Introduced by Michael Faraday in 1845, the Faraday rotation or Faraday effects is a magneto-optical phenomenon. The phenomenon means an interaction between light and a magnetic field in a medium.
Faraday rotation was not a direct development by Michael Faraday. He was searching for experimental evidence that the forces in nature were interconnected. In this process, he made a remarkable discovery by carefully examining the polarization of light when it passed through a transparent material in the presence of a magnetic field. It was observed by him that linearly polarized light propagated through matter parallel to a static magnetic field, causing a rotation of the plane of polarization. Here, the effect was very small. But, with his knowledge and experience, Michael Faraday identified the phenomenon, which is known as Faraday Rotation or Faraday Effect.
Generally, the Faraday rotation occurs in optically transparent dielectric materials, including liquids, under the influence of magnetic fields.
What is the physical interpretation of Faraday rotation?
The linear polarization that rotates in the Faraday Effect consists of the superposition of a right and left-circularly polarized beam. The direction of the electric field rotates at the frequency of the light in a clockwise or counter-clockwise direction in the circularly polarized light.
When you use material, the electric field causes a force on the charged particles known as electrons. The motion effect is circular, circular moving charges creating their own magnetic field along with the external magnetic field.
This creates two conditions: one, the created field is parallel to the external field for one circular polarization and in the opposing direction for the other polarization direction. Here, the net field is enhanced in one direction and diminished in the opposite direction. This leads to dynamic changes in the interactions for each beam. One of the beams slows down more than the other, causing a phase difference between the left and right-polarized beams. When the two beams are added after the phase shift, it results in a linearly polarized beam with a rotation in the polarization direction.
The physical properties of the material affect the direction and the intensity of polarization rotation.
Which devices are based on Faraday rotation?
Faraday isolator- Faraday rotation is needed in Faraday isolators to protect lasers and amplifiers against back-reflected light. For the right use in Faraday isolators, the rotation angle should be close to 45 degrees in the spectral region of interest. It’s said a large attenuation for back-reflected light is obtained by a highly uniform polarization rotation.
Ring laser resonator– In a ring laser resonator, a Faraday effect or rotation is used to introduce round-trip losses, depending on the direction. This enforces unidirectional operation. A Faraday rotator provides only a very small rotation angle but it’s sufficient because a very small loss difference is considered sufficient.
Faraday mirror– When a 45-degree rotator combines with an end mirror, it forms a Faraday mirror. A laser beam sent through some amplifier, then reflected at a Faraday mirror and sent back through the amplifier has a polarization directional on returning, which is orthogonal to that of the input beam. This happens even if the polarization state is not preserved within the amplifier.
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