Elliptical Dichroism Research Articles

The analogy between the optical theory of a twisted nematic liquid crystal and the dynamical theory of diffraction

A previously unnoticed connection between the classical optical theory of a twisted nematic liquid crystal and the two beam dynamical theory of diffraction in the transmission Laue case (as is applied to calculate beam intensities in X-ray, electron and neutron diffraction), has been uncovered. Both physical situations are described by an equation of the form (d/d z)Φ = 2π AΦ, where A is an Hermitian matrix, and Φ is a two component column vector representing beam amplitudes in the diffraction case and light polarization components in the nematic case. The counterpart of the concept of the scattering matrix in diffraction theory is the Jones calculus of crystal optics theory. Anomalous (Borrmann) transmission in crystal diffraction (different absorption of the two Bloch waves) also has a counterpart in elliptical dichroism in crystal optics. Crystal optical theory as formulated by Mauguin in 1911 predates the first formulation by Darwin in 1914 of the dynamical theory of diffraction.

Coupling between linear dichroism and piezobirefringence in CdS at the isotropic point

The eigenelectromagnetic modes characterizing a uniaxial and dichroic crystal exhibiting an isotropic point for refractive indices and submitted to a natural (optical activity) or artificial perturbation are theoretically investigated. It is demonstrated that these modes are two elliptical and linearly independent vibrations whose respective axes are not perpendicular. Expressions of elliptical birefringence and elliptical dichroism are derived, and it is demonstrated that, for low perturbations, the linear dichroism of the unperturbed crystal cannot be neglected. The experimental study is performed at room temperature with a crystal of CdS submitted to a uniaxial pressure parallel to the [110] direction. The radiation transmitted by the crystal in the filter configuration and the transmission spectrum in light polarized parallel and perpendicular to the optical axis are analyzed, with the linear dichroism of the unperturbed sample taken into account. Finally, the value of the elasto-optic coefficient p44 at the isotropic point is reexamined.

Single- and multi-photon circular and elliptical dichroism — the effects of permanent multipole moments and molecular structure

Recently, time-dependent perturbation theory has been used to investigate single- and multi-photon elliptical and circular dichroism for molecules having permanent electric dipole, and both permanent and transition electric quadrupole, moments in addition to the more commonly treated transition electric and magnetic dipole moments. In this paper it is shown how analytic rotating-wave approximation (RWA) expressions for the transition probabilities for a two-level molecule, having all the relevant transition and permanent moments and interacting with elliptically polarized light, can be employed to augment the perturbative treatments of chiroptical effects. Examples are given which illustrate how the RWA can be used, unlike the perturbative treatments, to study circular dichroism for long times, and for absorption spectra, including effects of permanent multipole moments absent in the perturbative analysis. Some of the effects of permanent moments on the single- and multi-photon circular and elliptical dichroism for chiral molecules are discussed. For example, two-photon dichroism, for molecules with permanent multipole moments, can offer a very different and more detailed probe of molecular structure than the usual one-photon circular dichroism.

Intrinsic and photoinduced elliptical dichroism and birefringence in glassy semiconductors

The paper presents a study of photoinduced changes of the optical activity and ellipticity in As 2 S 3 glasses. The results cannot be described by taking into account of the properties of anisotropic and gyrotropic media only. It is argued that structural native defects play a decisive role.

Circular and elliptical dichroism of partially oriented helical polymers

A linear response theory for the circular and elliptical dichroism of partially oriented helical polymers is presented. Circular and elliptical dichroism arise for light propagating parallel and perpendicular, respectively, to the direction of orientation. Expressions are presented for the difference of the oriented and unoriented circular dichroism. Specializing to polypeptide systems we provide a firm theoretical foundation for resolution of oriented circular dichroism spectra into component bands.

Elliptical dichroism of oriented helical polymers

A linear response theory of elliptical dichroism is obtained for light propagating perpendicular to the helix axes of a system of oriented helical polymers by solving the equation of motion for the radiation field's average vector potential. A nonlinear mixture of circular and linear dichroism and birefringence is obtained.

Optical activity of oriented molecules III. The absorption process and the vibrational coupling effects for the tensor of rotation

Equations for the rotation and the ellipticity of samples exhibiting linear and circular birefringence and dichroism (i.e. for small elliptical birefringence and dichroism) are given. By this a second presentation of the frequency dependence for linear and circular birefringence of anisotropic solutions is developed. The absorption coefficients of anisotropic solutions for linearly polarized light are calculated from the H-matrix of Gō. For a special type of orientational distribution function the absorption behaviour is analysed. Within the adiabatic approximation and by neglecting the variation of the electronic transition moments with the nuclear distances, the structure of a circular dichroism band is independent of the orientational distribution function. Empirically, this is not the case for α, β-unsaturated keto steroids. By including this variation in the adiabatic approximation the coordinates of the tensor of rotation R NnKk ij are altered by different amounts due to the coupling by vibrations of the involved states with other electronic states of the system (Herzberg-Teller method). As a consequence of this coupling electric dipole and magnetic dipole/electric quadrupole transition moments not belonging to the observed transition | K 0> ← | N 0> are added to the zeroth order tensor of rotation R NK ij . Le., first order tensor of rotation R LM/PR ij = 1 4 <μ k > LM |ε kli < C tj > PR + ε klj < C lj > PR }, multiplied by a mixing parameter depending on the type of vibration have to be added to R NK ij . As a consequence, the structure of the CD band will depend on the orientational distribution function. The model developed is demonstrated for molecules with symmetry D 2.