Giant electron–phonon coupling detected under surface plasmon resonance in Au film

Surface plasmon resonance (SPR) is a powerful tool to amplify coherent phonon signals in metal films. In a 40 nm Au film excited with a 400 nm pump, we observed an abnormally large electron–phonon coupling constant of about 8 × 10¹⁷ W∕m³K, almost 40× larger than those reported in noble metals, and even comparable to transition metals. We attribute this phenomenon to quantum confinement and interband excitation. With SPR, we also observed two coherent phonon modes in a GaAs/AlAs quantum well thin film. Our finding provides a new approach to generate high-frequency acoustic phonons in noble metals and to study their nonlinear nature of propagation.