The design and fabrication of ever smaller and faster magnetic devices for data storage, sensorics and information processing entail the development of efficient tools to control the dynamic behavior of the magnetization. In particular, femtosecond laser-induced magnetic excitations, originating in thermal and nonthermal effects, offer the possibility to study magnetic systems on time scales down to $10-100$ femtoseconds.
Schematic of laser pulse-induced electron motion.
Nonthermal opto-magnetic effects bear high potential for promising magnetic devices and applications and pose interesting questions for theory. In the present study we worked out the role of nonlinear effects which are not negligible for intensities in the range of $10^{14} - 10^{16}$ W/cm$^2$. Utilizing a classical treatment of the laser-driven carrier in a given symmetry environment, it is possible to separate the electron motion into a first-order displacement that is directly proportional to the electrical field of the optical pulse and a second-order displacement that depends on the square of the electrical field. Considering magnetic insulators with a certain symmetry configuration we found that both the first-order and the second-order electron displacement create current loops that generate the light-induced magnetic fields $B^{(1)}$ and $B^{(2)}$. As the direction of rotation of the first-order electron displacement always opposes that of the second-order displacement, the light-induced fields $B^{(1)}$ and $B^{(2)}$ also carry opposite signs. Applying Laguerre-Gaussian laser beams which carry orbital angular momentum characterized by the number L, the total optically-generated magnetic field $B^{(1)} + B^{(2)}$ can be controlled by varying |L|.
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