Wave-mixing origin and optimization in single and compact aluminum nanoantennas

Abstract

The outstanding optical properties for plasmon resonances in noble metal nanoparticles enable the observation of non-linear optical processes such as second-harmonic generation (SHG) at the nanoscale. Here, we investigate the SHG process in single rectangular aluminum nanoantennas and demonstrate that i) a doubly resonant regime can be achieved in very compact nanostructures, yielding a 7.5 enhancement compared to singly resonant structures and ii) the \(χ_{\perp\perp\perp}\) local surface and \(γ_{bulk}\) nonlocal bulk contributions can be separated while imaging resonant nanostructures excited by a tightly focused beam, provided the \(χ_{\perp\parallel\parallel}\) local surface is assumed to be zero, as it is the case in all existing models for metals. Thanks to the quantitative agreement between experimental and simulated far-field SHG maps, taking into account the real experimental configuration (focusing and substrate), we identify the physical origin of the SHG in aluminum nanoantennas as arising mainly from \(χ_{\perp\perp\perp}\) local surface sources.

Figures (5)

• Figure 1: SHG intensity spectrum of single 225 nm-long aluminum (blue) and gold (red) antennas illuminated at 850 nm. In inset, the SEM image of a single 425 nm aluminium nanoantenna (right) and typical topographic profile measured with the probe of a scanning near-field optical microscope implemented in the setup (left).
• Figure 2: a) Simulated absorption spectra for a 225 nm (orange curve) and a 425 nm (green curve) aluminum nanoantennas excited by an electric point dipole in its vicinity. Plasmonic mode orders n are indicated over each resonance. Red and blue dashed lines indicate excitation and detection wavelengths used for all measurements, respectively. b) Simulated absorption spectra for a 225 nm scanned along its long axis under a focused beam at the fundamental frequency (850 nm). Inset: 2D maps of the scattered electric field norm at the fundamental and harmonic frequencies obtained inside the antenna when the focus of the microscope objective is centered on the antenna. They evidence the $n=1$ fundamental and $n=2$ excited plasmon modes, respectively. The excitation configurations for the absorption spectra are sketched by a) the red dipole and b) focused beam, respectively.
• Figure 3: SHG excitation intensity maps in counts/s excited at 850 nm for aluminum nanoantennas of increasing lengths from 125 to 525 nm. White rectangles are guides to the eyes indicating the position of corresponding nanostructures. Both excitation and detection linear polarizations are set along the x axis.
• Figure 4: Measured and simulated SHG maps obtained for a single 225 nm aluminum antenna excited at 850 nm with an electric field parallel to the antenna axis (red arrows). The detection polarization is changed between panels (a) and (b) as indicated by the blue arrows. The SHG maps simulated with $\chi_{\perp\perp\perp}$ local surface and $\gamma_{bulk}$ nonlocal bulk contributions are labeled Surf and Bulk, respectively. The intensity profiles along the dashed green lines in panels (a) and (b) are shown in panels (c) and (d). The simulated curves for the $\chi_{\perp\perp\perp}$ local surface (red dashed line) and $\gamma_{bulk}$ nonlocal bulk (violet dashed line) contributions are divided by 79 (see text) to match the experimental data in panel (c).
• Figure 5: Same as in Figure 4 for a single 425 nm antenna. Note that the intensity profiles of panel (d) have been obtained perpendicularly to the antenna long axis, as shown by the green dashed line in panel (b), and multiplied by a factor of two for a better visibility.