Resonant Coupling and Gain Singularities in Metal/Dielectric Multishells: Quasi-Static Versus T‑Matrix Calculations Luigia Pezzi, †,∥ Maria Antonia Iatì,* ,‡,∥ Rosalba Saija, § Antonio De Luca,* ,† and Onofrio M. Maragò ‡ † Department of Physics and CNRNanotec, University of Calabria, 87036 Rende, Italy ‡ CNR-IPCF, Istituto per i Processi Chimico-Fisici, I-98158 Messina, Italy § Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Universitá di Messina, I-98166 Messina, Italy ABSTRACT: We investigate the resonant gain response in doped multishell hybrid nanoparticles made of concentric and alternated doped dielectric and metal shells. In particular, we compare the enhanced extinction properties calculated in a quasi-static approximation with accurate light scattering calculations in the T-matrix formalism. We show that, even for small hybrid particles, a difference in the calculated optoplasmonic mode yields a dramatic change in the resonant coupling with the doped molecular system. Thus, although a simple dipole approach gives a fast qualitative view of the multishell gain-assisted response, a complete light scattering framework is crucial for a quantitative investigation of these hybrid nanosystems. ■ INTRODUCTION The optical collective excitation of electrons on the surface of metallic nanostructures results in the well-known surface plasmon resonances 1,2 (SPRs). One of the most interesting properties of SPRs is their associated near-field enhancement, accompanied by a strong confinement which overcomes the diffraction limit of conventional optics and exhibits great potential in numerous areas. 3 The applications of hybrid or composite nanostructures, which combine the properties of (different) metallic and dielectric materials, span from fundamental aspects such as quantum plasmonics 4−6 and enhanced optical forces 7−9 to the realization of nanolasers. 10 In particular, in nanomedicine, core/shell nanoparticles (NPs) are greatly used for controlled drug delivery, 11,12 bioimaging, 13,14 cell labeling, 14,15 biosensing, 13,14 and in tissue engineering applications. 16 These resonances can be represented as lossy cavity modes determined by the coupling between nanostructures and their environment, exhibiting a high sensitivity to their dielectric properties. 17−20 Recently, increased interest has been man- ifested toward experimental and theoretical studies of particular plasmonic nanostructures, called “nanomatryosh- kas” 21,22 (NMs), which consist of spherical metallic cores surrounded by concentric metallic shells separated by dielectric layers. Gold NPs 23−28 and, in particular, NMs, are efficient near-infrared photothermal transducers that can be used for cancer treatment. 21 The plasmonic response of these NPs can be easily tuned by controlling the thickness of the dielectric spacers (junction nanogaps). This phenomenon has been well explained by classical electromagnetic theories, which predict strong electric field enhancements. 17−20 However, when these junction gaps reach subnanometer separation distances, recent studies have shown that quantum mechanical effects may play an increasingly important role and lead to a reduction in the electric field enhancements. 29 By adding gain materials to the dielectric layers (e.g., fluorescent molecules), the interaction with the plasmonic field modifies the resonance peak 17 and, consequently, the fluorescence behavior. Since Purcell’s work, 30 it is known that spontaneous emission could be modified by the resonant coupling to the external electro- magnetic environment. Recent works have shown how the plasmonic resonant field around the NPs is able to dramatically modify the spontaneous emission of nearby fluorescent molecules. 31 Enhancement fluorescence, for example, is of great interest in molecular fluorescence-based measurements and devices in fields such as chemistry, molecular biology, materials science, nanophotonics, and nanomedicine. 32 In particular, NMs can selectively provide either a strong enhancement or a quenching of the spontaneous emission of fluorophores dispersed within their internal dielectric layers. This behavior can be understood by taking into account the near-field enhancement induced by the Fano resonance 33 of the NM, 22,32 which is responsible for the enhanced absorption of the fluorophores incorporated into the nanocomplex. A Fano resonance 33 is a type of resonant scattering effect that gives rise to an asymmetric lineshape, called Fano lineshape. This can be seen as a quantum interference between a continuum or quasi-continuum background and a resonant scattering process 32 that can enhance or deplete light scattering depending on the excitation wavelength. The effect can be explained in a classical perspective as a system of externally driven coupled harmonic oscillators. 34,35 In the framework of NMs, a more interesting structure is a metal/dielectric onion- like NP, obtained from alternating metal and doped dielectric Received: August 6, 2019 Revised: November 11, 2019 Published: November 12, 2019 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C 2019, 123, 29291-29297 © 2019 American Chemical Society 29291 DOI: 10.1021/acs.jpcc.9b07489 J. Phys. Chem. C 2019, 123, 29291−29297 Downloaded via UNIV OF TECHNOLOGY SYDNEY on January 20, 2020 at 12:32:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.