High-Q Surface Lattice Resonances

Significance High-Q resonances that confine the light energy at subwavelength scales have applications in various fields such as micro/nano-lasers, fluorescence enhancement, and optical sensing. Extreme light localization has been realized by surface plasmons squeezed in plasmonic nanogaps, whereas there is intrinsic energy dissipation by electron oscillations on metal surfaces. In contrast, high-index dielectric nanostructures supporting Mie-type electric and magnetic resonances exhibit low optical dissipation but only moderate field confinement. When plasmonic or high-index dielectric nanoparticles are arranged into periodic arrays, diffractive coupling in the plane of periodic arrays may occur. This can suppress the radiative damping of individual nanoparticles, and produce surface lattice resonance (SLR) modes with significantly higher (vertical bar E vertical bar(2)/vertical bar E-0 vertical bar(2)>10(3)) field enhancements and much higher quality factors compared with isolated nanoparticles. The last two decades have seen significant progress in SLRs supported by metallic and high-index dielectric nanoparticle arrays under normal incident excitation. However, due to limitations in the involved materials and available nanofabrication methods, there is still a series of challenges in achieving a high Q-factor in the visible regime, especially in asymmetric refractive-index environments. Thus, it is necessary to summarize the existing studies to guide the future development of this field more rationally. Progress We first introduce the basic properties of SLRs in metallic and high-index dielectric nanoparticle arrays under normal incident excitation. The periodic lattices are usually generated by various top-down lithography methods. The difficulty in experimentally achieving SLRs with a high Q-factor from noble metal nanoparticle arrays is that the precise fabrication of defect-free nanoparticle arrays is hard. One strategy to overcome optical dissipation and reduce the linewidth of SLRs is to shrink the particle size relative to the lattice spacing. Reshef et al. at the University of Ottawa reported a Q-factor of 2340 in the telecommunication C band, and this is the ever reported highest value (Fig. 2). Another strategy is to make the particles smooth and uniform. Odom et al. at Northwestern University reported that thermal annealing can improve the uniformity, surface roughness, and crystallinity of metal nanoparticles produced by physical vapor deposition methods, which can lead to SLRs with dramatically improved Q factors (Fig. 3). Nie et al. from Fudan University proposed a method to produce metal nanoparticle arrays by combining solvent-assisted soft lithography and wet chemical with annealing processes, and thus a metal deposition process in a vacuum is not required (Fig. 4). Furthermore, SLRs can also be supported by arrays composed of complex basis or localized surface plasmons with multipolar characteristics, and these arrays show much richer optical responses compared with arrays with only one particle in a unit cell (Figs. 5 and 6). Arrays of high-index dielectric nanoparticles can support SLRs with characteristics of magnetic dipole (MD) besides electric dipole (ED), and both types of SLRs can be tuned independently (Fig. 7). By choosing lattice periods independently in each mutually perpendicular direction, Babicheva et al. from Georgia State University found that it is possible to make the ED-SLR and MD-SLR overlapped in a certain spectral range, which leads to the resonant suppression of the backward scattering (lattice Kerker effect). Subsequently, we summarize the progress in achieving high-Q SLRs based on mirror-backed high-aspect-ratio dielectric nanopillar arrays in asymmetric refractive-index environments (Fig. 9). In this hybrid system, dielectric nanopillars are arranged periodically on an optically thick metal film, which blocks the light transmission completely. Therefore, the issue of a symmetric dielectric environment between the substrate and the upper cladding does not exist, in contrast to the requirement of a symmetric environment for realizing sharp lattice resonances in all-plasmonic or all-dielectric systems. Meanwhile, the electric field enhancements are comparable to lattice plasmon modes from arrays of noble metal nanoparticles, but with strongly reduced plasmonic dissipation, since the enhanced fields are away from the metal surface. The narrow linewidth resonances can be tuned over a wide wavelength range from ultraviolet to mid-infrared by simply scaling the dielectric lattices and combining them with appropriate highly reflective metals. Additionally, numerical simulations show that it is possible to achieve a Q-factor of tens of thousands on this hybrid platform (Fig. 10). Conclusions and Prospects SLRs arise from the diffractive coupling in periodic arrays, which can theoretically achieve a high Q-factor and greatly enhance the interactions between light and matter in the background media. This prominence has brought about the development of potentially practical devices for optoelectronics, biosensing, and other applications, using common materials such as noble metals and transparent dielectrics. Nanoparticle arrays of other functional materials like magnetic metals and newly emerging materials such as two-dimensional layered materials still need new design principles to mitigate their intrinsic optical dissipation to achieve high-quality surface lattice resonances with fascinating properties.