One of the main purposes of optics is to control the propagation and confinement of light. Advances in optics began historically with the development of large lenses and mirrors, then prisms and gratings, etc. The improvement in these devices slowed as the diffraction limit approached. Nanophotonics aims to manipulate electromagnetic waves at a sub-wavelength scale to overcome this limit. Recent developments in manufacturing technologies, digital tools and theoretical models have paved the way for new devices with unprecedented performance.
Optical metasurfaces are optical antenna arrays, with sub-wavelength size and separation. They represent an original concept of flat optics without classic analogues. They allow the ultimate miniaturization of optical components, as well as the possibility of new functionalities not possible before. During the last two decades, the optical properties of metasurfaces have been intensely studied in the linear regime, with metallic or amorphous dielectric nanostructures.
Recently, nonlinear flat optics have received increasing attention, with frequency conversion effects observed first in hotspots associated with localized plasmon resonances in metallic nanoantennas, and later in association with multipole resonances. Mie type in dielectric nanostructures. In this transition to the nanoscale, the role of phase matching has been replaced by that of near-field resonances occurring in non-Hermitian open nanostructures.
In the new field of nonlinear metasurfaces, for which the name nonlinear meta-optics has been proposed, dielectric implementations have provided the highest nonlinear generation efficiencies: first with the third generation of harmonics in amorphous or silicon-on-insulator platforms, then with the second generation of harmonics and spontaneous parametric down-conversion in non-centrosymmetric materials such as III-V semiconductors and lithium niobate. More recently, the scientific community has also become interested in the wavefront shaping of harmonic fields, ranging from simple meta-grids and meta-lenses to the non-linear generation of complex meta-holograms and special beams.
The authors of this article review recent advances in nonlinear optics with dielectric metasurfaces, focusing on the paradigmatic effect of second harmonic generation. They discuss the most widely used technology platforms that have supported these advances and analyze different control approaches. Their article begins with an introduction on the topic of nonlinear generation in sub-wavelength “Mie” resonators, highlighting the main figures of merit for high efficiency in non-Hermitian systems. Then, they review the main approaches adopted in recent years to control or stimulate the generation of harmonics in metasurfaces. Finally, they compare their performance with other well-established technologies, illustrate the current state of the art and imagine some scenarios in which these devices could soon offer unprecedented opportunities. In their conclusion, two possible perspectives emerge for the burgeoning field of dielectric nonlinear metasurfaces.
On the one hand, the use of coupled nanoantennas and collective resonances seems the most judicious strategy to maximize the nonlinear generation. However, metasurfaces with the largest quality factors are also orders of magnitude less efficient than other platforms. This result stems from the fact that to date most studies taking this approach have focused on creating a single high-Q resonance around FF. Instead, it is to be expected that such a gap with other technologies can be bridged by careful design that strikes a good balance between free-space coupling and mode quality factors, while emphasizing implements a double resonance condition and optimizing the nonlinear overlap integral.
On the other hand, low-Q antennas represent a true paradigm shift with respect to both guided structures and photonic crystals. Their currently lower nonlinear generation efficiency is largely offset by intriguing possibilities ranging from dynamic tunability of individual meta-atom emission to pulse shaping, broadband parametric devices, imaging nonlinear, wavefront shaping and meta-holography. Their rapid development is currently supported by continued advances in nanofabrication, promising new nonlinear materials like TMDCs, and analytical and numerical methods to model nonlinear generation in leaky cavities. The improvement of such mathematical tools seems particularly important for the non-intuitive design and the optimization of highly multimode nanoresonators.
Based on the impressive achievements of this new branch of nonlinear optics, it is reasonable to expect that a new class of nonlinear photonic meta-devices will arise in the coming years, for large-scale switching. velocity, entangled photon sources, supercontinuum generation and nonlinear imaging.