Quality factor of 3D Metal-Insulator-Metal nanoresonators

We study the quality factor variation of three-dimensional Metal-Insulator-Metal nanoresonators when their volume is shrunk from the diffraction limit down to the quasi-static limit. In addition to rigorous fully-vectorial calculations, we provide a comprehensive study of the cavity mode and a closed-form expression of the quality factor Q obtained with a semi-analytical Fabry-Perot model. The latter quantitatively predicts the absorption and radiation losses of the nanoresonator over the whole length scale, and it also provides an in-depth understanding of the mode lifetime that cannot be obtained with brute-force computations. In particular, it highlights the impact of slow-wave effects on the Q-factor as the size of the resonator is decreased. The Fabry-Perot model also evidences that, unexpectedly, wave retardation effects are present in metallic nanoparticles, even for deep subwavelength dimensions in the quasi-static regime.