We used the particle swarm optimization algorithm, an evolutionary computational technique, to design metal nanoparticle arrays that produce broadband plasmonic field enhancement over the entire visible spectral range. The resulting structures turn out to be aperiodic and feature dense Fourier spectra with many closely packed particle clusters. We conclude that broadband field-enhancement effects in nanoplasmonics can be achieved by engineering aperiodic arrays with a large number of spatial frequencies that provide the necessary interplay between long-range diffractive interactions at multiple length scales and near-field quasi-static coupling within small nanoparticle clusters.

In this paper, we develop a quasielectrostatic theory describing the coupling of plasmon modes and we discuss its implications for the analysis and the design of the scattering and absorption spectra of complex metal nanostructures. In particular, we show that the interaction of bright plasmon modes determines the onset of zeros in the scattering spectra of nanoscale coupled systems. Under well-defined conditions, these zeros give rise to asymmetric scattering line shapes similar to the spectral signatures described by Ugo Fano in the context of atomic physics. We provide rigorous conditions in which Fano-like resonances occur, and we introduce a method for the direct calculation of their spectral position. In addition, we investigate the role of dark and bright modes in the power absorption near a Fano-like resonance. Our analysis demonstrates the quasielectrostatic origin of Fano-like resonances in subwavelength plasmonic structures.

Over the last 30 years, optical biosensors based on nanostructured materials have obtained increasing interest since they allow the screening of a wide variety of biomolecules with high specificity, low limits of detection, and great sensitivity. Among them, flexible optical platforms have the advantage of adapting to non-planar surfaces, suitable for in vivo and real-time monitoring of diseases and assessment of food safety. In this review, we summarize the newest and most advanced platforms coupling optically active materials (noble metal nanoparticles) and flexible substrates giving rise to hybrid nanomaterials and/or nanocomposites, whose performances are comparable to the ones obtained with hard substrates (e.g., glass and semiconductors). We focus on localized surface plasmon resonance (LSPR)-based and surface-enhanced Raman spectroscopy (SERS)-based biosensors. We show that large-scale, cost-effective plasmonic platforms can be realized with the currently available techniques and we emphasize the open issues associated with this topic.