Linear and nonlinear diffraction of dipolar spin waves in yttrium iron garnet films observed by space- and time-resolved Brillouin light scattering

An advanced spaceand time-resolved Brillouin light-scattering technique is used to study diffraction of two-dimensional beams and pulses of dipolar spin waves excited by strip-line antennas in tangentially magnetized garnet films. The technique is an effective tool for investigation of two-dimensional spin-wave propagation with high spatial and temporal resolution. Linear effects, such as the unidirectional excitation of magnetostatic surface waves and the propagation of backward volume magnetostatic waves ~BVMSW! in two preferential directions due to the noncollinearity of their phase and group velocities, are investigated in detail. In the nonlinear regime, stationary and nonstationary self-focusing effects are studied. It is shown that nonlinear evolution of a stationary BVMSW beam, having a finite transverse aperture, leads to self-focusing of the beam at one spatial point. Evolution of a finite-duration ~nonstationary! BVMSW pulse leads to space-time self-focusing and formation of a strongly localized two-dimensional wave packet ~spin-wave bullet!. Theoretical modeling of the self-focusing and diffraction processes by using a variational approach and direct numerical integration of the two-dimensional nonlinear Schrödinger equation provides a good qualitative description of the observed phenomena.