Plasmonic nanoantennas emit two-photon photoluminescence, which is much stronger than their second harmonic generation. Unfortunately, luminescence is an incoherent process and therefore generally not explored for nanoscale coherent control of the antenna response. Here, we demonstrate that, in resonant gold nanoantennas, the two-photon absorption process can be coherent, provided that the excitation pulse duration is shorter than the dephasing time of plasmon mode oscillation. Exploiting this coherent response, we show the pure spectral phase control of resonant gold nanoantennas, with effective read-out of the two-photon photoluminescence.

High-index dielectric nanoantennas have recently emerged as promising alternatives to plasmonic nanoantennas for concentrating and manipulating light at the nanoscale. For example, Gallium Phosphide (GaP) nanoantennas display extremely low losses in the visible range, high nonlinearities, support both electric and magnetic resonances, and are nicely compatible with CMOS fabrication technology. Especially, the low losses and large nonlinearities are promising for ultrafast optical switching and truly all-optical control of GaP nanodevices. Here first we use two-and three-photon excitation of GaP nanodisks to probe the size-dependent resonance enhancement of second harmonic and bandgap emission. Next, we show, by spectral phase control of broadband pulsed excitation, that GaP nanoantennas outperform their metal counterparts in supporting nonlinear optical coherences.

For a scalable photonic device producing entangled photons, it is desirable to have multiple quantum emitters in an ensemble that can be collectively excited, despite their spectral variability. For quantum dots, Rabi rotation, the most popular method for resonant excitation, cannot assure a universal, highly efficient excited state preparation, because of its sensitivity to the excitation parameters. In contrast, Adiabatic Rapid Passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we advocate the robustness of ARP for simultaneous excitation of the biexciton states of multiple quantum dots. For positive chirps, we find that there is also regime of phonon advantage that widens the tolerance range of spectral detunings. Using the same laser pulse we demonstrate the simultaneous excitation of energetically and spatially distinct quantum dots. Being able to generate spatially multiplexed entangled photon pairs is a big step towards the scalability of photonic devices.