Enhanced electronic emission by excitation of an interface resonance in metallic bilayer gratings irradiated by short laser pulses

Strong electronic emission (1 A/cm²) has been observed during the irradiation of metal-metal targets (Au overlayer on Al) by short femtosecond laser pulses. The electron signal, detected both by an electron multiplier and by a time-of-flight spectrometer, is found to vary with the Au thickness and to maximize for an optimum value of about 40 nm, with laser operating in p polarization. The results are interpreted on the basis of an interface resonance driven by a surface plasmon excited on the externally periodically modulated surface. The time duration of the electron signal is measured to be about 100 fs, comparable with the laser duration. When the laser intensity I varies, the electronic current exhibits a different behavior according to the excitation of the surface plasmon only or of the interface resonance driven by the surface plasmon. In the first case, the electronic current is increased with respect to the current characteristic of the multiphoton photoelectric process and changes according to an I⁴ law, suggesting a plasmon-assisted photoemission process. In the second case, the current is further increased and shifts its dependence towards an I^p law with p ranging from 3 to 1 here, showing the onset of additional mechanisms contributing to the electron ejection. The experimental results are compared with a simple theoretical model estimating surface and interface waves in the relevant experimental conditions.