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Atomic and Molecular Physics
Published in Walter Fox Smith, Experimental Physics, 2020
Intimately related to Faraday rotation is the phenomenon of circular dichroism, which describes the differences in absorption of RCP and LCP light by a solid, liquid, or gas. Circular dichroism finds wide application in the study of the structures of biological molecules that exhibit chirality (e.g., the double helix of nucleic acids). In this laboratory, you will measure an atomic effect induced by a magnetic field: the circular dichroism of rubidium (Rb) vapor. The Rb circular dichroism signal is dramatically enhanced by observing the differences in RCP and LCP light absorption in the vicinity of a strongly allowed Rb transition. The physics behind this “resonant” effect is best understood by the Zeeman effect – the splitting of atomic energy levels in a magnetic field. The underlying physical mechanisms of resonant circular dichroism are key elements in a variety of modern experimental techniques in atomic physics, including saturated absorption spectroscopy, frequency stabilization of laser light, and atom cooling and trapping.
Electronic and Optical Properties of Phosphorene
Published in Yongqing Cai, Gang Zhang, Yong-Wei Zhang, Phosphorene, 2019
Yongqing Cai, Gang Zhang, Yong-Wei Zhang
Two-dimensional materials like graphene and MoS2 have been found to show promising applications in optoelectronics as photodetectors. However, the ability to detect polarized light is elusive. Linear dichroism is a phenomenon related to different absorptions of parallel or perpendicular polarized light to an orientation axis. Normally, it was realized in an extrinsic way by creating a proper conformation and orientation of material structures to be oriented in anisotropic patterns69, 70 or in an intrinsic way relying on an anisotropic crystal structure.71, 72 Black phosphorus, owing to its highly anisotropic structure along the armchair and zigzag directions, allows a highly anisotropic interaction of electrons and photons within the layers. The presence of mirror reflection symmetry only in the armchair direction allows photodetection of linearly polarized lights.
Polarization
Published in Myeongkyu Lee, Optics for Materials Scientists, 2019
Some crystals such as tourmaline and herapathite exhibit dichroism, preferential absorption of light that is polarized in particular directions. While the original meaning of dichroism is the split of a light beam into two beams of different wavelength (i.e., color), polarization-dependent absorption is also referred to as dichroism. Despite the preferential absorption, these dichroic crystals are seldom used as a linear polarizer. Since the dichroic effect is wavelength-dependent, the crystals appear colored. Another practical limitation is the difficulty of growing them into large sizes. The most widely used linear polarizer is Polaroid H-sheet, which was invented by Land in 1938. When a sheet of polyvinyl alcohol (PVA) is heated and stretched in a certain direction, its long, hydrocarbon molecules are aligned in the direction of stretching and form an array of linear molecular chains. If the sheet is then dipped into a solution of iodine, the iodine attaches to the PVA molecules and makes them conducting along the length of the chains. Valence electrons from the iodine dopant are able to move along the polymer chains, but not transverse to them. Just as in a wire-grid polarizer, light polarized perpendicular to the conducting chains is transmitted while light polarized parallel to them is absorbed. The TA of the polarizer is thus perpendicular to the direction in which the sheet is stretched. The Polaroid H-sheet polarizer, much cheaper than other types of linear polarizer, is widely used for sunglasses, photographic filters, and display devices.
Optimal design of electromagnetic metamaterial electronic device sensor with specific performance based on multivariate big data fusion
Published in Journal of Experimental Nanoscience, 2023
An important research direction in the field of metamaterials is to study chiral and related electromagnetic phenomena [6]. Chirality refers to the geometric property that a structure cannot coincide with its mirror image after translation and rotation. Chiral metamaterials are one kind of chiral materials, which can exhibit two important electromagnetic properties: circular birefringence and circular dichroism. Circular birefringence refers to the ability of a structure to rotate the polarization plane of electromagnetic waves. Circular dichroism refers to the difference in the propagation of right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) waves in chiral media. Subsequent studies have shown that planar chiral metamaterials can also produce another novel phenomenon: asymmetric transmission (AT). This special phenomenon was first discovered by Fedotov and others in 2006 [7]. Due to these special electromagnetic characteristics of electromagnetic metamaterials, many researchers have designed a variety of miniature microwave components and applied them in the field of wireless communication and defense industry [8]. The structural loss of metamaterials has become a major problem in its application field [9]. It has certain practical value and is beneficial to broaden the application of electromagnetic metamaterial in the field of antenna and microwave devices [10].
Linear and circular dichroisms of cholesteric liquid crystals in a longitudinal external magnetic field
Published in Liquid Crystals, 2023
A.H. Gevorgyan, N.A. Vanyushkin, I.M. Efimov, K.B. Oganesyan
The existence of dichroism means that the reflection, transmission and absorption of an electromagnetic wave in a medium or in a composite structure depend on the polarisation of the incident wave. Usually, linear, circular and elliptical dichroisms are distinguished depending on the type of electromagnetic wave (light) polarisation; however, in practice, linear dichroism (LD) and circular dichroism (CD) are mainly considered. It is well known that the manipulation and establishment of the polarisation state of light play an important role in many applications, including spin optical coupling, spectroscopic measurements, chemical analysis, biomedical diagnostics, polarisation selectors and polarisation imaging [1–6]. Systems with high LD and CD can find applications in innovative polarised optoelectronic devices. CD is exhibited by chiral molecules or chiral structures that display different absorption (refraction) for circular polarised light of either handedness. CD spectroscopy is a widely used optical tool for studying the 3D conformation of molecules and nanostructure, such as chirality of molecules [7,8], conformation of proteins [9–11], chiroptical properties of nanostructures [12,13], etc. LD is caused by the difference in the absorption (refraction) coefficient for orthogonal linearly polarised optical radiations during their propagation in the medium. Systems with high LD can find applications such as wavelength-controllable detectors, and polarisers, in multispectral imaging technology, such as optical communication equipment, and in thermal imaging detection [14] and so on.
Cholesterol-based nonsymmetric dimers comprising phenyl 4-(benzoyloxy)benzoate core: the occurrence of frustrated phases
Published in Liquid Crystals, 2021
Channabasaveshwara V. Yelamaggad, Sachin A. Bhat
Circular dichroism (CD) is realised when the absorption of left- and right-hand circularly polarised light by an optically active (chiral absorbing) media differs. Thus, CD, being an absorption spectroscopic technique, uses circularly polarised light to probe the macroscopic structural aspects of the chiral media, i.e. the analysis of the CD spectrum provides invaluable information of the chiral (helical) structure under investigation. The chiral nematic (N*) phase is a defect-free helical superstructure and hence exhibits the CD phenomenon intrinsically wherein the incident light gets resolved into its two circularly polarised (CP) components, left and right at a given wavelength. In general, the helical twist sense (handedness or helicity), which is either left or right of the N* phase, depends on the chirality (absolute configuration), R and S of the constituent conventional chiral mesogens. However, interestingly, the handedness of the N* phase formed by the cholesterol-based dimers has been reported to be dictated by the parity of the central spacer connecting the two mesogenic segments. To verify this observation, the dimers CPD-3,10, CPD-4,10, CPD-5,10 and CPD-7,10 were chosen where the length and parity of the spacer vary markedly.