Dipolar and quadrupolar plasmon LASER modes for core-shell

International audienceSpasers are based on a combination of the localized plasmon field properties of the metallic material with an amplification medium [1]. The optical properties of these compound systems are based on a compensation of the metallic losses trough the optical gain of the amplification medium. Recently it was demonstrated experimentally that coating a gold nanoparticle with a silica shell containing Oregon Green 488 allows to overcome the loss-of-surface plasmon resonance [2]. One of the consequences of the compensation of the losses is an extremely large enhancement of the local field amplitudes (giant resonances). Rising the gain leads to a singularity stricking the transition between amplified spontaneous emission (SE) to SPASING regime [3,4,5]. Based on Mie Theory, the optical properties of the core-shell systems are determined by the scattering coefficients. The poles of the Mie coefficients give the critical resonances frequencies (!n ), and the critical value of gain ! ("n ) giving the SPASER threshold. Based on the fact that dipolar and quadrupolar modes for spherical nanoparticle own high Purcell factors [6], we analyze the critical gain values of these modes for two configurations: the metallic core-gain medium shell nanoparticle and metallic shell-gain medium core nanoparticle. Finally, we discuss the applications