(Invited) High-Index Dielectric Nanoantennas for Light Management, Nonlinear Optics, and Controlled Photoluminescence of Quantum Emitters
Authors: Wiecha Peter R. , Cuche Aurélien , Girard Christian , Arbouet Arnaud , Paillard Vincent 
Published: February 2020

Over the last twenty years, nanoscale optics was mainly driven by plasmonics since noble metal nanostructures, thanks to localized surface plasmon resonances (LSPR), can be used to manipulate, concentrate or redirect visible light. Such properties have led to numerous actual or potential applications in integrated optics, sensors, nonlinear optics, field-enhanced spectroscopies, or photovoltaics.
Recently, an alternative to plasmonics emerged with high refractive index dielectric nanostructures (e.g. silicon, n≈4 in the visible range), which offer the same range of applications as plasmonics by manipulating Mie optical resonances instead of LSPRs. The Mie resonances can be adjusted by modifying the size, shape, and material of those nanostructures. Furthermore, high index dielectric nanostructures offer several key advantages: absorption losses are far weaker than in metals for wavelengths longer than the direct band gap (near UV to near IR for Si), presence of strong magnetic resonances much more difficult to obtain in metals, and access to semiconductor (CMOS) technology for nanostructure fabrication (essential for easier transfer towards applications).
We present three examples describing the interest of using silicon nanoantennas: enhancement and directionality control of light scattering, enhancement and polarization control of second harmonic generation (SHG), and emission rate modification of quantum emitters placed in the silicon nanoantenna near-field. We focus also on the important role of both electric and magnetic components of light.
First, we show that silicon nanospheres and nanowires are able to scatter and/or absorb very efficiently visible light. Furthermore, due to a kind of Fano process involving the interference between a broad resonance and a sharp higher mode simultaneously excited, the direction of light scattering can be very different (either dominating forward or backward scattering).
Second, we investigate the nonlinear emission of silicon nanowires (Si-NWs). We show that SHG is strongly increased if a Mie optical mode corresponding to the fundamental (laser) wavelength is supported by the nanowire. According to nonlinear scattering theory and the so-called Miller's rule, the highest SHG is predicted for an optimum overlap of resonant modes and linear susceptibilities, at both the fundamental and harmonic wavelengths, respectively. Contrary, no SHG can be detected in absence of any resonance. This behavior, and experimental SHG mappings of individual Si-NWs, are well reproduced by numerical simulations (Green Dyadic Method - GDM). We also show that, depending on the Si-NW diameter, different surface of bulk-like contributions to the SHG are addressed. This also results in different polarizations of SHG as function of both the Si-NW diameter and incident light polarization.
Third, we show the effect of a high index dielectric nanoantenna on the spontaneous emission of quantum emitters placed in its vicinity. The emission rate corresponds to the local density of photonic states (LDOS), which is modified by the nanostructure. Thus, it can be controlled (enhancement or quenching) by properly designing the nanoantenna. For application in field-enhanced spectroscopy and single molecule detection, the goal is to obtain the highest enhancement. We discuss results of photoluminescence enhancement of molecules or quantum dots placed around simple Si nanoantennas (nanowires, nanodisk dimers). We also present preliminary results of the photoluminescence of Eu3+-doped clusters deposited on top of Si nanoantennas. These rare earth ions exhibit both electric dipole (ED) and magnetic dipole (MD) transitions of nearly equal strength. We show that these ED and MD transitions are very sensitive to the electric and magnetic LDOS, respectively. This can be evidenced using Si nanoantennas, which support both electric and magnetic resonances of comparable strength. Due to the very high index, they also give the unique opportunity to separate and redistribute in the nearfield the energy of the magnetic and electric parts of the electromagnetic field, otherwise inextricably connected in the farfield.