Nondiffractive Subwavelength Wave Beams in a Medium with Externally Controlled Anisotropy

Abstract

We predict and experimentally demonstrate that in a medium with externally induced anisotropy, a wave source of a sufficiently small size can excite practically nondiffractive wave beams with stable subwavelength transverse aperture. The direction of beam propagation is controlled by rotating the induced anisotropy axis. Nondiffractive wave beam propagation, reflection, and scattering, as well as beam steering have been directly observed by optically probing dipolar spin waves in yttrium iron garnet films, where the uniaxial anisotropy was created by an in-plane bias magnetic field.

Towards graded-index magnonics: Steering spin waves in magnonic networks

“…Apart from the small region at small wave vectors, the curve consists of nearly straight lines. This leads to the virtually same direction of the magnonic group velocity for a wide range of wave vectors, giving rise to the formation of spin-wave caustic beams [24,26]. This explains our observation of the strongly directional beam emitted from the leg-arm boundary (but not the absence of the other beam).…”

“…The symmetry axis of the anisotropic magnetostatic dispersion coincides with the direction of the magnetization [21][22][23]. This anisotropic dispersion leads to the formation of nondiffracting caustic spin-wave beams * Corresponding author: v.v.kruglyak@exeter.ac.uk [24][25][26][27][28][29][30] and to anomalous spin-wave reflection, refraction, and diffraction [31][32][33][34][35]. Here, we explore these ideas in networks of magnonic waveguides [6,8,[12][13][14][15][16][17][18][19][20]24,29,36], in which not only the internal magnetic field and the magnetization, but also the associated anisotropic dispersion are nonuniform and contribute significantly to the observed switching of the direction of spin-wave propagation.…”

“…Then, for each k x value, the isofrequency curve corresponding to the excitation frequency returns an allowed (by the magnetostatic dispersion relation) value of k y , while the normal to the isofrequency curve shows the direction of the group velocity [ Fig. 3(c)] [23][24][25][26]. The uniformity of the field and magnetization distributions in the arms along the x axis (starting from about 1 µm from the leg-arm boundary) ensures conservation of the k x value of the spin wave propagating across the arm's width.…”

Generation of propagating spin waves from regions of increased dynamic demagnetising field near magnetic antidots

“…Spin waves excited by a focused optical pulse 4 were observed to have the caustic character, but their frequency is difficult, if not impossible, to control. Spin-wave caustics have also been generated as a by-product of the collapse of non-linear spin-wave "bullets," 21 via the diffraction of plane spin waves from narrow apertures [22][23][24] and antidots, 25 and via the excitation of localized edge modes. 26 In this letter, we demonstrate excitation of propagating spin waves (and more specifically, spin-wave caustics) using magnetic non-uniformities created by patterning in magnetic films.…”

Electric Field Control of Magnon Power Flow in Thin Ferromagnet Films

“…The magnitude of the energy sent in that direction is proportional to the square root of the curvature of the surface at a given point. When the curvature is zero, one finds the so-called caustics beams, when for waves having different wave vectors k the group velocity is the same and, formally, in those directions the power flow diverges [8][9][10][11].…”