Oscillating Behavior of Inverse Faraday Effect in YFeO3

Abstract

Studying spin system dynamics concerns different rapidly growing fields of technology such as developing new types of information transfer systems [1], quantum computing [2, 3], new approaches for magnetic recording and reading [4]. The most common way to excite spin oscillations is by using the inverse Faraday effect (IFE). It is based on a Raman-like coherent optical scattering process and does not require absorption of light. This gives us two main advantages of this effect: it is instantaneous and non-thermal [5]. IFE results in magnetization induced by high-power laser radiation according to the following formula:Min=χ/16π [E×E*], (1)where χ is magneto-optical susceptibility, E is the electric field of the incident light, and E* is its complex conjugate. The cross product in formula (1) is zero for linearly polarized incident light and is maximum for circularly polarized light.The excitation of high order spin waves requires the nonuniform distribution of the electromagnetic field in the medium. It is commonly achieved by using the nanostructured materials in which the resonant phenomena can occur breaking the uniformity of the field. For example, in work [6] the all-dielectric 1D grating was used to generate spin waves due to the excitation of propagating waveguide modes.Another way to excite spin waves is the utilization of anisotropic materials in which the polarization conversion of the incident light occurs. Such a transformation leads to the spatial dependency of magnetization induced via IFE. This nonuniformity of the magnetic torque allows one to excite the spin waves of a high order.Let us consider the case of biaxial crystals the influence on the incident light polarization of which is described by the permittivity tensor εij. The diagonal components of this tensor are assumed not to be equal ε11≠ε22≠ε33 and connected with cartesian coordinates x, y, and z respectively. Let the light fall under the normal incidence to the plane of the material and propagate along the z-axis. Placing the material under study in an external magnetic field along the z-axis leads to the appearance of additional off-diagonal components which are ε12=-i×g and ε21=i×g. Polarization of the light propagating in such a medium transforms due to the anisotropy and Faraday effect. This process is described theoretically in [7].To be more specific, consider the YFeO3 crystal with an orthorhombic cell. Its crystallographic axes a, b, and c are oriented along x, y, and z axes respectively. The main refractive indices of such material are n1=2.365, n2=2.4, n3=2.337 at the wavelength of 633 nm (ε'' is assumed to be zero). The transformation of the incident light with circular polarization when it passes through anisotropic medium results in oscillations of the magnetization induced via IFE (blue line on Fig. 1). It worth noting that oscillations of magnetization occur for linearly polarized light. The maximum value of cross-product in formula (1) can be achieved for the polarization of incident light at an angle of 45° to the a crystal axis (green line on Fig. 1). In this case, the material behaves as a quarter-wave plate fully transforming the initial linear polarization to circular. One can observe the oscillations even for light polarized linearly along the crystal axis but their maximum value is two times smaller compared to the previous scenarios (Fig. 2). In this situation, the polarization transformation due to birefringence must disappear but the Faraday effect slightly rotates the polarization off the axis allowing the anisotropy to take its place.The oscillations of IFE due to the birefringent properties of the material under study have been theoretically demonstrated. Such a behavior of the induced magnetization is important for generating spin waves of a high order allowing one to avoid additional nanostructuring of the material.This work was supported by the Ministry of Science and Higher Education of the Russian Federation (mega-grant no. 075-15-2019-1934). **