Fanoshells: Nanoparticles with Built-in Fano Resonances Shaunak Mukherjee, †,‡ Heidar Sobhani, ‡,| J. Britt Lassiter, ‡,§ Rizia Bardhan, †,‡ Peter Nordlander* ,‡,§,| and Naomi J. Halas* ,†,‡,§,| † Department of Chemistry, ‡ Laboratory for Nanophotonics, § Department of Physics and Astronomy, and | Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005 ABSTRACT A nanoparticle consisting of a dielectric (SiO 2 ) and metallic (Au) shell layer surrounding a solid Au nanoparticle core can be designed with its superradiant and subradiant plasmon modes overlapping in energy, resulting in a Fano resonance in its optical response. Synthesis of this nanoparticle around an asymmetric core yields a structure that possesses additional Fano resonances as revealed by single particle dark field microspectroscopy. A mass-and-spring coupled oscillator model provides an excellent description of the plasmon interactions and resultant optical response of this nanoparticle. KEYWORDS Plasmon, Fano resonance, nanoshell, dark field, harmonic oscillator, coherent phenomena N oble metal nanoparticles possess a range of inter- esting optical properties due to localized electro- magnetic resonances, known as surface plasmons. The dependence of nanoparticle plasmon resonances on geometry and local dielectric environment 1,2 has led to a variety of strategies for the systematic design of optical properties into nanoparticles and nanostructures. Applica- tions of plasmonic nanoparticles and nanostructures range from therapeutics and biomedical imaging, 3–5 surface- enhanced spectroscopies, 6–8 and ultrasensitive chemical and biological sensing 9,10 to developing optical frequency metamaterials. 11–13 Many of the optical properties of plasmonic nanoparticles result from the interaction between multiple plasmon modes of the same nanostructure. In the core-shell geometry of a nanoshell, for example, plasmon resonance frequencies are determined by the interaction between the two primitive plasmons supported by the structure, namely, the sphere and cavity plasmon modes. Symmetry breaking can en- hance the interaction between plasmon modes. 14,15 In highly symmetric nanostructures such as nanoshells, offset- ting the dielectric core with respect to the metal shell causes mixing of the bright dipole mode with higher order dark multipolar modes, so named because they do not couple directly to the far field and therefore cannot be optically excited. 14,16 Since localized plasmons behave remarkably like simple classical damped oscillators, these systems provide a unique opportunity to study, and ultimately de- sign, coherent, coupled-oscillator phenomena using plas- monic nanoparticles and complexes. While coherent effects such as subradiance and superradiance, 17 Fano reso- nances, 18 and electromagnetically induced transparency (EIT) 19 have long been of interest in atomic physics, plas- monic nanoparticles and nanostructures provide a very practical testbed where coherent effects can be designed, examined, and optimized. The ability to design nanoscale structures and complexes that support specific coherent plasmonic effects has become a topic of intense current interest. Recently, a variety of reduced-symmetry plasmonic nano- structure complexes such as nanoparticle heterodimers, 20,21 septamers, 22 and ring/disk nanocavities 23–25 with Fano resonances present in their optical response, have been reported. Each of these systems supports both broad super- radiant plasmon modes and substantially narrower subra- diant modes. The coexistence of a broad bright mode and a narrow dark mode resonant over the same range of energies can result in a coupling between these two coherent modes, producing a Fano resonance. In strongly coupled systems, the modulation depth of the asymmetric Fano line shape may give rise to plasmon-induced transparency. 26–29 This particular phenomenon is similar to EIT, observed in atomic systems. 30,31 In plasmonics, plasmon-induced transparency over a short-range of frequencies has great potential for the design of low-loss metamaterials and subwavelength wave- guides with low radiative losses. 12,32 A simple multilayered plasmonic nanoparticle consisting of an Au nanocrystalline core, a silica spacer layer, and a metallic shell, has recently been fabricated and analyzed within the plasmon hybridization picture. 33 The hybridized plasmonic response of this nanoparticle, originating from the coupling between the primitive dipolar Au sphere and shell plasmons, gives rise to three hybridized plasmon modes: in increasing energy, an antisymmetric bonding mode, a sym- metric antibonding mode, and a nonbonding mode. The lowest energy antisymmetric bonding mode is a subradiant, dark mode, where the individual dipole moments of the Au * Corresponding authors, (N.J.H.) halas@rice.edu and (P.N.) nordland@rice.edu. Received for review: 05/8/2010 Published on Web: 05/28/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 2694 DOI: 10.1021/nl1016392 | Nano Lett. 2010, 10, 2694–2701