Effects of Spatial Confinement on Nonlinear Light Emission from Plasmonic Nanostructures

Photoluminescence (PL) from plasmonic nanostructures with spatial inhomogeneities have been demonstrated to exhibit an interesting nonlinear character [1,2]. Previous studies on rough metal films have shown the PL signal to have a wavelength dependent nonlinear scaling exponent which increases linearly with emitted photon energy [2]. This result was attributed to recombination of nonequilibrium electrons within the conduction band mediated by the breakdown of momentum restrictions as a consequence of the spatial inhomogeneities of the surface. However, in these experiments the spatial structures under study were random in nature. Thus, the degree of momentum breakdown and its effect on PL are not obvious from these results. To address this, we use a geometry of Au nanowires separated from Au films by nanometric SiO₂ layers via ALD to control the degree of spatial confinement of the resonant gap-mode plasmons. Our results indicate that the PL signal from this nanoprecise system has a nonlinear power law exhibiting two distinct linear regimes, differing from that of rough films. This indicates that the physical mechanism of the nonlinear PL signal originating from thermalized hot electron relaxation needs to be revisited.
[1] Phys. Rev. B 68, 115433
[2] Phys. Rev. Lett. 115, 067403