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

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Abstract
An advanced space- and 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.