatively similar to the results for a first order diffraction order CEM and have the accuracy of order of δ², i.e., practically exact. In particular, second order diffraction CEM polarization properties are related to the linear σ and π polarizations. The known experimental results on the CEM are discussed and optimal conditions for the second order diffraction CEM observations are formulated. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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11 pages, 4560 KiB

Open AccessArticle

Miniaturized Metalens Based Optical Tweezers on Liquid Crystal Droplets for Lab-on-a-Chip Optical Motors

by Satayu Suwannasopon, Fabian Meyer, Christian Schlickriede, Papichaya Chaisakul, Jiraroj T-Thienprasert, Jumras Limtrakul, Thomas Zentgraf and Nattaporn Chattham

Crystals 2019, 9(10), 515; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9100515 - 07 Oct 2019

Cited by 12 | Viewed by 4164

Abstract

Surfaces covered with layers of ultrathin nanoantenna structures—so called metasurfaces have recently been proven capable of completely controlling phase of light. Metalenses have emerged from the advance in the development of metasurfaces providing a new basis for recasting traditional lenses into thin, planar [...] Read more.

Surfaces covered with layers of ultrathin nanoantenna structures—so called metasurfaces have recently been proven capable of completely controlling phase of light. Metalenses have emerged from the advance in the development of metasurfaces providing a new basis for recasting traditional lenses into thin, planar optical components capable of focusing light. The lens made of arrays of plasmonic gold nanorods were fabricated on a glass substrate by using electron beam lithography. A 1064 nm laser was used to create a high intensity circularly polarized light focal spot through metalens of focal length 800 µm, N.A. = 0.6 fabricated based on Pancharatnam-Berry phase principle. We demonstrated that optical rotation of birefringent nematic liquid crystal droplets trapped in the laser beam was possible through this metalens. The rotation of birefringent droplets convinced that the optical trap possesses strong enough angular momentum of light from radiation of each nanostructure acting like a local half waveplate and introducing an orientation-dependent phase to light. Here, we show the success in creating a miniaturized and robust metalens based optical tweezers system capable of rotating liquid crystals droplets to imitate an optical motor for future lab-on-a-chip applications. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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12 pages, 3215 KiB

Open AccessArticle

Chiral Optical Tamm States at the Interface between an All-Dielectric Polarization-Preserving Anisotropic Mirror and a Cholesteric Liquid Crystal

by Natalya V. Rudakova, Ivan V. Timofeev, Rashid G. Bikbaev, Maxim V. Pyatnov, Stepan Ya. Vetrov and Wei Lee

Crystals 2019, 9(10), 502; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9100502 - 26 Sep 2019

Cited by 10 | Viewed by 3422

Abstract

As a new localized state of light, the chiral optical Tamm state exists at the interface between a polarization-retaining anisotropic mirror and a substance with optical activity. Considering a hybrid structure comprising a metal-free polarization-preserving mirror and a cholesteric liquid crystal, we highlight [...] Read more.

As a new localized state of light, the chiral optical Tamm state exists at the interface between a polarization-retaining anisotropic mirror and a substance with optical activity. Considering a hybrid structure comprising a metal-free polarization-preserving mirror and a cholesteric liquid crystal, we highlight the high Q factor arising from the all-dielectric framework. The intensity of localized light decreases exponentially with increasing distance from the interface. The penetration of the field into the cholesteric liquid crystal is essentially prohibited for wavelengths lying in the photonic bandgap and close to the cholesteric pitch length. The dielectric mirror has its own photonic bandgap. The energy transfer along the interface can be effectively switched off by setting the tangential wave vector to zero. The spectral behavior of the chiral optical Tamm state is observed both as reflection and transmission resonance. This Fano resonance is analogous to the Kopp–Genack effect. Our analytics are well in line with precise calculations, which may pave a new route for the future development of intelligent design for laser and sensing applications. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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15 pages, 5153 KiB

Open AccessArticle

Magnetic Field-Induced Macroscopic Alignment of Liquid-Crystalline Lanthanide Complexes

by Elena Aksenova, Liliya Dobrun, Alexander Kovshik, Evgeny Ryumtsev and Ivan Tambovtcev

Crystals 2019, 9(10), 499; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9100499 - 25 Sep 2019

Cited by 5 | Viewed by 2357

Abstract

We propose a theoretical approach and a numerical method for determining the Frank elastic constants based on the experimental dependence of the effective values of the permittivity components on the magnetic field. The theoretical task was to find the minimum of the free [...] Read more.

We propose a theoretical approach and a numerical method for determining the Frank elastic constants based on the experimental dependence of the effective values of the permittivity components on the magnetic field. The theoretical task was to find the minimum of the free energy and then to solve the inverse problem on finding elastic constants by the least squares root minimizing with experimental data. The proposed approach combines strong and weak models with various pretilt conditions at the boundaries. This model also describes the inhomogeneity of the electric field inside the sample. The proposed method allows to achieve higher accuracy using a small amount of experimental data. This statement is confirmed by the error estimation study, which is also presented in this research. As an experimental sample, we used the gadolinium-based liquid crystal complex, since there are no data on the Frank elastic constants for this complex. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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13 pages, 2312 KiB

Open AccessArticle

The Field-Induced Stop-Bands and Lasing Modes in CLC Layers with Deformed Lying Helix

by Serguei P. Palto

Crystals 2019, 9(9), 469; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst9090469 - 08 Sep 2019

Cited by 7 | Viewed by 2250

Abstract

Waveguide optical properties of a cholesteric liquid crystal (CLC) layer with a deformed lying helix (DLH) have been studied by numerical simulations using the finite difference time domain method. The DLH structure, when the helix’s axis is oriented in plane of a CLC [...] Read more.

Waveguide optical properties of a cholesteric liquid crystal (CLC) layer with a deformed lying helix (DLH) have been studied by numerical simulations using the finite difference time domain method. The DLH structure, when the helix’s axis is oriented in plane of a CLC layer, is induced by an electric field in a virtual CLC cell with periodic (planar/homeotropic) boundary conditions at one of the alignment surfaces. This in-plane helical structure is stable only in a permanently applied electric field providing the helix deformation. In this work the polarized light reflectance spectra have been studied at different electric fields and light impingement into a waveguide formed by the DLH layer. It is found that for light propagating along the helix axis the reflectance spectrum has multiple stop-bands centred at wavelengths

λ_{j} = \frac{2 p ⟨ n ⟩}{j}

, which is different from set of bands located at

λ_{j} = \frac{p ⟨ n ⟩}{j}

, and characteristic of CLC spectra for the Grandjean-plane textures subjected to distortion by an electric or magnetic field perpendicular to the helix axis, where j is a natural number, p is the helix pitch and

⟨ n ⟩

is the average refractive index. Each of the higher order (j > 1) bands consists of three polarization-dependent sub-bands. In the case of an amplifying CLC DLH layer, depending on an extent of the helix deformation, the lasing modes can be excited at different edges of the sub-bands. While at the strongest deformation the lasing is preferable at the edges of the central sub-band; a lower extent of deformation makes favourable conditions for the lasing at edges of the two other sub-bands. Full article

(This article belongs to the Special Issue Localized Optical Modes in Liquid Crystals)

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