Spin-wave optics fabricated by FIB irradiation of YIG films

Abstract

We present focusing of spin waves with spherical lenses in YIG by creating a magnonic refractive index change via Ga+ focused ion beam (FIB) irradiation. Direct FIB irradiation in YIG induces an ion-dose-dependent change in the effective magnetization Meff [1], and we use this to locally modify the refractive index. The lenses were designed using the lensmaker’s equation where the relative refraction index is estimated from experiments as nrel=λ0/λFIB= 4.2μm/2.5μm = 1.68.In the experiments, we use an in-house sputtered 100 nm thick YIG film on GGG and obtain the largest △Meff for a dose of 1×1013 ions/cm2. For larger doses, the change in △Meff is reversed and damping increases significantly. The measured spin-wave pattern of two plano-convex spherical lenses is presented in Fig. 1. We use longitudinal time-resolved MOKE microscopy to image the spin-wave patterns in forward volume configuration (isotropic propagation). Bending of the wavefront is visible inside the lenses, as well as the shorter wavelengths.Ion irradiation in thin films to manipulate their magnetic properties has been used in many magnetic materials/compositions but is particularly challenging in insulators. Researchers in our field, to our knowledge, have not FIB irradiated YIG to locally modify the spin-wave dispersion relation, or to pattern YIG without etching. Using spin waves for analog computing (such as optical signal processing) requires precise local modification of the magnetic properties, and we recently demonstrated the potential of FIB patterning by fabricating a concave grating for spin waves realized with higher ion doses that locally destroy magnetic properties of YIG [2]. Furthermore, gradually changing refractive-index maps can be realized by irradiating a dose-gradient, giving access to create gradient-index magnonic elements like GRIN lenses [3],[4],[5].  Time-resolved MOKE image of a 20 (left) and a 10 μm (right) thick spin-wave lens fabricated via FIB. The irradiation pattern d-shaped lens is indicated in semitransparent red, the groundline of a coplanar antenna for coherent excitation in gray. The initially applied wavelength is 4.3 μm, and shortened to about 2.5 μm inside the lenses due to the FIB induced Meff increase.