Surface Plasmon Polariton Propagation and Coupling in Gold Nanostructures

Abstract

Surface plasmon polariton (SPP) propagation in chemically synthesized gold nanobars was investigated using scanning transient absorption microscopy. The SPP propagation lengths were correlated to the size and shape of the nanobars, which were determined by atomic force microscopy. The average propagation length was found to be 12 ± 4 μm, and the measured values were independent of the excitation polarization. Finite element calculations were performed for gold nanobars in the presence of a glass substrate to model the experimental results. Comparison between the experimental and theoretical results shows that both bound and leaky SPP modes can be excited. The leaky mode is detected in the larger nanobars (widths greater than 500 nm), and the bound mode is detected in smaller nanobars. Highly directional and efficient plasmon coupling between a gold nanobar and a nearby gold nanoplate was also observed in the experiments.

Imaging nano-objects by linear and nonlinear optical absorption microscopies

“…Control experiments on long nanowires have shown that this effect is not significant in our measurements. [75, 98,177] These experiments also show that the finite group velocity of the SPP does not create a problem.…”

“…[181,182] The finite element analysis shows that the propagation lengths are very sensitive to the nanowire dimensions, and factors such as the aspect ratio and degree of rounding at the corners of the structure. [177] The comparison between experiment and simulation demonstrates that, in general, both the bound and leaky modes are excited by the pump laser. The transient absorption measurements provide information about the mode that has the longest propagation length, which could be either the bound or the leaky mode.…”

“…[75, 98,177] SPPs are electromagnetic waves that are confined at metal-dielectric interfaces. [129] They cannot be directly excited by free space photons because of wavevector miss-match.…”

“…An example image for a gold "nanobar" (a thick gold nanowire with a rectangular cross-section) is shown in Figure 5(a), and a line profile of the signal along the nanobar is shown in Figure 5(b). [98,177] Note that the signal is presented on a natural log scale in both figures, and the pump and probe pulses are temporally overlapped in these measurements. The polarizations of the pump and probe beams were parallel and perpendicular, respectively, to the long axis of the nanobar.…”

“…This is optimal for exciting the propagating SPP modes, and probing the response through the transverse plasmon band of the nanobar. [152,177] The signal in these experiments can be understood in the following way: the pump laser launches the SPP mode that travels down the nanowire. As the SPP moves it dephases, this creates excited electron-hole pairs that produce a transient absorption signal which is detected by the probe.…”