Brillouin light scattering of spin waves inaccessible with free-space light

Open Access

Brillouin light scattering of spin waves inaccessible with free-space light

Ryan Freeman, Robert Lemasters, Tomi Kalejaiye, Feng Wang, Guanxiong Chen, Jinjun Ding, Mingzhong Wu, Vladislav E. Demidov, Sergej O. Demokritov, Hayk Harutyunyan, and Sergei Urazhdin

Phys. Rev. Research 2, 033427 – Published 16 September 2020

Abstract

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.

Received 21 January 2020
Revised 21 June 2020
Accepted 20 July 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.033427

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Light-induced magnetic effects Magnons Nanoantennas Nanophotonics Spin density waves Spin dynamics

Condensed Matter, Materials & Applied PhysicsGeneral Physics

Authors & Affiliations

Ryan Freeman¹, Robert Lemasters¹, Tomi Kalejaiye¹, Feng Wang¹, Guanxiong Chen¹, Jinjun Ding², Mingzhong Wu², Vladislav E. Demidov³, Sergej O. Demokritov³, Hayk Harutyunyan^1,*, and Sergei Urazhdin^1,†

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

^*Corresponding author: hayk.harutyunyan@emory.edu
^†Corresponding author: sergei.urazhdin@emory.edu

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Vol. 2, Iss. 3 — September - November 2020

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Images

Figure 1
(a) Schematic of BLS. (b) Spin-wave manifold for small wave numbers, calculated using the analytical spin wave theory [14]. The vertical dashed line indicates the largest wave vector accessible to BLS at the probing light wavelength $λ = 473$ nm, due to momentum conservation. The horizontal lines show the accessible spectral range. (c) Calculated spectral sensitivity of diffraction-limited $μ − BLS$ (dashed curve) and the spectral density of thermal magnons in YIG(30) at $T = 295 K$ (solid curve). (d) Experimental (symbols) and calculated (curve) $μ − BLS$ spectrum of thermal spin 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.
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Figure 2
(a, c) Pseudo-color maps of the $μ − BLS$ spectra of thermal spin waves at 295 K, as a function of the position of the probing laser spot along the biwire antenna, with the beam incident from above (a) and from the substrate side (c). (b, d) Corresponding integral BLS intensity as a function of position along the antenna (bottom scale) and $w$ (top scale). Insets in panels (a) and (c) are schematics of the corresponding experimental layouts. In all measurements, the incident beam polarization and the in-plane field $H = 2$ kOe were parallel to the wires.
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Figure 3
(a) $μ − BLS$ spectrum of thermal spin waves for $w = 160$ nm (symbols) and its fitting with the sum of three Gaussian functions $g_{i} (f) = \frac{I_{i}}{\sqrt{2 π} Δ f_{i}} e^{- {(f - f_{i})}^{2} / 2 Δ f_{i}^{2}}$ (curve). The individual Gaussian curves are shown with dotted curves. (b, d) Central frequencies $f_{i}$ (b) and integral intensities $I_{i}$ (d) of the Gaussians vs $w$ . Where omitted, the error bars are smaller than symbols sizes. The peaks are labeled 1, 2, and 3, as indicated in panel (a). The frequency of peak 3 is not shown for $w > 200$ nm, because of the large fitting uncertainty. (c) Spin wave manifold superimposed with the central frequencies of the Gaussian peaks (horizontal lines), for $w = 160$ nm. Vertical lines are guides for the eye.
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Figure 4
(a) Cross section of the spatial distribution of the component of the incident optical E-field directed along the antenna, for $w = d = 120$ nm, and the polarization along the wire length. The boundaries of the antenna and the interfaces of the magnetic film are indicated by white lines. (b) Pseudo-color plot of the calculated $μ − BLS$ spectrum of thermal spin waves at 295 K, as a function of $w$ , for $d = 120$ nm. (c, d) Dependencies on the wire width of the central frequencies (c) and integral intensities (d) of the peaks obtained by fitting the spectra in panel (b) with a sum of three Gaussians. (e) Spatial Fourier spectra of the component of the incident E-field along the wire, for polarization along the wire, vs $w$ . (f) Spatial Fourier spectra of the out-of-plane component of the electric field, for polarization perpendicular to the wires. Dashed curves in panels (e) and (f) show $k = 2 π / a$ and $4 π / a$ , as labeled, where $a = w + d$ . The dotted vertical line in (f) shows $k = π / d$
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