Brillouin Light Scattering of Spin Waves Inaccessible with Free-Space Light

PHYSICAL REVIEW RESEARCH 2, 033427 (2020) Brillouin light scattering of spin waves inaccessible with free-space light Ryan Freeman,1 Robert Lemasters,1 Tomi Kalejaiye,1 Feng Wang,1 Guanxiong Chen,1 Jinjun Ding,2 Mingzhong Wu,2 Vladislav E. Demidov,3 Sergej O. Demokritov,3 Hayk Harutyunyan,1,* and Sergei Urazhdin1,† 1Department of Physics, Emory University, Atlanta, Georgia 30322, USA 2Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USA 3Department of Physics, University of Münster, Münster 46321, Germany (Received 21 January 2020; revised 21 June 2020; accepted 20 July 2020; published 16 September 2020) Microfocus Brillouin light scattering is a powerful technique for the spectroscopic and spatial characterization of elementary excitations in materials. However, the small momentum of light limits the accessible excitations to the center of the Brillouin zone. Here we utilize a metallic nanoantenna fabricated on the archetypal ferrimagnet yttrium iron garnet to demonstrate the possibility of Brillouin light scattering from large-wave-vector, high- frequency spin wave excitations that are inaccessible with free-space light. The antenna facilitates subdiffraction confinement of the electromagnetic field, which enhances the local field intensity and generates momentum components significantly larger than those of free-space light. Our approach provides access to high-frequency spin waves important for fast nanomagnetic devices, and can be generalized to other types of excitations and light-scattering techniques. DOI: 10.1103/PhysRevResearch.2.033427 I. INTRODUCTION ponents of an electromagnetic field not present in free-space light, providing information about the high-frequency states Light-matter interaction provides an indispensable ap- not accessible to the standard BLS. Our approach can be ex- proach to studying physical, electronic, and magnetic prop- tended to other types of excitations such as phonons, and other erties of materials. In inelastic light scattering, exemplified by inelastic scattering techniques such as Raman spectroscopy. Brillouin light spectroscopy (BLS), the spectrum of scattered In an elementary scattering event underlying the anti- light is analyzed to determine the dispersion of elementary Stokes BLS in magnetic materials, a magnon is annihilated, excitations such as phonons, excitons, or magnons (quanta transferring its energy and momentum to the scattered pho- of spin waves in magnetic materials), as well as their trans- μ ton [Fig. 1(a)]. This process satisfies energy and momentum port and relaxation [1–7]. Micro-focus BLS ( -BLS), which conservation, expressed in terms of the frequencies and the utilizes a focused probing light spot [8–11], has provided wave vectors as f = f + f , and k = k + k .Here f , an unprecedented ability to study magnetization dynamics at 2 1 sw 2 1 sw sw μ f1, and f2 are the frequencies of the spin wave and the incident nanoscale [7–9,12]. -BLS is sensitive exclusively to long- wavelength spin waves, which represents a significant ob- and the scattered photons, respectively, and ksw, k1, k2 are the stacle to progress in the understanding of short-wavelength corresponding wave vectors. Conservation laws impose strin- gent constraints on the quasiparticle states accessible to BLS. spin wave modes that control fast magnetization dynamics For the probing light wavelength λ = 473 nm, the largest at nanoscale [13], which is important for the development of wave number of spin waves accessible to BLS, achieved in faster and more efficient magnetic nanodevices. − the backscattering geometry, is k = 4π/λ = 26.6 μm 1 The spectroscopic limitations of μ-BLS originate from the max [dashed vertical line in Fig. 1(b)], less than 0.1% of the small momentum of visible light, which appears to be inherent typical Brillouin zone size. The corresponding spectral range to this technique. Here we demonstrate that sub-diffraction is limited to the states only very close to the bottom of the confinement of electromagnetic field by a metallic nanoan- spin-wave spectrum. For example, in a 30 nm thick film of tenna fabricated on top of the magnetic film can efficiently an archetypical insulating ferrimagnet, yttrium iron garnet overcome this limitation. We show experimentally, and with [YIG(30)] [15], the accessible range, counted from the lowest simulations, that the antenna generates large-momentum com- spin wave frequency to the highest observable frequency, is only 1.38 GHz at an in-plane field H = 2kOe[Fig.1(b)], even though the spectrum itself extends into the THz range *Corresponding author: [email protected] [16,17]. †Corresponding author: [email protected] II. GENERATION OF LARGE-MOMENTUM Published by the American Physical Society under the terms of the COMPONENTS OF LIGHT Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) Here we experimentally demonstrate that the spectroscopic and the published article’s title, journal citation, and DOI. limits of BLS