The Faraday effect: materials modify the direction of polarization of light 

Polarized light as a powerful tool to investigate material properties


https://en.wikipedia.org/wiki/Faraday_effect

 

"In circularly polarized light the direction of the electric field rotates at the frequency of the light, either clockwise or counter-clockwise. In a material, this electric field causes a force on the charged particles comprising the material (because of their low mass, the electrons are most heavily affected). The motion thus effected will be circular, and circularly moving charges will create their own (magnetic) field in addition to the external magnetic field. There will thus be two different cases: the created field will be parallel to the external field for one (circular) polarization, and in the opposing direction for the other polarization direction – thus the net B field is enhanced in one direction and diminished in the opposite direction."

 

 

Faraday rotation in the interstellar medium
"The effect is imposed on light over the course of its propagation from its origin to the Earth, through the interstellar medium. Here, the effect is caused by free electrons (...)"

 

 

Faraday rotation in the ionosphere
"Radio waves passing through the Earth's ionosphere are likewise subject to the Faraday effect. The ionosphere consists of a plasma containing free electrons which contribute to Faraday rotation (...)."

"Faraday rotation is an important tool in astronomy for the measurement of magnetic fields" (e.g. pulsars).

 

 

 


Circular Dichroism

 

https://en.wikipedia.org/wiki/Dichroism

A dichroic (two-color) prism is a prism that splits in two colors or two wavelengths (cf. beam splitter for two different wavelengths).
Dichroism refers to the differential absorption of light in different polarization states when travelling in a material.
When the polarization states in question are right and left-handed circular polarization, we then refer to circular dichroism.


https://en.wikipedia.org/wiki/Circular_dichroism
Left-hand circular (LHC) and right-hand circular (RHC) polarized light represent two possible spin angular momentum states for a photon, and so circular dichroism is also referred to as dichroism for spin angular momentum.[3] 


"Interaction of circularly polarized light with matter"
"When circularly polarized light passes through an absorbing optically active medium, the speeds between right and left polarizations differ (cL ≠ cR) as well as their wavelength (λL ≠ λR) and the extent to which they are absorbed (εL≠εR). Circular dichroism is the difference Δε ≡ εL- εR.[5] The electric field of a light beam causes a linear displacement of charge when interacting with a molecule (electric dipole), whereas its magnetic field causes a circulation of charge (magnetic dipole). These two motions combined cause an excitation of an electron in a helical motion, which includes translation and rotation and their associated operators."

 

A circular dichroism experiment will induce excitation and light absorbance (photoabsorption). "(...) the two types of circularly polarized light are absorbed to different extents. In a CD experiment, equal amounts of left and right circularly polarized light of a selected wavelength are alternately radiated into a (chiral) sample. One of the two polarizations is absorbed more than the other one, and this wavelength-dependent difference of absorption is measured, yielding the CD spectrum of the sample." 

 

CD spectroscopy has a wide range of applications in molecular structure studies in biology, chemistry and material sciences.

 

 

 

Photoexcitation Circular Dichroism


A new CD technique termed "Photoexcitation Circular Dichroism" is presented at the paper "Photoexcitation Circular Dichroism in Chiral Molecules"

https://www.nature.com/articles/s41567-017-0038-z.
The corresponding arXiv.org paper is found at https://arxiv.org/abs/1612.08764.

 

This study initially notes the helical motion described above. 

First, the coherent excitation of electronic states leads to a charge displacement in the light propagation direction. Hence, a dipole moment, a macroscopic dipole and the corresponding electron density, a chiral density (asymmetric pattern), are created in the excited states, with a chiral current oscillating out of phase for the two enantiomers. 

 

The resulting chiral dynamics can be probed/detected with photoelectron circular dichroism arising from the ionization of excited molecules by linearly polarized light pulses (PXECD).

 

PXCD could be used to drive molecular reactions in chiral systems in a stereospecific way, by imprinting a chiral torque via the helicity of the exciting circularly polarized pulse. 

 

 

As mentioned in a relevant article in FR http://www.cea.fr/drf/Pages/Actualites/En-direct-des-labos/2018/quand-les-electrons-partent-en-vrille.aspx?utm_source=newsletter&utm_medium=email&utm_campaign=CEADRF19, the electrons are ejected from the molecules following two different directions.

 

(From the arXiv.org paper: "The photoelectrons are accelerated and projected by an electrostatic lens onto a set of dual microchannel plates and imaged by a phosphor screen and a CCD camera."