Chiral Lattice Resonances in 2.5-Dimensional Periodic Arrays with Achiral Unit Cells

Abstract

Lattice resonances are collective electromagnetic modes supported by periodic arrays of metallic nanostructures. These excitations arise from the coherent multiple scattering between the elements of the array and, thanks to their collective origin, produce very strong and spectrally narrow optical responses. In recent years, there has been significant effort dedicated to characterizing the lattice resonances supported by arrays built from complex unit cells containing multiple nanostructures. Simultaneously, periodic arrays with chiral unit cells, made of either an individual nanostructure with a chiral morphology or a group of nanostructures placed in a chiral arrangement, have been shown to exhibit lattice resonances with different responses to right- and left-handed circularly polarized light. Motivated by this, here, we investigate the lattice resonances supported by square bipartite arrays in which the relative positions of the nanostructures can vary in all three spatial dimensions, effectively functioning as 2.5-dimensional arrays. We find that these systems can support lattice resonances with almost perfect chiral responses and very large quality factors, despite the achirality of the unit cell. Furthermore, we show that the chiral response of the lattice resonances originates from the constructive and destructive interference between the electric and magnetic dipoles induced in the two nanostructures of the unit cell. Our results serve to establish a theoretical framework to describe the optical response of 2.5-dimensional arrays and provide an approach to obtain chiral lattice resonances in periodic arrays with achiral unit cells.

Normal Incidence Excitation of Out-of-Plane Lattice Resonances in Bipartite Arrays of Metallic Nanostructures

“…40 The quality factor of the out-of-plane lattice resonance, which is analyzed in Figure 5b, has a simpler dependence on a and D. It monotonically increases with the period and decreases with the size of the particle. This behavior, which is in accordance with previous works on lattice resonances, 34,79 is associated with the increase of the collective nature of the lattice resonance. It is worth highlighting that the quality factor of the out-of-plane mode takes values in the range of 10 3 to 10 4 , reaching a value over 2.2 × 10 4 for the optimum case considered in our calculations.…”

“…We use a coupled dipole model (CDM) ,,− to study the properties of the periodic array. This method is valid provided that D is significantly smaller than both a and the wavelength of the light λ.…”

“…Lattice resonances appear at wavelengths that match the periodicity of the array − and, thanks to their collective nature, exhibit strong optical responses with lineshapes much narrower than those associated with the individual nanostructures composing the array. − In particular, arrays supporting lattice resonances can reach values of reflectance and absorbance that saturate the theoretical limits − , with quality factors well beyond one thousand. − At the same time, they produce very strong near-field enhancements, , only limited by the number of elements of the array that are coherently coupled . As a result of their exceptional properties, lattice resonances are being explored for the development of different optical systems such as color filters, , lenses, light-emitting devices, − and chiral elements, − as well as ultrasensitive biosensors, − light-to-heat transducers, , and even platforms to mediate long-range energy transfer, − strong coupling, , or to achieve Bose-Einstein condensation. , …”

Maximum Chirality Empowered by a Bound State in a Continuum in a Plasmonic Metasurface

“…In the present work, we exploit a BIC in a plasmonic nanohole array in order to enhance CD in absorption. Thus, we do not need to leverage the coupling to lattice modes, , and the present structure is expected to preserve the advantage of the smaller footprint granted by plasmonic platforms. There is room for improving the Q -factor of BIC resonances by proper optimization.…”

Chiral Optical Properties of Plasmonic Kagome Lattices

“…The in-phase oscillations of the NPs reduce radiative losses while supporting strong, localized electric fields around the NPs. − Theory has predicted that non-Bravais plasmonic lattices (i.e., lattices with nonprimitive unit cells consisting of at least two particles) can form topological edge states at different band edges as well as at high symmetry points. − Moreover, such lattice symmetries exhibit complex near-field characteristics from interactions between multiparticle unit cells. , For example, honeycomb lattices treated as 2-NP unit cells on a hexagonal lattice and square lattices with multiparticle unit cells exhibit SLR modes from the hybridization of inequivalent LSPs. In addition, the arrangement of particles within a unit cell of non-Bravais lattices can lead to polarization-dependent properties such as chiral lattice resonances in square lattices with dimer unit cells , or the lattice Kerker effect in lattices of plasmonic trimer oligomers . Expanding or shrinking the hexamer unit cells of a hexagonal lattice has also been shown to change the topological charge of the system and introduce polarization-tunable lasing .…”