Coherent spin waves driven by optical spin-orbit torque

Phase-coherent spin waves can be generated by magnetic field pulse, spin current pulse, or optical pulse. Here we use optical spin-orbit torque, originating from the conversion of an optical pulse into a spin-polarized current pulse, to excite spin waves in the frequency range from GHz to THz. We investigate the frequency, amplitude, and damping of the spin waves of Co thin films. From the frequency analysis, we determine the stiffness of Co spin waves to be $5\phantom{\rule{0.16em}{0ex}}\mathrm{meV}\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{2}$. From the amplitude analysis, we show that the Co layer acts as a cavity for spin waves. From the damping analysis, we observe that the damping enhancement due to the spin pumping effect is about two times larger in spin waves than in uniform precession.