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Lattice resonances are collective modes that appear in two-dimensional periodic arrays of metallic nanostructures. These resonances are characterized by very strong optical responses with a high quality factor. Among the different types of lattice resonances, out-of-plane resonances, for which the nanostructures are polarized in a direction perpendicular to the nanoparticle array, are particularly interesting. Thanks to their geometrical configuration, these modes exhibit minimal radiative losses, allowing very high quality factors and near-field strengths to be obtained. However, until now their use has been very difficult and has only been achieved with oblique incidence, which limits their applicability in integrated devices.

In this study, led by Juan José Álvarez Serrano and Juan Ramón Deop Ruano under the direction of Dr. A. Manjavacas, a method for the excitation of out-of-plane lattice resonances under normal incidence is proposed for the first time. The method consists of taking advantage of the coupling between the electrical and magnetic response of nanostructures, a phenomenon that has been underestimated until now in the study of arrays composed of metallic nanostructures.

Schematic of the system under consideration, consisting of a square array of period a = 800 nm, with two silver nanospheres per unit cell of diameter D = 160 nm.

The system they have proposed consists of a square array with two silver nanospheres per unit cell. Using the coupled dipole model developed in-house, the researchers have shown that electric-magnetic coupling provides a general mechanism for the excitation of out-of-plane lattice resonances under normal incidence conditions. In addition, they have characterized the properties of these modes, demonstrating that extremely high quality factors and near-field strengths are possible. This discovery paves the way for the exploitation of the special properties of out-of-plane lattice resonances in a wide range of applications such as ultrasensitive biosensors and laser emitters at the nanoscale.

The article also includes an analysis of the results in a nanodisk array with the inclusion of a substrate and a coating, which demonstrates the applicability of the results in a realistic system. This scientific advance represents an important step forward in the understanding of out-of-plane lattice resonances and their efficient excitation. The study provides a solid foundation for future research and opens new perspectives in the design and development of state-of-the-art optical devices.

Communication IO-CSIC
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