Spatially Modulated Two-Photon Luminescence from Si-Au Core-Shell Nanowires

Si-Au core-shell nanowires were synthesized with a two-step strategy combining chemical vapor deposition and wet chemistry methods. Si core diameter, Au shell thickness, and wire length were controlled during synthesis. Under a far-field twophoton excitation scanning microscope, spatially modulated twophoton luminescence (TPL) was observed from the core-shell nanowires prepared with several micrometers in length. The mode spacing scales linearly with the length of nanowires for samples with 15 and 40 nm Au shells. The spatial modulation pattern was found to be independent of the excitation wavelength. The TPL intensity displayed a cos dependence on the excitation field polarization, indicating a sequential two-photon excitation process enhanced by the longitudinal plasmon field. The spatially modulated TPL is interpreted by a standing wave model of surface plasmon polaritons along the core-shell nanowire which acts as a Fabry-P erot resonator. These results suggest TPL imaging of Si-Au core-shell nanowires as a new system providing useful insights regarding a plasmonic wave in a 1-D nanostructure.