The Faraday Effect (Faraday Rotation)

 
A magnetic field rotates the polarization of light 
The rotation is proportional to the magnetic field strength.
 
Interpretation: The superposition of a left circularly polarized light beam and a right circularly polarized beam provides a linearly polarized light beam. The physical interpretation of the Faraday rotation is based on this superposition. For the purpose of this analysis, we consider that the linearly polarized beam consists of the above beams. As mentioned by Wikipedia:
 
 
"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. This changes the dynamics of the interaction for each beam and one of the beams will be slowed down more than the other, causing a phase difference between the left- and right-polarized beam. When the two beams are added after this phase shift, the result is again a linearly polarized beam, but with a rotation in the polarization direction."

 

 

Faraday rotation in the interstellar medium

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

"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

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

"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).

 

 

 

Surface magneto-optic Kerr effect (SMOKE)

 

Light reflected from a magnetized surface (object) can change in polarization and intensity.

https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect

 

 

 

 

Remote Sensing of Magnetic fields - Remote Optical Magnetometry

 

In optical magnetometry, we ofter use the Faraday rotation of light caused by its interaction with a spin-polarized atomic population to measure the strenght of a magnetic field.

 

Remote detection of underwater objects at distances larger than 100 meters from the sensor
 
 
Magnetic Anomaly Detection - Perturbation of the Earth's magnetic field due to the presence of a metallic object.
If measurements of the Earth's magnetic field* are conducted at different locations, a local field deviation could indicate the presence of an underwater object.
 
1) Surface Magneto-Optical Kerr Effect (SMOKE)
The presence of an underwater object induces magnetization of the sea surface above it due to the alteration of the dielectric properties of water. Light incident on and reflected from the surface will have its polarization rotated to a degree that is proportional to the magnetic field. This is termed the Surface Magneto-Optical Kerr Effect (SMOKE).
 
2) Faraday rotation effect
Light will penetrate water and will be reflected off the object itself. Its propagation and its reflection will be influenced by the altered magnetic field and as a result its polarization will be rotated due to the Faraday effect to a degree that is proportional to the magnetic field.
 
The purpose of this paper is to estimate the polarization rotation angle which is proportional to B addressing the issue of whether it is large enough to be measured.
 
The reflected field which has its polarization modified by both the SMOKE and Faraday effect is obtained (analytical expression). It is concluded that the contribution of SMOKE in the polarization rotation is small.
 
Interpretation: It is noted that the electric field of light exerts a force on the electrons of water. The displacement of electrons of water modelled as a harmonic oscillator motion induces an additional magnetic field, other than the Earth's magnetic field, which influences the polarization of the electric field of light. We refer to a change in the refractive index of the water.
 
It is suggested to alternatively measure the un-rotated component of the reflected laser field as this can be significantly larger in amplitude than the polarization-rotated component.
 
*magnitude of polarization-rotated component

 

 

 

 

 

 


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.")