  Terahertz (THz) spin waves or their quanta, magnons, can be efficiently
excited by acoustic phonons because these excitations have similar wavevectors
in the THz regime. THz acoustic phonons can be produced using photoacoustic
phenomena but typically have a low population and thus a relatively low
displacement amplitude. The magnetization amplitude and population of the
acoustically excited THz magnons are thus usually small. Using analytical
calculations and dynamical phase-field simulations, we show that a freestanding
metal/magnetic-insulator (MI)/dielectric multilayer can be designed to produce
large-amplitude THz spin wave via cavity-enhanced magnon-phonon interaction.
The amplitude of the acoustically excited THz spin wave in the freestanding
multilayer is predicted to be more than ten times larger than in a
substrate-supported multilayer. Acoustically excited nonlinear magnon-magnon
interaction is demonstrated in the freestanding multilayer. The simulations
also indicate that the magnon modes can be detected by probing the charge
current in the metal layer generated via spin-charge conversion across the
MI/metal interface and the resulting THz radiation. Applications of the
freestanding multilayer in THz optoelectronic transduction are computationally
demonstrated.
