Measurements of angular and spatial characteristics of magnetostatic wave beams by an optical guided-wave probe

Abstract

Optical measurements of the angular and spatial characteristics of magnetostatic forward volume wave (MSFVW) beams propagating both in a linear and in a non-linear regime in yttrium–iron–garnet (YIG) film have been performed. The MSFVW beams were excited by a single-element microstrip transducer, within the microwave frequency range 2–3 GHz. The measurements were conducted with the help of an optical probing technique based on the magneto-optical (MO) interaction between guided optical waves and the MSFVWs in the YIG film. The MO probing system was built on the principle of transverse Bragg diffraction geometry, allowing us to measure the spatial evolution of the partial plane waves making up the MSFVW beam. In the linear regime, the measurements of the longitudinal profiles of the partial waves have shown that the MSFVW attenuation coefficient can increase following a linear law, from 2 to 10 cm −1, as the MSFVW wave number increases from 200 to 2000 cm −1. Also, it was shown that MSFVW beams with different frequencies can be spatially separated at a small distance from the microstrip transducer (≈2 mm) by using a weak non-uniform magnetic bias field with a gradient of ≈30 Oe/cm. In the non-linear regime, there are, at least, three specific levels of the incident microwave power which correspond to different manifestations of non-linearity of the MSFVW beams. At a certain value of the power, the partial plane waves of the beam can periodically change the direction of propagation (we observed a periodic collision of these waves). A small increase of the power level can lead to a spatial-periodic energy exchange between the different groups of the partial plane waves. At high microwave powers, non-linear MSFVW can excite a discrete spectrum of new spin-wave modes, the in-plane wave number of which is less than the wave number of the MSFVW. These non-linearly excited waves exist within a small spatial region situated near the microstrip transducer.