Phonon Engineering in Ultrathin Sn Films

ABSTRACT Nanostructuring offers unique opportunities to manipulate vibrational properties of materials through reduced dimensionality. This is widely exploited in materials science to engineer thermal conductivity in nanostructures, for example. Similarly, in quantum optics, nanoscale engineering of vibrational states can suppress thermal decoherence and enhance coherent light–matter interactions. In x‐ray quantum optics, vibrational properties are especially critical because light–matter interaction often occurs via coherent excitations of ultrasharp nuclear resonances, requiring a recoilless interaction, i.e., no energy exchange between nuclei and lattice. This raises the question: can nanoscale engineering also enhance coherent light–matter interactions in x‐ray quantum optics, and thereby enable the use of nuclear resonances with typically low recoilless interaction fractions, such as the resonance of . Here, we demonstrate the tunability of the vibrational properties of nanometer‐thin Sn films by embedding them in multilayer structures, achieving nearly an order‐of‐magnitude increase in the recoilless fraction. Using nuclear inelastic x‐ray scattering, we studied vibrational density of states of Sn layers with respect to interlayer diffusion, structural disorder and interfacial intermetallic compound formation. Our results show that suitable embedding materials can substantially modify Sn's vibrational behavior and thus potentially enable the nuclear resonance for x‐ray quantum optics applications.