When nanoparticles are arranged into a periodic lattice, their individual optical properties can profoundly change in comparison to those of a single nanoparticle. In particular, collective behavior can emerge due to the existence of grazing diffracted orders (the so-called Rayleigh anomalies). If the period of the array is commensurate with the wavelength of light at which the particle resonates, sharp and intense additional resonances can appear. These lattice resonances stem from a Fano-type interference effect between the sharp diffraction anomaly and the broad resonance associated with each nanoparticle (Mie resonances of dielectric or metal nanoparticle, and often called localized surface plasmon resonances in the case of metal). The spectral position and the linewidth of this surface lattice resonance (SLR) can be controlled through the period of the array [1].

The broad nanoparticle resonances involved in Fano-type interference resulting in SLR can have an origin of various multipoles. While plasmonic nanoparticles with electric dipole resonance have been studied in earlier works, dielectric nanoparticles with magnetic dipole resonances have been shown to support strong lattice effect in subsequent research [2]. Periodic lattices of higher order multipoles such as electric and magnetic quadrupoles can also SLR and even exhibit cross-multipole coupling [3]. The high quality factor and the relatively “delocalized” near-field distribution associated with SLRs make them very appealing for enhancement of various phenomena, including luminescence emission, light concentration, nonlinear effects, and for applications in metasurfaces, beaming, and nanolasers, to name only a few (see for instance [4,5] for recent reviews).

This feature issue highlights recent research on SLRs, and more generally, collective resonances in optical arrays and metasurfaces. It contains three invited papers (including two reviews) and 14 regular research articles. The two reviews offer a perspective on some of the most exciting aspects of this field of research. First, Ramezani et al. [6] wrote a comprehensive perspective article on the strong coupling between molecules and SLRs, including effects such as plasmon-exciton-polariton condensation and lasing. The paper also contains a very clear explanation of the formation of SLRs in arrays of nanoparticles that newcomers might use as a comprehensive introduction to the field. The second review article, written by Maccaferri [7], describes the link between these collective resonances and magnetism, and how they can be merged to create magneto-optic or opto-magnetic metamaterials.

The research articles in this feature issue cover three main broad topics. The first topic is the understanding of the optical properties of arrays of nanoparticles, notably to the role of polarization. Walmsness et al. [8] report on the study of arrays of elliptical metallic nanoparticles using Mueller matrix ellipsometry. Muravitskaya et al. [9] study rectangular arrays of aluminum nanoparticles and analyze collective resonances excited in orthogonal and parallel directions with respect to incidence light polarization. It is demonstrated that if the substrate is present, scattered fields from nanoparticles can interact with each other in directions both parallel and orthogonal to the incident field. Ragheb et al. [10] study the behavior of surface lattice resonance as a function of the period, comparing the near- and far-field response. Dai et al. [11] suggest an improved vector diffraction theory and utilize it for calculating the distribution of circularly-polarized light transmitted through a metasurface. Interestingly, collective resonances can also appear in disordered systems. Zakomirnyi et al. [12] theoretically study collective lattice resonances in disordered two-dimensional arrays of spherical silicon nanoparticles. Three different ways of introducing disorder in the lattice are considered, namely positional disorder, size disorder, and random removal of nanoparticles. The results clearly show that the effect of disorder on the collective resonances strongly depends on the disorder type. Finally, Xiao et al. [13] numerically study a metal-graphene hybrid metasurface with the aim to utilize it for active tailoring of slow light in the terahertz regime. They consider four mirror-symmetrically arranged aluminum split ring resonators in the unit cell of the metasurface and analyze coupling of bright and dark modes and strong electromagnetically induced transparency resonances in the structure.

Another important field of application of nanoparticles, their arrays, and nanostructures is resonant enhancement and coupling of the optical modes with emitters, either organic or inorganic. This research direction is considered promising for enhancement of luminescence or applications in sensing. Gollmer et al. [14] experimentally demonstrate that the fluorescence emission from a thin film of small organic molecules can be polarized and their emission spectrum shaped using a periodic array of gold nanostrips. Peters et al. [15] analyze chemiluminescence of rhodamine 6G dye in a series of R6G:DNPO:PMMA films deposited onto different metallic and dielectric substrates. It is shown that chemiluminescence kinetics on top of gold and silver films covered by insulating magnesium fluoride layer exhibits longer decay times than those on top of a dielectric substrate. Prayakarao et al. [16] experimentally investigate the non-resonant enhancement of spontaneous emission of HITC laser dye in metal-insulator-metal waveguides. The authors find that the emission intensity increases when the cavity size is reduced. However, in the cavities that are too small to support a fundamental or any higher order resonance, the emission decay rate increase by nearly an order of magnitude. Kawachiya et al. [17] explore the effect of layer thickness on the photoluminescence decay rate of an emitter layer on an Al nanocylinder array. An ${\mathrm{Eu}}^{3+}$ complex was selected as an emitter with high quantum yield. The authors find that the layer thickness critically affects the energy transfer from the emitters to the metal or higher order, dark plasmonic modes of the system acting as nonradiative decay paths. Furthermore, they suggest that there is an optimal thickness, where this energy transfer is suppressed, thus resulting in a maximized photoluminescence yield. Murai et al. [18] use the photoluminescence signal from a polymer known as the chameleon complex to monitor the local temperature inside an array of aluminum nanocylinders. While all the previous works focused on the spontaneous emission, SLRs can also be used to enhanced stimulated emission and hence lasing. In their invited article, Li et al. [19] compare plasmon lasing in gold and aluminum arrays. It is shown that aluminum—a metal of interest for plasmonics, due to its widespread availability and low cost—exhibits similar to the gold lasing threshold and ultrafast dynamics.

The third topic explored in this feature issue is linked to the enhancement of non-linear effects. Three articles report on the increase of second- and third-harmonic generation using collective resonances. Gigli et al. [20] experimentally demonstrate polarization-controlled optical second harmonic generation from AlGaAs-on-insulator metasurfaces. Two different methods are shown to allow for effective second harmonic polarization control, namely the orientation of the employed elliptical meta-atoms and the polarization direction of the incident fundamental frequency laser beam with respect to the crystal axis. Furthermore, the polarization-engineered second harmonic field emitted in the zeroth-diffraction order emerges within a small solid angle around the surface normal. Doron et al. [21] make use of collective interactions in plasmonic metasurfaces to manipulate the interplay between direct and cascaded third-harmonic generation. They find that the direct contribution is mainly enhanced by the local plasmonic resonances, while the enhancement of the cascaded contribution can be influenced by nonlocal interactions in a suitable metasurface geometry. In addition, they show that the relative phase between the direct and cascaded contributions allows tailoring their interference. Finally, Huttunen et al. [22] propose to exploit the high quality factors associated with SLRs to enhance the nonlinear frequency conversion. Providing the array is properly designed, the authors predict huge enhancements (over one million-fold) of the second-harmonic intensity. This theoretical prediction needs to be experimentally validated, but it is an important advance toward efficient non-linear metasurfaces.

We hope that the overview of the current research in collective resonances in optical metasurfaces presented in this feature issue will foster new directions of research in this fascinating field.