Being able to confine and localize light in small volumes is of paramount importance in several scenarios, e.g., for sensing, data storage and processing. In finite-size open systems, however, any optical state is known to gradually lose its energy by coupling with radiation modes in the surrounding environment, resulting in finite oscillation lifetime. For this reason, light confinement is conventionally achieved by suppressing these radiation channels, “closing” the source region with reflectors or photonic band-gap materials. It has recently been pointed out that ideal optical bound states with infinite lifetime may also interestingly exist within the continuum of radiation modes in open unbounded 2D structures (photonic crystal slabs) [C.W. Hsu et al. Nature 499, 188 (2013)], in analogy with so-called “embedded eigenstates” in quantum systems [F.Capasso et al. Nature 358, 565 (1992)]. These setups however require infinitely large apertures. In recent works [1,2], we have shown that ideal light confinement can be surprisingly achieved also in finite-size three-dimensional open structures, even in the presence of compatible radiation channels.
Notably, we have theoretically demonstrated ideal light trapping with infinite lifetime in open metallodielectric nanocavities in the limit of vanishing material loss. It was previously shown that composite multi-layered nanoparticles may exhibit Fano scattering resonances, arising from the interference of different plasmon modes [F.Monticone et al. Phys. Rev. Lett. 110, 113901 (2013)]. Interestingly, we observed that, by varying the composition of plasmonic and dielectric materials, the resonance lifetime of these resonances can diverge at specific singular frequencies (a), as the coupling to free-space radiation is suppressed. This feature represents the fingerprint of an optical bound state with zero radiation loss – and therefore infinite lifetime – remarkably realized in an open system without altering the photonic density of states of the surrounding environment. Our investigations shed light on the generation and dynamics of these embedded scattering eigenstates existing within the radiation continuum (b). This phenomenon may lead to extreme light localization and enhancement, as the impinging energy is trapped in a self-sustained power flow within the open cavity (c-d). These findings demonstrate a fundamental mechanism for light confinement in open systems, enabled by plasmonic materials, with exciting possibilities for enhanced nonlinearities, thermal ablation, nanolasing, data storage and sensing.
(a) Scattering cross section for the composite nanosphere in the inset, as a function of wavelength and aspect ratio. Numbers denote disappearing Fano features, as their lifetime diverges. (b) Evolution of the complex eigenfrequency of the nanoparticle eigenmode 4, for different aspect ratios (insets: corresponding scattering spectra). An ideally bound state arises when its eigenfrequency becomes real. (c) Power distribution under plane wave illumination, at the embedded eigenstate frequency (right) and slightly off-resonance (left); (d) power flow distribution. From Refs. [1,2].
 F. Monticone, and A. Alù, “Trapping Light in Plain Sight: Embedded Eigenstates in Open 3D Nanostructures,” Forum for Electromagnetic Research Methods and Application Technologies (FERMAT), Vol. 6, No. 1, November 3, 2014. (web)
 F. Monticone, and A. Alù, “Embedded Photonic Eigenvalues in 3D Nanostructures,” Physical Review Letters, Vol. 112, No. 21, 213903 (5 pages), May 29, 2014. (web) [This paper has been selected as PRL Editor’s Suggestion].
 F. Monticone,and A. Alù, “Scattering at the Extreme with Metamaterials and Plasmonics,” in A Handbook of Metamaterials and Nanophotonics, S. Maier, K. Shamonina, S. Guenneau, O. Hess, J. Aizpurua, eds., World Scientific, in press.
 F. Monticone, and A. Alù, “Leaky-Wave Theory, Techniques and Applications: From Microwaves to Visible Frequencies,” Proceedings of the IEEE, Vol. 103, No. 5, pp. 793-821, May 26, 2015, (invited paper). (web) [The paper has been featured on the cover. A prolog by J. Esch, introducing our paper, has also appeared on the same issue].