NOnlinear photonics with MEtal-less Nanoantennas and metasurfaces (NOMEN)

NOMEN sets its roots on the recently introduced concept of enhanced second order nonlinear optics in metal-less nanoantennas and metasurfaces. The past three years have witnessed a significant growth in this research area; however, the field is still in its infancy and many exciting developments are foreseen.

Impressive results have been reported in isolated dielectric nanoresonators, with several orders of magnitude increase in the efficiency of second harmonic generation (SHG) as compared to plasmonic nanoantennas. “Nonlinear metasurfaces are causing a paradigm shift in nonlinear optics”, since they are going to become the long-sought breakthrough in the quest for miniaturized, robust, and efficient nonlinear photonic devices.

Our key goal is to demonstrate the complete control of the spatial, polarization and frequency properties of photons generated by second order nonlinear processes in all-dielectric metasurfaces. Sum- and difference-frequency generation (SFG, DFG) as well as spontaneous parametric down conversion (SPDC) will be addressed. To tackle this exciting field of research, we will engineer the near and far fields of isolated dielectric nanoantennas and exploit the coherence and resonant effects that arise in properly designed periodic or quasi periodic planar arrays, i.e. metal-less metasurfaces and/or planar photonic crystals. We shall tackle the following crucial challenges:

Efficiency: Team members of this consortium were the first to recently demonstrate all-dielectric nonlinear nanoantennas with SHG efficiencies up to ~10^-4 for Al_0.18Ga_0.82As at 1 GW/cm² pump intensity.

The huge potential of dielectric resonant nanoantennas in nonlinear optics has been only partially unveiled so far, as recent results on anapole and supercavity modes have shown: clearly there is plenty of room for further improvements. The DFG process will particularly benefit from this optimization. Exploiting the mathematical correspondence between DFG and SFG, the angular distribution of the generated sum frequency beam will be measured to define the best excitation geometry to maximize the conversion efficiency and attain DFG with gain.

Resonant enhancement: Team members of the consortium were among the first to demonstrate SHG and THG with a cw pump at ultra-low power (sub-mW) in Si and GaN photonic crystal nanocavities with high quality factors, together with the efficient control of the electromagnetic far-field profile. Also, they were the first to demonstrate entangled photon sources by SPDC in high-Q silicon resonators. Our goal here is to apply these concepts for the realization of a new generation of metasurfaces for SHG and SPDC at low operation powers exploiting the strong nonlinearities of AlGaAs as well as its transparency at 1550 nm and 775 nm wavelengths.

Complete beam control: This is a major challenge for the proposal: we aim at carefully engineering the radiation of the nonlinear emission from the dielectric nanoantennas, tuning the multipolar properties of Mie resonators through the recently demonstrated optimization process. By a proper design of the nanoantennas and the metasurfaces, we plan to obtain control of the polarization and the wave-front forming properties of the nonlinearly generated optical beams.

Tunability: Several technologies are currently emerging to provide modulation of metasurface response using mechanical, electrical, or optical control. We have recently developed a quantitative model to identify and disentangle the three physical processes that govern the ultrafast changes in amorphous silicon particles exhibiting Mie-type resonances, namely two-photon absorption, free-carrier relaxation and lattice heating. Exploiting the ps-scale optical response of III-V semiconductors, we plan to obtain high-contrast, ultrafast all-optical modulation of the nonlinear emission with full return to zero below 10 ps.

The scientific outcome of our project will dramatically advance the knowledge on classical and quantum frequency conversion processes driven by second order nonlinearities in all-dielectric metasurfaces, offering many novel opportunities in several branches of science. In particular, it will deliver a new class of compact and lightweight room temperature devices generating entangled photons for quantumcommunication, imaging and sensing at the nanoscale.