By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Here we exploit the interference among the electric dipole, quadrupole, and magnetic dipole moments of a split-ring resonator to experimentally realize a compact directional optical antenna. This single-element antenna design robustly directs emission even when covered with nanometric emitters at random positions, outperforming previously demonstrated nanoantennas with a bandwidth of 200 nm and a directivity of 10.1 dB from a subwavelength structure. The advantages of this approach bring directional optical antennas closer to practical applications.

Resonant optical antennas are ideal for nanoscale nonlinear optical interactions due to their inherent strong local field enhancement. Indeed second- and third-order nonlinear response of gold nanoparticles has been reported. Here we compare the on- and off-resonance properties of aluminum, silver, and gold nanoantennas, by measuring two-photon photoluminescence. Remarkably, aluminum shows 2 orders of magnitude higher luminescence efficiency than silver or gold. Moreover, in striking contrast to gold, the aluminum emission largely preserves the linear incident polarization. Finally, we show the systematic resonance control of two-photon excitation and luminescence polarization by tuning the antenna width and length independently. Our findings point to aluminum as a promising metal for nonlinear plasmonics.

Broadband excitation of plasmons allows control of light-matter interaction with nanometric precision at femtosecond timescales. Research in the field has spiked in the past decade in an effort to turn ultrafast plasmonics into a diagnostic, microscopy, computational, and engineering tool for this novel nanometric-femtosecond regime. Despite great developments, this goal has yet to materialize. Previous work failed to provide the ability to engineer and control the ultrafast response of a plasmonic system at will, needed to fully realize the potential of ultrafast nanophotonics in physical, biological, and chemical applications. Here, we perform systematic measurements of the coherent response of plasmonic nanoantennas at femtosecond timescales and use them as building blocks in ultrafast plasmonic structures. We determine the coherent response of individual nanoantennas to femtosecond excitation. By mixing localized resonances of characterized antennas, we design coupled plasmonic structures to achieve welldefined ultrafast and phase-stable field dynamics in a predetermined nanoscale hotspot. We present two examples of the application of such structures: control of the spectral amplitude and phase of a pulse in the near field, and ultrafast switching of mutually coherent hotspots. This simple, reproducible and scalable approach transforms ultrafast plasmonics into a straightforward tool for use in fields as diverse as room temperature quantum optics, nanoscale solid-state physics, and quantum biology. coherent control | phase shaping | nonlinear optics | nanoscopy T he intriguing prospect of resolving and using nanoscale and quantum-mechanical processes in large, complex, and disordered systems pushes physics, biology, chemistry, and engineering to ever smaller length scales and ever shorter time scales (1-3). A promising route to unlocking this regime is the marriage of ultrafast spectroscopy with nanoplasmonics (4-6), as evidenced by various experiments aimed at controlling localization or measuring ultrafast dynamics of hotspots, such as polarization control of localization (7,8), measurements of plasmon dephasing (9-11), and adiabatic compression of pulses at plasmonic tips (12). However, for ultrafast nanoplasmonics to find widespread application in physics, biology, and material sciences, the ability to engineer a plasmonic system at will to provide a desired ultrafast response in a predetermined nanoscale hotspot is crucial: only then will the technique reach the necessary reproducibility, flexibility, and simplicity to be broadly usable. The achievement of this goal requires three conditions be met: localization of a broadband pulse in a nanoscale volume, deterministic near-field dynamics for a given plasmonic structure, and the ability to tune the near-field dynamics by plasmonic design. To prove the achievement of these first three goals, a fourth ability, measuring the ultrafast field dynamics in a given hotspot, is also required.Large inroads have been made toward achieving those goals individually....

All antennas presented in this study were nanorod metallic structures made by e-beam lithography using PMMA as positive photo-resist and depositing the metals by thermal evaporation. The height of these nanorods was fixed at 40 nm for all the experiments described in this paper.

Investigations of ultrafast processes occurring on the nanoscale require a combination of femtosecond pulses and nanometer spatial resolution. However, controlling femtosecond pulses with nanometer accuracy is very challenging, as the limitations imposed both by dispersive optics on the time duration of a pulse and by the spatial diffraction limit on the focusing of light must be overcome simultaneously. In this paper, we provide a universal method that allows full femtosecond pulse control in subdiffraction-limited areas. We achieve this aim by exploiting the intrinsic coherence of the second harmonic emission from a single nonlinear nanoparticle of deep subwavelength dimensions. The method is proven to be highly sensitive, easy to use, quick, robust and versatile. This approach allows measurements of minimal phase distortions and the delivery of tunable higher harmonic light in a nanometric volume. Moreover, the method is shown to be compatible with a wide range of particle sizes, shapes and materials, allowing easy optimization for any given sample. This method will facilitate the investigation of light-matter interactions on the femtosecond-nanometer level in various areas of scientific study.