Lattice resonances, the collective modes supported by periodic arrays of metallic nanoparticles, give rise to very strong and spectrally narrow optical responses. Thanks to these properties, which emerge from the coherent multiple scattering enabled by the periodic ordering of the array, lattice resonances are used in a variety of applications such as nanoscale lasing and biosensing. Here, we investigate the lattice resonances supported by bipartite nanoparticle arrays. We find that, depending on the relative position of the two particles within the unit cell, these arrays can support lattice resonances with a super-or subradiant character. While the former result in large values of reflectance with broad lineshapes due to the increased radiative losses, the latter give rise to very small linewidths and maximum absorbance, consistent with a reduction of the radiative losses. Furthermore, by analyzing the response of arrays with finite dimensions, we demonstrate that the subradiant lattice resonances of bipartite arrays require a much smaller number of elements to reach a given quality factor than the lattice resonances of arrays with single-particle unit cells. The results of this work, in addition to advancing our knowledge of the optical response of periodic arrays of nanostructures, provide an efficient approach to obtain narrow lattice resonances that are robust to fabrication imperfections.

Periodic arrays are an exceptionally interesting arrangement for metallic nanostructures because of their ability to support collective lattice resonances. These modes, which arise from the coherent multiple scattering enabled by the lattice periodicity, give rise to very strong and spectrally narrow optical responses. Here, we investigate the enhancement of the near-field produced by the lattice resonances of arrays of metallic nanoparticles when illuminated with a plane wave. We find that, for infinite arrays, this enhancement can be made arbitrarily large by appropriately designing the geometrical characteristics of the array. On the other hand, in the case of finite arrays, the near-field enhancement is limited by the number of elements of the array that interact coherently. Furthermore, we show that, as the near-field enhancement increases, the length scale over which it extends above and below the array becomes larger and its spectral linewidth narrows. We also analyze the impact that material losses have on these behaviors. As a direct application of our results, we investigate the interaction between a nanoparticle array and a dielectric slab placed a certain distance above it and show that the extraordinary near-field enhancement produced by the lattice resonance can lead to very strong interactions, even at significantly large separations. This work provides a detailed characterization of the limits of the near-field produced by lattice resonances and, therefore, advances our knowledge of the optical response of periodic arrays of nanostructures, which can be used to design and develop applications exploiting the extraordinary properties of these systems.

Arrays of nanostructures have emerged as exceptional tools for the manipulation and control of light. Oftentimes, despite the fact that real implementations of nanostructure arrays must be finite, these systems are modeled as perfectly periodic, and therefore infinite. Here, we investigate the legitimacy of this approximation by studying the evolution of the optical response of finite arrays of nanostructures as their number of elements is increased. We find that the number of elements necessary to reach the infinite array limit is determined by the strength of the coupling between them, and that, even when that limit is reached, the individual responses of the elements may still display significant variations. In addition, we show that, when retardation is negligible, the resonance frequency for the infinite array is always redshifted compared to the single particle. However, in the opposite situation, there could be either a blue-or a redshift. We also study the effects of inhomogeneity in size and position of the elements on the optical response of the array. This work advances the understanding of the behavior of finite and infinite arrays of nanostructures, and therefore provides guidance to design applications that utilize these systems.

The ability of graphene nanostructures to support strong plasmonic resonances in the infrared part of the spectrum makes them an ideal platform for plasmon-enhanced spectroscopy techniques. Here we propose to exploit the exceptional tunability of graphene plasmons to perform infrared detection of molecules with subwavelength spatial resolution. To that end, we investigate the optical response of finite arrays of graphene nanodisks that are divided into a number of identical subarrays, or pixels, each of them with a uniform level of doping. Using realistic conditions, we show that, by adjusting individually the doping level of each of these pixels, it is possible to bring them sequentially into resonance with the vibrational spectrum of the analyte. This enables the identification of the analyte and the simultaneous detection of its spatial location with a resolution determined by the size of the pixels. Our work brings new possibilities to plasmon-enhanced infrared sensing by combining the already demonstrated sensing abilities of graphene nanostructures with subwavelength spatial resolution. This could be exploited to develop actively tunable substrates for multiplexed sensing, which could be used to analyze the chemical composition of complex biological systems and to follow their temporal evolution with spatial resolution.

Periodic arrays of nanoparticles are capable of supporting lattice resonances, collective modes arising from the coherent interaction of the particles in the array. These resonances, whose spectral position is determined by the array periodicity, are spectrally narrow and lead to strong optical responses, making them useful for a wide range of applications, from nanoscale light sources to ultrasensitive biosensors. Here, we report that, by removing particles from an array in a periodic fashion, it is possible to induce lattice resonances at wavelengths commensurate with the periodicity of these vacancies, which would otherwise not be present in the system. Using a coupled dipole approach, we perform a comprehensive analysis of how the properties of these vacancy-induced lattice resonances depend on the array periodicity, the particle size, and the number of vacancies per unit of area. Furthermore, we find that these lattice resonances have a subradiant character and originate from the symmetry breaking introduced in the unit cell by the presence of the vacancies. Finally, we investigate a potential implementation of an array with vacancies made of nanocylinders embedded in a homogeneous dielectric environment. The results of this work serve to advance our understanding of lattice resonances and provide an alternative method for controlling the optical response of periodic arrays of nanostructures.