Three-dimensional canonical quantum plasmonics for finite media: exact solution in terms of the classical Green tensor
This article presents a comprehensive three-dimensional canonical quantization to treat quantum plasmonics for finite metallic or dielectric media of arbitrary shape. We use a microscopic model for the dissipative and dispersive medium coupled with the electromagnetic field, which is justified by the fact that if one integrates the degrees of freedom of the medium, one obtains the macroscopic Maxwell equations. Its quantization features a Hamiltonian formulation having the form of two infinite harmonic oscillators characterized by a double continuum. The diagonalized Hamiltonian is quantized by the correspondence principle, introducing creation-annihilation operators in a bosonic Fock space. The diagonal quantum Hamiltonian is the sum of two terms corresponding to the two continua. The physical observables, like, e.g., the electric field, are also the sum of two terms corresponding to the two continua, one of which had been omitted in the literature geared for an infinite bulk medium. In a second step, we show that the electric field operator can by written as linear combinations of the creation-annihilation operators with coefficients that satisfy integral equations of Fredholm type. We show that the solution of these equations can be expressed in terms of the classical Green tensor of the medium satisfying the Sommerfeld radiation condition. Finally, we consider the Purcell effect for the spontaneous emission of an atom close to the medium. We show that through an exact compensation of some terms, the Purcell factor for the system with the double continuum is proportional to the imaginary part of the Green tensor, which defines the local density of states. This result has the same form as the one obtained in the literature for bulk systems that involve a single continuum and a small dissipative background extending to infinity, and can be seen as a justification of this approach.