Hybrid magnon-phonon cavity for large-amplitude terahertz spin-wave excitation

Terahertz (THz) spin waves or their quanta, magnons, can be efficiently excited by acoustic phonons because these excitations have similar wave vectors 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 waves via cavity-enhanced magnon-phonon interactions. The amplitude of the acoustically excited THz spin wave in the freestanding multilayer is predicted to be more than 10 times larger than in a substrate-supported multilayer. Acoustically excited nonlinear magnon-magnon interactions are 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.