Plasmonic Horizon in Gold Nanosponges

Abstract

An electromagnetic wave impinging on a gold nanosponge coherently excites many electromagnetic hot-spots inside the nanosponge, yielding a polarization-dependent scattering spectrum. In contrast, a hole, recombining with an electron, can locally excite plasmonic hot-spots only within a horizon given by the lifetime of localized plasmons and the speed carrying the information that a plasmon has been created. This horizon is about 57 nm, decreasing with increasing size of the nanosponge. Consequently, photoluminescence from large gold nanosponges appears unpolarized.

Plasmonic nanosponges

“…This is even more pronounced for the largest nanosponge shown in Figure 9(g) and (h). So, it is clearly different from the case of plane wave excitation, that a plasmonic horizon with finite distance of about 57 nm is formed via local excitation from inside the nanosponge [23].…”

“…Furthermore, the gold nanosponges demonstrate an interesting and strong polarization effect [22,23]. Figure 7 shows the SEM images of two individual gold nanosponges and the corresponding polarization-dependent scattering spectra.…”

“…In other words, electron-hole recombinations can be practically localized by exciting a plasmonic hot spot, which will decay radiatively with enhanced quantum yield. Recently, both polarization-dependent scattering and PL of the same individual gold nanosponges with different particle or sponge size have been studied for comparison [23]. Figure 8(a)-(c) shows the polarization-dependent scattering spectra of three individual nanosponges with increasing particle or sponge size.…”

“…The difference of the polarization anisotropy between PL and scattering shown in Figure 8 indicates that the excitations of localized plasmons by the two different sources from outside and inside, are clearly different. In order to investigate the difference in depth, FDTD simulation has been performed [23]. As shown in Figure 9, three gold nanosponges with increasing particle size have been excited by both plane wave from outside ( Figure 9(a)) and dipole from inside ( Figure 9(b)), respectively.…”

“…The plasmonic properties can be tuned further by forming the structure of hybrid nanosponges [18,21]. The nanosponges show strong polarization dependence due to the structural anisotropy contributed from the constraint of the nanoporous structure within a limited nanosponge or particle size [22,23]. Strong nonlinear optical behavior and even long-lived plasmon modes can be also observed in the gold nanosponges [24], which can be therefore used as efficient nanoantenna for many spectroscopic and sensing applications.…”