Spontaneous Emission Beyond Dipolar Approximation: Two-Photon Emission, Interferences and Large Emitters

Generally, the spontaneous emission rate of an emitter is computed as a point-dipole. The approximation is usually valid since the size of the quantum emitter (QE) is orders of magnitude smaller than the wavelength and the dominant transition is usually the electric dipolar (ED) one. However, close to nanophotonic structures sustaining strongly confined fields and/or sharp cavity resonances, the approximation is not valid anymore: the ED transition is no longer dominant and can be surpassed by the rates of higher-order transitions such as quadrupolar electric (EQ), two-photons… [1]. For two-photons transitions that are typically 8 to 10 orders of magnitude slower than the spontaneous emission of a single photon, for example, the dispersion of hBN makes the two-photon transition of an emitter in proximity dominant [2]. In addition, large quantum emitters such as long conjugated chains break the dipolar approximation [3] and make forbidden transitions happen.