Direct Excitation of Propagating Spin Waves by Focused Ultrashort Optical Pulses

An all-optical experiment long utilized to image phonons excited by ultrashort optical pulses has been applied to a magnetic sample. In addition to circular ripples due to surface acoustic waves, we observe an $X$-shaped pattern formed by propagating spin waves. The emission of spin waves from the optical pulse epicenter in the form of collimated beams is qualitatively reproduced by micromagnetic simulations. We explain the observed pattern in terms of the group velocity distribution of Damon-Eshbach magnetostatic spin waves in the reciprocal space and the wave vector spectrum of the focused ultrafast laser pulse.

Figure 1

(a) Cartoon illustration of the experimental setup: (1) 800 nm wavelength pump beam, (2) pump beam objective, (3) 50 nm thick permalloy film, (4) $430\mu \mathrm{m}$ thick sapphire substrate, (5) 0.17 mm thick glass coverslip, (6) probe beam objective, (7) nonpolarizing beam splitter, (8) 400 nm wavelength probe beam, (9) 400 nm bandpass filter, (10) spatial filter, (11) diaphragm with a $50\mu \mathrm{m}$ pinhole, (12) polarizing beam splitter, and (13) photodiodes. (b)–(j) Time resolved $10\times 10\mu {\mathrm{m}}^2$ Kerr images at time delays of 0.667, 1.000, 1.200, 1.267, 1.367, 1.450, 1.567, 1.733, and 2.017 ns, respectively. The pump-probe time delay of 1 ns corresponds to the simultaneous arrival of the pump and probe pulses to the sample [27].

Figure 2

(a) The initial Gaussian profile of the absolute value of the magnetization assumed in the simulations is shown. (b) The simulated recovery of the absolute value of the magnetization $⟨M⟩$ averaged over the $5\times 5\mu {\mathrm{m}}^2$ sample is shown as a function of time (bottom axis) together with the net dynamic out-of-plane component of the magnetization $\Delta ⟨m_z⟩$ presented for longer time scales (top axis) for the epicenter of the pump pulse. (c) The simulated snapshots of the spatial profiles of the out-of-plane component of the magnetization are shown. The color contrast is given by $m=C(M_z/M_s-M_{z0}/M_{s0})$, where $C=100$ is a contrast adjustment constant and $M_{z0}$ and $M_{s0}$ are the equilibrium out-of-plane component and absolute value of the magnetization, respectively. ${\mathbf{H}}_{\mathrm{ext}}$ and ${\mathbf{M}}_0$ denote the directions of the in-plane components of the applied static magnetic field and the initial magnetization, respectively.

Figure 3

(a) Constant frequency curves ranging from 4 to 13 GHz (from left to right) are shown in steps of 1 GHz for magnetostatic spin waves on a 50 nm thick permalloy film under a field of 150 Oe applied along the $y$ direction. (b) The spatial profile of the pump spot is shown on the sample surface of a $10\times 10\mu {\mathrm{m}}^2$ area. (c) The Fourier transform of the pump spot profile from (b) is shown in the reciprocal space. (d) The distribution of the efficiency of excitation of magnetostatic spin waves by the pump pulse from (b) and (c) is shown in the group velocity space.