Optical excitation and probing of the antiferromagnetic modes with non-uniform in-depth distribution in birefringent antiferromagnetic crystals

Optical pump-probe setups are commonly used for excitation and investigation of the spin dynamics in various types of magnetic materials. However, usually the spatially homogeneous excitation is considered. In the present study we describe an approach for optical excitation of the nonuniform THz spin dynamics and for probing its spatial distribution inside a magnetic crystal. We propose to illuminate a crystal with laser pulses of properly adjusted polarization to benefit from a strong optical birefringence inherent to the crystal. It results in an 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 exemplary for yttrium orthoferrite crystal although the proposed approach is applicable for various magnetic materials with optical anisotropy.