Let's Learn the Application Principle of Magneto Optic Crystal Materials Together!

With the development of optical communication and high-power laser technology, the research and application of magneto-optical isolators have become more and more extensive, which has directly promoted the development of magneto-optical materials, especially Magneto Optic Crystal. Among them, magneto-optical crystals such as rare earth orthoferrite, rare earth molybdate, rare earth tungstate, yttrium iron garnet (YIG), terbium aluminum garnet (TAG) have higher Verdet constants, showing unique magneto-optical performance advantages and broad application prospects.

Magneto-optical effects can be divided into three types: Faraday effect, Zeeman effect, and Kerr effect.

Faraday effect or Faraday rotation, sometimes called magneto-optical Faraday effect (MOFE), is a physical magneto-optical phenomenon. The polarization rotation caused by the Faraday effect is proportional to the projection of the magnetic field along the direction of light propagation. Formally, this is a special case of gyroelectromagnetism obtained when the dielectric constant tensor is diagonal. When a beam of plane polarized light passes through a magneto-optical medium placed in a magnetic field, the polarization plane of the plane polarized light rotates with the magnetic field parallel to the direction of the light, and the angle of deflection is called the Faraday rotation angle.

The Zeeman effect (/ˈzeɪmən/, Dutch pronunciation [ˈzeːmɑn]), named after the Dutch physicist Pieter Zeeman, is the effect of the spectrum splitting into several components in the presence of a static magnetic field. It is similar to the Stark effect, that is, the spectrum splits into several components under the action of an electric field. Also similar to the Stark effect, the transitions between different components usually have different intensities, and some of them are completely prohibited (under the dipole approximation), depending on the selection rules.

The Zeeman effect is the change in the frequency and polarization direction of the spectrum generated by the atom due to the change of the orbital plane and the movement frequency around the nucleus of the electron in the atom by the external magnetic field.

The Kerr effect, also known as the secondary electro-optic effect (QEO), refers to the phenomenon that the refractive index of a material changes with the change of the external electric field. The Kerr effect is different from the Pockels effect because the induced refractive index change is proportional to the square of the electric field, rather than a linear change. All materials exhibit the Kerr effect, but some liquids exhibit it more strongly than others.

Rare earth ferrite ReFeO3 (Re is a rare earth element), also known as orthoferrite, was discovered by Forestier et al. in 1950 and is one of the earliest discovered Magneto Optic Crystals.

This type of Magneto Optic Crystal is difficult to grow in a directional manner due to its very strong melt convection, severe non-steady-state oscillations and high surface tension. It is not suitable for growth using the Czochralski method, and the crystals obtained using the hydrothermal method and the co-solvent method have poor purity. The current relatively effective growth method is the optical floating zone method, so it is difficult to grow large-sized, high-quality rare earth orthoferrite single crystals. Because rare earth orthoferrite crystals have a high Curie temperature (up to 643K), a rectangular hysteresis loop and a small coercive force (about 0.2emu/g at room temperature), they have the potential to be used in small magneto-optical isolators when the transmittance is high (above 75%).

Among the rare earth molybdate systems, the most studied ones are scheelite-type two-fold molybdate (ARe(MoO4)2, A is a non-rare earth metal ion), three-fold molybdate (Ｒe2(MoO4)3), four-fold molybdate (A2Re2(MoO4)4) and seven-fold molybdate (A2Ｒe4(MoO4)7).

Most of these Magneto Optic Crystals are molten compounds of the same composition and can be grown by the Czochralski method. However, due to the volatilization of MoO3 during the growth process, it is necessary to optimize the temperature field and material preparation process to reduce its influence. The growth defect problem of rare earth molybdate under large temperature gradients has not been effectively solved, and large-sized crystal growth cannot be achieved, so it cannot be used in large-sized magneto-optical isolators. Because its Verdet constant and transmittance are relatively high (more than 75%) in the visible-infrared band, it is suitable for miniaturized magneto-optical devices.