User:Adirocco/sandbox/Optically Induced Magnetism

= Introduction =

Optically induced magnetism is essentially the combination of optics and induced magnetism. Optics is the study of the behavior of light and induced magnetism is when an object is kept near a magnet and the object itself becomes magnetic.

Optically induced magnetism works when an electric current passes through a magnetic layer and the electric current becomes spin-polarized. The spin-polarized current will exert a spin-transfer torque (STT) on the magnetization. This phenomena can also be generated inside a non-magnetic metal due to the spin–orbit coupling (SOC), and the corresponding torque (spin–orbit torque (SOT).

Method
Optically induced magnetism occurs when an initial photon establishes an electrical polarization within a material and that causes an orbital angular momentum. This occurs on all electric dipoles within the material that transition between L = 0 and L = 1. A second photon can exert a magnetic torque on the orbital angular momentum, and that causes an exchange of orbital angular momentum to rotational angular momentum. The change from orbital angular momentum to rotational angular momentum de-excites the molecule and increases the radius of charge motion. When the radius of charge motion increases, the magnetic dipole increases[9]. This is because the magnetic dipole depends on the area enclosed by the current within the molecule (m = ids). This type of magnetism can occur in materials that are thought to be "non magnetic," such as diamagnets, as long as the material is dielectric.

The more you optically excite the dielectric material, the more magnetic dipoles are formed, and therefore the more magnetic the material becomes. However, the electric dipole magnitude will always be larger than the magnetic dipole magnitude, and the magnetic dipole moment will always be relative to the electric dipole moment.

History
Optically induced magnetization dates back to the beginning of nonlinear optics

In 1961, Pitaevski, Shen and Bloembergen, and others described theoretically how a static magnetization could be generated by an “effectively magnetic” interaction proportional to E(ω) x E*(-ω) in the case of circularly polarized fields [5].

Inverse Faraday Effect
The Inverse Faraday Effect, abbreviated as IFE, is open used as a way to optically induce magnetism.

For the purpose of optically induced magnetism, IFE is an opto-magnetic phenomenon that manifests as an induced magnetization that is parallel or anti-parallel with the axis of circularly polarized excitation based on the helicity of the radiation [2]. This tells us that the light must be circularly polarized in order to induce a magnetization.

= Different Media = Optically induced magnetism can occur in various media. This page discusses a few of those media.

Plasmonic Nanostructures
Plasmonic nanostructures provide routes to control light matter, and is observed as a rotation of the polarization plane of light transmitted or reflected through a magnetized magnetic medium. The magnetization in this media is induced using IFE.

An experiment, conduced by S.M. Hamidi, M. Razavinia, and M.M. Tehranchi [1], was done on three structures.

This experiment showed us that circularly polarized light can induce a dc magnetic field, the maxima of the magnetic field aligns with the maxima of the light intensity (roughly), and the direction of the magnetization depends on the direction of the light.

Plasmonic Au Nanoparticles
To optically induce magnetism in Plasmonic Au Nanoparticles, again, the Inverse Faraday Effect is used.

An experiment related to this media was conducted by Oscar Hsu-Cheng Cheng, Dong Hee Son, and Matthew Sheldon [2]. Their experiment showed us that optically induced magnetization that is ~1,000 times larger than in bulk Au due to plasmonic field enhancement.

Dilute Magnetic Semiconductors
An experiment on dilute magnetic semiconductors was conducted by J. Warnock and D. D. Awschalom.

The results of their experiment demonstrated that optically induced magnetism is self limiting (it has a saturating limit), nonlinear effects were significant (at least in dilute magnetic semiconductors ), and the magneto-optical susceptibility saturates at values which were several order smaller than normal magnetic susceptibility.

Hg1-xMnxTe
An experiment on a specific dilute magnetic semiconductor, Hg1-xMnxTe, was also done. This experiment was carried out by H. Krenn, W. Zawadzki, and G. Bauer [4].

This experiment taught us that circularly polarized light shows a net optically induced magnetic flux but linearly polarized light has no observable flux. It also showed us that magnetic flux increased with increased laser power, and that optically induced magnetization is temperature dependent.

Homogenous, Undoped Dielectric Media
This experiment was completed by S. C. Rand, W. M. Fisher, and S. L. Oliveira [5].

The homogenous, undoped dielectric media that they used was CCl4, water , and C6H6.

The results of this experiment were that the incident polarizations that produce maximum electric dipole and magnetic dipole scattered intensities are orthogonal and the maximum ratio for magnetic dipole scattering is 0.25.

Oxygen-Deficient Strontium Titanite
SrTiO3 is a foundational material in the emerging field of complex oxide electronics. This has made exploring it's optical properties a real interest.

An experiment about optically inducing magnetism in oxygen-deficient strontium titanite was conducted by W. Rice,P. Ambwani,M. Bombeck, et al top explore these properties [6].

They found that optically induced magnetism is extremely long-lived at low temperatures (the magnetization persists long after the pump illumination is turned off) in SrTiO3, and that optically induced magnetization can be controlled by the light wavelength.

Transparent Dielectrics
A transparent dielectric can efficiently control the light behavior in the device without parasitic absorption or electrical property degradation. It is a dielectric that is clear in color.

W.M. Fisher and S.C. Rand carried out an experiment on transparent dielectrics, such as CCl4, water, and benzene [7].

The results of their experiment show us that magnetic intensities rise quadratically as a function of input power in several dielectric liquids, the magnetic dipole intensity saturates at a universal value of 0.25 the intensity of electric dipole scattering, and the optically induced magnetism is more intense in some materials than others at a given incident power.

=References=
 * 1) Hamidi, S.M., Razavinia, M., & Tehranchi, M.M. (2015). Enhanced optically induced magnetization due to inverse Faraday effect in plasmonic nanostructures. Optics Communications, 338, 240-245
 * 2) Cheng, Oscar & Son, Dong Hee & Sheldon, Matthew. (2019). 1,000-Fold Enhancement of Light-Induced Magnetism in Plasmonic Au Nanoparticles
 * 3) J. Warnock and D. D. Awschalom 1987 Jpn. J. Appl. Phys. 26 819
 * 4) H. Krenn, W. Zawadzki, and G. Bauer Phys. Rev. Lett. 55, 1510 – Published 30 September 1985
 * 5) S. C. Rand, W. M. Fisher, and S. L. Oliveira, "Optically induced magnetization in homogeneous, undoped dielectric media," J. Opt. Soc. Am. B 25, 1106-1117 (2008)
 * 6) Rice, W., Ambwani, P., Bombeck, M. et al. Persistent optically induced magnetism in oxygen-deficient strontium titanate. Nature Mater 13, 481–487 (2014)
 * 7) W.M. Fisher, S.C. Rand, Dependence of optically induced magnetism on molecular electronic structure Journal of Luminescence,Volume 129, Issue 12, 2009, Pages 1407-1409, ISSN 0022-2313
 * 8) A. A. Fisher et al., Opt. Express, 24, 23, 26055-26063 (Nov. 14, 2016)
 * 9) A. A. Fisher et al., Opt. Express, 24, 23, 26064-26079 (Nov. 14, 2016)