image: An array of several hundred chair-shaped nanostructures (on the right in an electron-microscope picture) is able to halve the wavelength of an incident "red" beam and focus the generated "blue" beam at a desired distance. view more
Credit: OEA
A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2022.210093 consider the present and future of nonlinear optical metasurfaces.
One of the main objectives of optics is the control of light propagation and confinement. Progress in optics historically started with the development of bulky lenses and mirrors, then prisms and gratings, and so on. The improvement of these devices slew down as the diffraction limit was approached. Nanophotonics aims at manipulating electromagnetic waves at sub-wavelength scale to go beyond this limit. The recent evolution of fabrication technologies, numerical tools and theoretical models opened the way to novel devices with unprecedented performances.
Optical metasurfaces are arrays of optical antennas, with sub-wavelength size and separation. They represent an original concept of flat optics with no classical analogs. They allow for the ultimate miniaturization of optical components, as well as the enabler of new functionalities not possible to date. In the past two decades, the optical properties of metasurfaces have been intensely studied in the linear regime, with either metallic or amorphous dielectric nanostructures.
Recently, nonlinear flat optics has gained increasing attention, with frequency conversion effects observed firstly in the hot spots associated to localized plasmon resonances in metal nanoantennas and then in association to Mie-type multipolar resonances in dielectric nanostructures. In this transition to the nano-scale, the role of phase matching has been replaced by that of near-field resonances occurring in open non-Hermitian nanostructures.
In the new field of nonlinear metasurfaces, for which the name of nonlinear meta-optics has been proposed, dielectric implementations have provided the highest nonlinear generation efficiencies: firstly with third harmonic generation in amorphous or silicon-on-insulator platforms, and then with second harmonic generation and spontaneous parametric down-conversion in non-centrosymmetric materials like III-V semiconductors and lithium niobate. More recently, the scientific community became also interested in the wavefront shaping of the harmonic fields, ranging from simple meta-gratings and meta-lenses to nonlinear generation of complex meta-holograms and special beams.
The authors of this article review the recent progress in nonlinear optics with dielectric metasurfaces, focusing on the paradigmatic effect of second harmonic generation. They discuss the most used technological platforms which underpinned such advances and analyze different control approaches. Their paper begins with an introduction on the topic of nonlinear generation in sub-wavelength “Mie” resonators, pointing out the main figures of merit for a high efficiency in non-Hermitian systems. Then, they overview the main approaches adopted in recent years to control or boost harmonic generation in metasurfaces. They finally compare their performances with other well-established technologies, illustrating the current state-of-the-art and figure out a few scenarios in which these devices might soon offer unprecedented opportunities. In their conclusion, two possible outlooks emerge for the rapidly emerging domain of dielectric nonlinear metasurfaces.
On the one hand, resorting to coupled nanoantennas and collective resonances seems the wisest strategy to maximize nonlinear generation. However, also the metasurfaces exhibiting the largest quality factors are orders of magnitude less efficient than the other platforms. This outcome stems from the fact that to date most of the studies adopting this approach, focused on the creation of a single high-Q resonance around FF. Instead, is has to be expected that such gap with the other technologies may be filled with a careful design providing a good balance between free-space coupling and mode quality factors, while implementing a doubly resonant condition and optimizing the nonlinear overlap integral.
On the other hand, low-Q antennas represent a true change of paradigm with respect to both guided structures and photonic crystals. Their presently lower nonlinear generation efficiency is largely counterbalanced by intriguing possibilities ranging from the dynamic tunability of individual
meta-atom emission, to pulse shaping, broadband parametric devices, nonlinear imaging, wavefront shaping and meta-holography. Their rapid development is currently underpinned by an ongoing progress in nanofabrication, new promising nonlinear materials like TMDCs, and both analytical and numerical methods to model nonlinear generation in leaky cavities. The improvement of such mathematical tools seems especially important for non-intuitive design and optimization of highly multi-mode nanoresonators.
Based on the impressive achievements of this new branch of nonlinear optics, it is reasonable to expect a new class of nonlinear photonic meta-devices to arise in the forthcoming years, for high-speed switching, entangled photon sources, supercontinuum generation and nonlinear imaging.
Article reference: Gigli C, Leo G. All-dielectric χ(2) metasurfaces: recent progress. Opto-Electron Adv 5, 210093 (2022) . doi: 10.29026/oea.2022.210093
Keywords: nonlinear meta-optics / all-dielectric metasurfaces / second harmonic generation / Mie resonators / nonlinear nanoantennas
# # # # # #
Within Université Paris Cité, France’s leading multidisciplinary university, the MPQ department aims at finding new links between fundamental aspects of the matter and quantum phenomena, in order to innovate and suggest quantum-enhanced devices for future technologies. Among its research groups, Giuseppe Leo’s group investigates the physics and the engineering of quantum and nonlinear photonic devices on chip, focusing on the design, fabrication and characterization of semiconductor waveguides, micro- and nano-cavities for parametric generation, and frequency conversion in metasurfaces. This group includes about 25 people, from permanent research staff to post-doc and PhD students, in the last years it has published an average total number of about 10 publications per year, and it presently participates in several national and EU research programs (ANR NOMOS, NANOPAIR and NANO-APPEARANCE; ASTRID SPAIN; FET-OPEN METAFAST). In both modeling and experimental work, it combines fundamental and applied research, in a framework of collaborations including among others C. De Angelis (University of Brescia), J.-M. Gérard (CEA), Ph. Lalanne (IOGS Bordeaux), D. Neshev (Australian National University), and S. Wabnitz (La Sapienza, Rome). Prof. Leo is an Optica (formerly OSA) Fellow since 2020 because of “pioneering contributions to nonlinear nanophotonics with monolithic aluminum gallium arsenide nanoantennas and metasurfaces”, and some of his former students have won several prestigious prizes in the last years.
# # # # # #
Opto-Electronic Advances (OEA) is a high-impact, open access, peer reviewed monthly SCI journal with an impact factor of 8.933 (Journal Citation Reports for IF2021). Since its launch in March 2018, OEA has been indexed in SCI, EI, DOAJ, Scopus, CA and ICI databases over the time and expanded its Editorial Board to 36 members from 17 countries and regions (average h-index 49).
The journal is published by The Institute of Optics and Electronics, Chinese Academy of Sciences, aiming at providing a platform for researchers, academicians, professionals, practitioners, and students to impart and share knowledge in the form of high quality empirical and theoretical research papers covering the topics of optics, photonics and optoelectronics.
# # # # # #
More information: http://www.oejournal.org/oea
Editorial Board: http://www.oejournal.org/oea/editorialboard/list
All issues available in the online archive (http://www.oejournal.org/oea/archive).
Submissions to OEA may be made using ScholarOne (https://mc03.manuscriptcentral.com/oea).
ISSN: 2096-4579
CN: 51-1781/TN
Contact Us: oea@ioe.ac.cn
Twitter: @OptoElectronAdv (https://twitter.com/OptoElectronAdv?lang=en)
WeChat: OE_Journal
# # # # # #