Optical Excitation and Probing of Antiferromagnetic Modes with Nonuniform-in-depth Distribution in Birefringent Antiferromagnetic Crystals

Optical pump-probe setups are commonly used for the excitation and investigation of spin dynamics in various types of magnetic materials. However, spatially homogeneous excitation is usually considered. In the present study, we describe an approach to optical excitation of nonuniform THz spin dynamics and to probing its spatial distribution inside a magnetic crystal. We propose to illuminate a crystal with laser pulses of properly adjusted polarization to benefit from the strong optical birefringence inherent in the crystal. This results in unusual behavior of the effective magnetic field generated by the pulses due to the inverse Faraday effect and the peculiar sign-changing dependence of the direct Faraday effect inside the crystal. The study is performed for an yttrium orthoferrite crystal as an example, although the proposed approach is applicable to various magnetic materials with optical anisotropy.

Figure 1

Schematic representation of the configuration under study and the dynamics of the antiferromagnetism and magnetization vectors in the quasiantiferromagnetic mode (inset).

Figure 2

IFE-induced effective magnetic field distribution inside $c$-cut yttrium orthoferrite (normalized to $H_{\mathrm{IFE}}^0$) vs (a) the ellipticity and (b) the wavelength of the incident light. (c)–(g) Effective magnetic field distributions generated by illumination with (c) right circularly polarized light ($\alpha ={45}^{\circ }$, $\psi ={90}^{\circ }$); (d) left circularly polarized light ($\alpha ={45}^{\circ }$, $\psi =-{90}^{\circ }$); (e) light linearly polarized at an angle of $\alpha ={45}^{\circ }$ to the $a$ crystal axis ($\psi =0$); (f) light linearly polarized at an angle of $\alpha =-{45}^{\circ }$ to the $a$ crystal axis ($\psi =0$).

Figure 3

Amplitude (taking account of sign) of the temporal oscillations in the probe polarization ${\mathrm{\Phi }}_{\mathrm{osc}}$ (a) and induced ellipticity ${\mathrm{\Psi }}_{\mathrm{osc}}$ (b) depending on the wavelength ${\lambda }_{\mathrm{pb}}$ of the probe and the ellipticity angle ${\psi }_{\mathrm{pm}}$ of the pump pulses. The wavelength of the pump pulse (${\lambda }_{\mathrm{pm}}=1.2\mu \mathrm{m}$) and the length of the crystal ($h=68.6\mu \mathrm{m}$) are chosen in such a way to provide excitation of the IQ-AFM mode.

Figure 4

The distribution of the magnetization precession amplitude induced by a pump pulse with a certain wavelength in an anisotropic material can be reconstructed (d) using the measured spectral dependences of the amplitude of the oscillations of the polarization plane (a) and ellipticity (b) of the probe pulse. (c) Real distribution of the magnetization precession amplitude in the medium.