Tuning the directionality of spin waves generated by femtosecond laser pulses in a garnet film by optically driven ferromagnetic resonance

Excitation of spin waves of a required frequency and directional spectrum is among the crucial tasks in optomagnonics. Here we investigate the generation of spin waves in an iron garnet thin film by a train of femtosecond laser pulses with ultimately high repetition rate of up to 10 GHz and compare it with the case of 1-GHz repetition rate. The periodic optical excitation with repetition rate close to the frequency of the ferromagnetic resonance amplifies spin waves with particular phase velocity and wavelength, which are tunable across a wide range by small variations of the frequency detuning, and can be adjusted by the magnitude of the applied external magnetic field. For pulses of the same fluence, the 10-GHz pulse rate provides a significant resonant increase of the spin-wave amplitude by 11.5 times with respect to single-pulse excitation while the 1-GHz pulse rate provides only a 1.5 times advancement. Moreover, variation of the detuning frequency provides different regimes of the spin-wave propagation: short- and long-distance propagation along the magnetic field direction and appearance of an ``X'' line shape in the directionality pattern, making the considered optical approach of spin-wave generation promising for designing magnonic devices.