Dispersion of the nonlinear susceptibility in gold nanoantennas

Femtosecond optical pulses tunable in the near infrared are exploited to drive third harmonic generation (THG) and incoherent multiphoton photoluminescence (MPPL) in gold plasmonic nanoantennas. By comparing the yield of the two processes concurrently occurring on the same nanostructure, we extract the coherent third-order response of the antenna. Its contribution is enhanced at shorter excitation wavelengths allowing the observation of dispersion in the nonlinear susceptibility of gold.

Figure 1

(a) Scanning electron micrograph of two polycrystalline nanorods resonant at 2000 nm (left) and 1400 nm (right), used in the experiments. (b) Sketch of the density of occupied states in gold. The solid arrows depict the sequence of individual absorption steps that lead to a hot electronic distribution in the case of excitation with near-IR photons. (c) One additional photon promotes an electron from the $d$ band to states that become available below the Fermi level. The residual hole (white circle) then recombines (dashed arrow) with the consequent emission of PL that results from a multiphoton process.

Figure 2

(a) Typical nonlinear emission spectra obtained from single monocrystalline nanorods, each custom-tailored to be resonant to the excitation wavelength between 1260 and 2020 nm. Consequently, the rod length varies from 170 to 360 nm. The spectra are normalized to their narrow THG maximum, residing on the broad background of MPPL emission. Spurious second harmonic generation (SHG) may be observed if within the recorded spectral window. (b) Separation of the coherent THG emission (blue) from the incoherent MPPL (red) background in the nonlinear response (gray) of a monocrystalline nanorod with a cubic spline (green).

Figure 3

Spectrally resolved power dependence of the nonlinear emission obtained from a gold nanorod of 320 nm length. (a) The nonlinear order $n$ of each corresponding emission wavelength in the case of a monocrystalline rod (black crosses) together with the nonlinear emission spectrum acquired at the maximum excitation power (gray). The excitation was tuned to a central wavelength of 2020 nm. The vertical lines (A, B, and C) mark positions for which the power dependence fits are shown in (b): the experimental data points of emission intensity are depicted versus excitation power (symbols). The solid lines in this double-logarithmic graph show the least-square exponential fits determining the nonlinear order $n$ plotted in (a).

Figure 4

Spectrally resolved power dependence of the nonlinear emission obtained from a gold nanorod of 275 nm length under excitation at a central wavelength of 1310 nm. (a) The nonlinear order $n$ of each corresponding emission wavelength (black crosses) together with the nonlinear emission spectrum acquired at the maximum excitation power (gray). The vertical lines A and B mark positions for which the power dependence fits are shown in (b): the experimental data points of emission intensity are depicted versus excitation power (symbols). The solid lines in this double-logarithmic graph show the least-square exponential fits determining the nonlinear order $n$ plotted in (a).

Figure 5

(a) Selected excitation spectra that were used to obtain THG and MPPL from gold nanorods. The integrated nonlinear emission is shown as a function of the center wavelength of the driving pulse for THG (b) and MPPL (c), respectively. Each data point marks the result from a single individual antenna. Both mono- and polycrystalline specimens are considered in these measurements with no significant differentiations. The green lines serve as a guide to the eye.

Figure 6

Ratio between THG and total nonlinear emission. Each data point is obtained from a single gold nanorod. The green line serves as a guide to the eye.