Electron pressure drives THz phonons in metal-metal superlattices

Light controlled strain engineering of functional quantum materials and modulation of materials at THz frequencies are two formidable challenges of nanotechnology. Cheap and robust metal-metal superlattices, where periodic repetitions of bilayers - each layer a few atoms thick - are deposited by simple sputtering, constitute thermoacoustic meta-materials. These meta-materials are largely underexplored, because the free electrons in metals are thought to delocalize even beyond the optical penetration depth. We use ultrafast X-ray diffraction to prove that in Pt/Cu SLs, the energy of optically excited electrons is rapidly localized in the Pt layers, faster than the electron-phonon coupling. The energy of the hot electron gas is confined to a few nm in Pt, although it is in direct contact with Cu. The concomitant ultrafast electron pressure drives coherent THz strain waves with a giant 1$\%$ lattice deformation. The frequency can be tailored by the sputtered SL structure, enabling new schemes of ultrafast strain-mediated resonant nano-(spin)-electronics.