China
Africa
Explore chapters and articles related to this topic
Quasi-Static Averaging of Microscopic Fields and the Concept of Bianisotropy
Published in Constantin Simovski, Composite Media with Weak Spatial Dispersion, 2018
Since we do not consider a colored quartz as an anisotropic chiral medium, we may refer to almost all natural BA media as isotropic chiral media. An only exception are cholesteric liquid crystals— they are anisotropic (uniaxal) chiral media. All natural chiral media exhibit their chirality in the visible range. It is a weak chirality because the MEC parameter χ is not resonant and very low. As to BA composites, the resonance of their MEC parameter—tensor χ¯¯ $ \overline{\overline{\chi }} $ — may be specially engineered and this resonance grants high values of the MEC parameter in the resonance band. To achieve this resonance in an optically small particle one should use a material having the sufficient optical contrast with the dielectric matrix. The highest contrast is offered by a metal. Therefore, BA composites (at least at microwaves) are composites of metal inclusions, as a rule, of those discussed in the previous subsection.
A Review of Chiral and Bianisotropic Composite Materials Providing Backward Waves and Negative Refractive Indices
Published in Filippo Capolino, Theory and Phenomena of Metamaterials, 2017
Cheng-Wei Qiu, Saïd Zouhdi, Ari Sihvola
In order to force chiral materials to fall in the backward-wave regime, one only needs to make either permittivity or permeability resonant, which will produce a very small value of the product of єμ. On the other hand, the effect of the chirality should be another solution, where the big chirality also favors the realization of backward waves and negative refraction. The optical activity and circular dichroism has been studied for chiral media, and the chirality of the medium’s molecules can be seen as the cause of optical activity. Born [79] put forward the interpretation of optical activity for a particular molecular model, in which a coupled-oscillator model was used. Condon [80] gave a single-oscillator model in dissymmetric field for optically active material, based on the molecular theories of Drude, Lorentz, and Livens. The constitutive relations were suggested as follows:
Faraday Rotation versus Natural Rotation
Published in Dikshitulu K. Kalluri, Principles of Electromagnetic Waves and Materials, 2017
Note that the chiral media also cause rotation of linearly polarized wave. In calculating the angle ψ = ψc, we use kcL and kcR instead of kpR and kpL. See Appendix 8A.
Monostatic microwave ellipsometry for material characterization
Published in Waves in Random and Complex Media, 2021
R. Izhar, M. Amin, O. Siddiqui, Farooq A. Tahir
We proposed the chiral metasurfaces which are the planar microwave analogs of the optically active crystals and organic compounds. The chiral media also possess circular dichroism which is the form of optical activity by virtue of which the left- and right-handed polarizations are absorbed in different proportions leading to different forms of elliptical polarizations. The natural chirality was initially discovered by Arago in 1811 while studying the optical properties of quartz [7,8] and later realized artificially by Lindman at the microwave frequencies [7] with randomly distributed small helicals. With the advances in metamaterial research, planar, more structured, and homogeneous chiral surfaces have gained popularity in the past two decades [9]. More recently, chiral metasurfaces have been designed to manipulate the polarization states of electromagnetic waves in both optical and microwave regimes [10–14]. The material changes can be detected by analyzing the changes in polarization states of an electromagnetic wave after it is reflected from or transmitted by a chiral metasurface.
Optical spectra for a cholesteric elastomer doped with metallic nanospheres with an inversion defect
Published in Journal of Modern Optics, 2023
Structurally chiral media (SCM) have been an object of deep investigation because of their potential applications. SCMs are chiral arrangements that form periodic and aperiodic helical structures. Structurally chiral media mimic cholesteric liquid crystals (CLCs) because, likewise CLCs, they also exhibit selective reflection of circularly polarized light. There appears a PBG for light that travels through the chiral structure of an SCM, whether the incident light has the same chirality handedness of the SCM [1]. Consequently, the reflectance is on the order of 50% of the unpolarized light and nothing for reverse-circularly polarized light to the chirality of the CLC. Such exclusive optical feature of CLCs permits building devices based on selective reflection of light circularly polarized, while it can be a limitation to other photonic purposes without polarization concerns. SCMs with polarization-independent PBGs or higher reflectivity beyond 50 % have been discovered in nature and commonly involve layers of both-handedness. For example, the beetle P. resplendens exhibits the hyper-reflections of light with any polarization, which is polarization-independent due to the existence of a three-layer structure based on a cuticle made of chitin [2–4]. Theoretical analyses have established the possibility of controlling those hyper-reflections by accumulating bilayer cholesteric slabs with the same pitch and complementary handedness, in which case some optical troubles might occur [5–7]. The multi-layering method is suitable for creating adjustable reflectors conformed to reflect light with both polarization handedness. Mitov and Dessaud initiated the elaboration of a single-layered hyper-reflective polymer-stabilized liquid crystal; the structure possesses a reflectivity beyond the CLC limit [8]. To obtain an ambichiral arrangement, Mitov and Dessaud used a temperature-sensitive chiral molecular switch to build a cholesteric blend, where helicity inversion appears at a critical temperature. An alternative procedure to obtain the mentioned ambichiral structure (ACS) is the physical vapour deposition, in which a vaporized anisotropic solid is progressively placed over a continuously rotating substrate to create a helical structure that emulates a cholesteric liquid crystal. Nevertheless, the rotation should switch its sense after some pitches to get two layers with opposed helicities [9,10]. Yang et al. introduced a ‘washout/refill’ technique to develop single-layer cholesteric samples with both handedness and hyper-reflection [11,12]. This technique is advantageous for creating cholesteric LCs with dynamic PBG control, such as broad-width multi-wavelength reflection or hyper-reflection through various external agents. As a result, the transmittance of unpolarized light in the subsequent cholesteric films diminishes from 50% to closely zero per cent, which implies that left- and right-handedness light are not allowed [13–20].