The Kerr effect is a combination of three phenomena. Moreover, the first and third phenomena in 1875 were discovered by J. Kerr. This is the so-called electro-optical effect. In 1876, the same scientist discovered a magneto-optical effect. As a result, such an effect was revealed that became similar to electro-optical. He behaved in strong optical fields in much the same way. They began to call it optical.
Classification
The Kerr effect is divided into two types:
- magneto-optical;
- light optic.
Let's consider each of them in more detail.
Electro-optical effect
Any isotropic (gas, liquid, or glass) optical medium that has previously been placed in an electric field is converted into an anisotropic medium with the properties of a uniaxial crystal. It is worth considering the features. The axis of the optics of such a crystal has a longitudinal direction. In other words, it is located along the electric field.
In order to detect the Kerr effect, it is necessary to pass a stream of monochromatic light through a polarizer (for example, it may be a Nicolas prism) . Then direct it into the zone of a flat capacitor filled with an isotropic substance.
The function of the polarizer is to convert naturally polarized light into a beam, but in a linear way.
What will happen if the conditions are changed? If the voltage is not connected to the capacitor, then, accordingly, the polarization of the light beam remains the same, and the luminous flux itself decays in the second Nicolas prism. The arrangement of the elements also matters. Nicolas prism is deployed at right angles to the first, that is, they are perpendicular to each other. Refractive index is taken into account.
If the voltage is connected, then the process of linear polarization of the light wave is divided, in turn, into two component parts. They turn out to be polarized longitudinally oriented extraordinary waves. In this case, both parts are at an angle of 90 ° to the ordinary wave, but move at different speeds.
This provides a phase difference of the oscillatory movements of the wave, both ordinary and extraordinary. Therefore, the total (resulting) stream of light partially passes through the analyzer. It becomes a polarized, elliptical image.
What happens if a compensator is placed in the gap between the adjacent analyzer and the installed Kerr cell? It is also possible to achieve the effect of complete extinction of the luminous flux by the action of the analyzer. This is due to the fact that the compensator converts the elliptical light polarization into a linearly polarized wave. What is artificial optical anisotropy? We will talk about this later.
Explanation of this property
It was proposed first in 1910 by Langevin, and then in 1918 by Born. At the same time, the studies were independent of each other. Their opinion is that by its efforts the electric field tries to expand the small molecules of substances in such a way that their direction moments (electric and dipole) become oriented along the direction of the electric field E. In addition, being within the electric field, the moments of the molecules not only make turns of its direction, but additional dipole moments also arise in them simultaneously. So, for example, in gas molecules in the absence of such an electric field they are not.
How does the refractive index affect it?
As a result, variously directed (in the direction along and across) streams of light rays are formed.
It should be noted that with increasing temperature, the process of changing direction slows down, since the obstructing effect of thermal motion on the orientation of atoms and molecules is revealed.
Therefore, by constantly measuring the values of the quantities used, it is possible to establish an ellipsoid polarization of the optics. It also allows you to identify the structural components of these molecules and atomic particles.
In addition, the Kerr effect will still depend on other indicators. First of all, this is the rate of reorientation of molecular and atomic particles. It is known that such an indicator of low molecular weight liquid has a sufficiently large numerical value. This is what the quadratic electro-optical effect is.
Magnetic optical effect
The magneto-optical effect is one of the main properties of magnetooptics. In other words, it reflects the result of the action of a magnetized medium on such properties of a light beam as its degree of intensity and ability to polarize. In this case, the light should be reflected from the entire surface of the medium.
This effect was described by Kerr in 1876 when conducting an experiment using reflected light from a pre-polished magnet surface.
What is its essence? It consists in the transformation of a plane-oriented polarized light beam from the surface of a ferromagnet, previously magnetized, into a polarized one, but already in the form of an ellipse.
What happens with this? At the same time, the largest axial component of the polarized ellipse deviates from the plane of polarization of the incident beam of the light beam by a certain angle.
The significance of this discovery and its application
In fact, it is the third magnetic-optical effect discovered by Kerrom that is fully used to study and study the electronic structures of those metals and alloys that have ferromagnetical properties. These substances are able to attract objects of a certain composition. Simply put, these are simple magnets. It can also be used to determine the basis of (domain) ferromagnets and the components of the surface layer of a polished metal object itself.
The relationships between the magnitude of the Kerr effect and the basic properties characterizing the optical system, which were in close contact with the adjacent surface of the magnet of the medium under study, are determined. So, for example, an increase in the value of the effect is possible when applied to the upper dielectric layer. In addition to this, we can also achieve the formation of a more pronounced picture of the study.
Law
Kerr's law is as follows:
n e - n o = Bλ 0 E 2 ,
Where:
λ 0 - the length in vacuum of a wave of light;
B - Kerr constant, depending on the nature of the substance, wavelength λ 0 and temperature.
For most substances, B> 0.
What is artificial optical anisotropy based on?
Birefringence can be in natural anisotropic environments. There are, however, various methods for its preparation.
Optically isotropic substances turn into optically anisotropic when they are affected by:
- unilateral compression or tension (crystals of a cubic system, glass, etc.);
- electric field (Kerr effect, liquid, amorphous body, gas);
- magnetic field (liquid, glass, colloid).
In these cases, the substance becomes a uniaxial crystal, the optical axis begins to coincide with the deformation, electric or magnetic field.