Metal nanoparticles can strongly enhance such nonlinear optical processes as harmonic generation and multiphoton photoluminescence due to their high electronic polarizabilities, intense optical resonances and high surface-to-volume ratio. Heavily doped semiconductor nanoparticles, such as metal chalcogenides, can also exhibit plasmonic resonances in addition to their excitonic response. Here we describe frequency upconversion in bilayer nanoparticle thin-film structures comprising copper sulfide and gold nanoparticles separated by insulating ligands or thin films when excited by near-infrared femtosecond laser pulses. The surprisingly large third-harmonic signal is due to coherent excitation of the LSPR in both semiconducting and metallic nanoparticles – a mechanism validated by a dipole-dipole model calculation
We describe the generation of second harmonic light (525 nm) from femtosecond near-infrared (NIR, 1050 nm) pulses from three nanoparticle films: gold nanospheres, hexagonal nanodisks of covellite (CuS), and a bilayer comprising covellite and gold films. Enhanced second-harmonic generation (SHG), fourth order in the NIR pump intensity, arises from coupling of plasmon resonance modes of CuS and Au. Above 6 GW/cm2, the enhanced SHG from the bilayer film is much larger than the incoherent sum of SHG from the Au and CuS films separately, and the SHG efficiency of the bilayer (Au:CuS) films is nine times larger than that of BBO per unit thickness.
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