Experiment and Theory: No Spasing from Core-Shell Spasers Günter Kewes,1 Kathrin Höfner,2 Andreas Ott,3 Rogelio Rodriguez-Oliveros,2 Alexander Kuhlicke,1 Yan Lu,3 Matthias Ballauff,3 Kurt Busch,2, 4 and Oliver Benson1 1AG Nanooptik, Institut für Physik, Humboldt-Universität zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany 2AG Theoretische Optik & Photonik, Institut für Physik, Humboldt-Universität zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany 3Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), Hahn-Meitner-Platz 1, 14109 Berlin, Germany 4Max-Born Institut, Max-Born-Strasse 2a, 12489 Berlin, Germany In 2009 the smallest laser-device consisting of a single 14 nm gold sphere and a dye-doped coating was “demonstrated” [1] based on the proposal of the “spaser” from 2003 [2]. Here, we critically evaluate so far reported results on both an experimental and on a theoretical basis. Experiments: Core-shell spasers were chemically synthesized or metal nanoparticles were coated with dye-doped polymer films. Various excitation schemes were tested, yielding laser or laser-like signatures. All of them, however, were later identified to stem from sources other than spasing. Theory: A fully analytic model for spherical core-shell spasers was developed: Within this model, we drop the widely used quasi-static approximation of the electromagnetic field in favor of fully electromagnetic Mie theory. This allows for precise incorporation of realistic gain relaxation rates (quenching and cavity-to-gain coupling) that so far have only been estimated (and massively underestimated) in spaser theory. The model unravels multiple severe limitations (that exist all over the VIS) concerning the threshold-pump, the extreme gain-medium requirements and the unavoidable heat production (independent of excitation scheme). These limits basically result from the poor quality-factors and the surprisingly poor β-factors (ratio of gain-to-cavity coupling to the overall decay rate) in such laser architectures. Beyond pessimism, our theoretical model gives advice on how to construct ultra-small lasers with reasonable performance. Especially nano-cavities made from high index dielectrics with low losses – hosting electric and magnetic Mie resonances - , e.g., Si or GaP may be the resonator materials in future efficient nano- or micro-laser devices. Thus, our model marks to ultimate limit in laser miniaturization that may impact current aims to realize more and more communication in networks or even on-chip with small lasers of high bandwidth like VCSELs. [1] M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, “Demonstration of a spaser-based nanolaser,” Nature, vol. 460, no. 7259, pp. 1110–1112, 2009. [2] D. J. Bergman and M. I. Stockman, “Surface plasmon amplification by stimulated emission of radiation: quantum generation of coherent surface plasmons in nanosystems,” Phys. Rev. Lett., no. January, pp. 1–4, 2003. Contact info: Günter Kewes Tel.: 030 2093 4901 Humboldt Universität zu Berlin Institut für Physik, AG Nanooptik Newtonstraße 15 12489 Berlin Education/ current position: Günter Kewes studied physics at the RWTH Aachen, however changed to the HU Berlin to do his Diploma in the group of Prof. Oliver Benson, where he also works on his PhD project in the SFB 951 (B2). He is currently just about to finish his PhD thesis. Research interests: • Plasmonic and dielectric nano-antennas (e.g.., see „A fully nanoscopic dielectric laser“, arXiv:1412:4549 (2014)) • Coupling of emitters with nano-localized cavity-modes or guided modes (e.g., see ”Single Defect Centers in Diamond Nanocrystals as Quantum Probes for Plasmonic Nanostructures”, Optics Express, 19, Issue 8, (2011)) • Plasmonic responses from 2D materials (e.g., see „Evidence for graphene plasmons in the visible spectral range probed by molecules“, arXiv:1404.6518 (2014)) • Numerical Maxwell-equation solving with FEM (e.g., see “Design and numerical optimization of an easy-to-fabricate photon-to-plasmon coupler for quantum plasmonics”, APL, 102:051104 (2013).) .