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Plasmonic quasi-periodic structures are well-known to exhibit several surprising phenomena with respect to their periodic counterparts, due to their long-range order and higher rotational symmetry. Thanks to their specific geometrical arrangement, plasmonic quasi-crystals offer unique possibilities in tailoring the coupling and propagation of surface plasmons through their lattice, a scenario in which a plethora of fascinating phenomena can take place. In this paper we investigate the extraordinary transmission phenomenon occurring in specifically patterned Thue–Morse nanocavities, demonstrating noticeable enhanced transmission, directly revealed by near-field optical experiments, performed by means of a scanning near-field optical microscope (SNOM). SNOM further provides an intuitive picture of confined plasmon modes inside the nanocavities and confirms that localization of plasmon modes is based on size and depth of nanocavities, while cross talk between close cavities viapropagating plasmons holds the polarization response of patterned quasi-crystals. Our performed numerical simulations are in good agreement with the experimental results. Thus, the control on cavity size and incident polarization can be used to alter the intensity and spatial properties of confined cavity modes in such structures, which can be exploited in order to design a plasmonic device with customized optical properties and desired functionalities, to be used for several applications in quantum plasmonics.

We present a detailed study on the resonant gain (RG) phenomena occurring in two nanostructures, in which the presence of dielectric singularities is used to reach a huge amplification of the emitted photons resonantly interacting with the system. The presence of gain molecules in the considered nanoresonator systems makes it possible to obtain optical features that are able to unlock several applications. Two noticeable cases have been investigated: a 1D nanoresonator based on hyperbolic metamaterials and a 3D metal/dielectric spherical multishell. The former has been designed in the framework of the effective medium theory, in order to behave as an epsilon-near-zero-and-polemetamaterial, showing extraordinary light confinement and collimation. Such a peculiarity represents the key to lead to a RG behavior, a condition in which the system is demonstrated to behave as a self-amplifying perfect lens. Very high enhancement and spectral sharpness of 1 nm of the emitted light are demonstrated by means of a transfer matrix method simulation. The latter system consists of a metal/doped-dielectric multishell. A dedicated theoretical approach has been set up to finely engineer its doubly tunable resonant nature. The RG condition has been demonstrated also in this case. Finite element method-based simulations, together with an analytical model, clarify the electric field distribution inside the multishell and suggest the opportunity to use this device as a self-enhanced loss compensatedmultishell, being a favorable scenario for low-threshold SPASER action. Counterintuitively, exceeding the resonant gain amount of molecules in both systems causes a significant drop in the amplitude of the resonance.