can be overcome by utilizing the near-field 2643-1564/2020/2(3)/033427(6) 033427-1 Published by the American Physical Society RYAN FREEMAN et al. PHYSICAL REVIEW RESEARCH 2, 033427 (2020) FIG. 1. (a) Schematic of BLS. (b) Spin-wave manifold for small FIG. 2. (a, c) Pseudo-color maps of the μ-BLS spectra of thermal wave numbers, calculated using the analytical spin wave theory spin waves at 295 K, as a function of the position of the probing laser [14]. The vertical dashed line indicates the largest wave vector spot along the biwire antenna, with the beam incident from above accessible to BLS at the probing light wavelength λ = 473 nm, due (a) and from the substrate side (c). (b, d) Corresponding integral to momentum conservation. The horizontal lines show the accessible BLS intensity as a function of position along the antenna (bottom spectral range. (c) Calculated spectral sensitivity of diffraction- scale) and w (top scale). Insets in panels (a) and (c) are schematics limited μ-BLS (dashed curve) and the spectral density of thermal of the corresponding experimental layouts. In all measurements, the magnons in YIG(30) at T = 295 K (solid curve). (d) Experimental incident beam polarization and the in-plane field H = 2 kOe were (symbols) and calculated (curve) μ-BLS spectrum of thermal spin parallel to the wires. waves in YIG(30), at an in-plane field H = 2 kOe, and T = 295 K. Dashed lines indicate the bottom of the spin-wave spectrum and the high-frequency BLS sensitivity cutoff. of light and electrons in the antenna is expected to result in both the concentration of the electromagnetic energy of the light into the antenna’s near field and the generation of high- redistribution of the electric field facilitated by a nanoscale momentum field components eld due to the spatial modulation metallic antenna fabricated on the magnetic system [18]. We of the field in the vicinity of the antenna [20–26]. These effects utilized an Al(30) biwire antenna fabricated on the YIG(30) are confirmed by the data and analysis presented below. film deposited on the GGG substrate [18]. The gap between the wires is d = 120 nm, and the width w of the wires was III. EXPERIMENTAL RESULTS gradually varied from 90 nm to 285 nm along the antenna [SEM micrograph in Fig. 2(a), left], allowing us to probe the Figure 2(a) shows a pseudo-color map of the μ-BLS dependence of BLS signals on w by scanning the probing spot spectra acquired with the probing spot incident from the free- along the antenna, to elucidate the mechanisms underlying the space side of the structure. The main feature of these data is observed effects. We observed similar effects for a metallic the abrupt broadening of the spectra, once the spot becomes ferromagnet Permalloy, confirming their generality [18]. positioned above the antenna, whose edge is indicated by To gain a qualitative insight into the inelastic light scatter- the dashed line. The intensity decreases with increasing w, ing in the presence of such a nanoantenna, we first approx- as expected due to the partial blocking of the beam by the imate its effects as simple partial blocking of the incident nanowires. These trends are reflected by the analysis of the light. Below we present a quantitative analysis that does total BLS intensity, which exhibits an overall decrease with not rely on such a simplifying assumption. We consider the increasing nanowire width as the probing spot is scanned diffraction-limited light spot with the half-width σ incident along the antenna [Fig. 2(b)]. on the film from above, and assume that w σ, so that only The intensity enhancement and the spectral broadening a strip of width d is not blocked. The resulting distribution of are significantly more pronounced for illumination from the the momentum of the E-field is described by the Fraunhofer substrate side [Fig. 2(c)]. In addition to the primary peak pattern, exhibiting maxima at k = (2n + 1)π/d, with peak observed without an antenna, a large secondary peak appears amplitude decreasing exponentially but remaining finite at any at frequencies around 9 GHz as the probing spot crosses the integer n [19]. Thus, the antenna generates large-momentum edge of the antenna. In contrast to illumination from above, near-field components of an electromagnetic field not present the intensity of the primary peak does not decrease with in the free-space light, which may overcome the restrictions increasing w. Meanwhile, the intensity of the secondary peak on the spectral range of BLS illustrated in Fig. 1. first increases, and then starts to decrease, and a third peak We note that the effects of the antenna are not limited emerges in the spectrum at higher frequencies. The additional to light blocking. The interaction between the electric field peaks lie outside the spectral range accessible to free-space 033427-2 BRILLOUIN LIGHT SCATTERING OF SPIN WAVES … PHYSICAL REVIEW RESEARCH 2, 033427 (2020) of the first peak corresponds to the bottom of the manifold.

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