Electric field operator of an ellipsoidal metal nanoparticle

Question

I learnt from a publication (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.105.263601) that the positive frequency component of a spherical metal nanoparticle can be written as $\hat{E}^+_m = i\mathcal{E}\hat{a}$ where $\hat{a}$ is the annihilation operator of the plasmon mode of the metal nanoparticle. I need to know if this form is valid for an ellipsoidal metal nanoparticle. Any insight is highly appreciated