Understanding spontaneous emission in the strong coupling regime of an emitter and absorbing matter

Abstract

This thesis proposes a partition of optical states into radiative and non-radiative parts when an emitter is proximal to resonant absorbing nanostructures. The conventional partition is valid only for the weak coupling regime (Purcell regime), and the proposed partition is significant to understand spontaneous emission in the (moderate or) strong coupling regime of an emitter and plasmonic metal nanostructures. It highlights and explains the anomalous large increase in the spontaneous emission of photons from emitters placed near fully absorbing plasmonic nanoparticles (< 10 nm in dimensions) that do not scatter light. Further, this work also explains the origins of the large gains observed in surface-enhanced-Raman-spectroscopy (SERS). In SERS, a rough metal surface (or metal nanopore) effects a near-field enhancement of the incident radiation exciting the proximal molecule by factors up to 10^5. But the radiation emitted from the molecule is predicted to be largely dissipated by the metal, making the observed large gains of emission anomalous in conventional theory. This remarkable divergence of SERS from theoretical predictions has been widening for four decades, during which the reported SERS enhancements have grown from 10^4 to 10^{14}.

The first objective of this work was to establish the divergence of multiple independent experimental observations from the theory, using quantitative evaluations of an emitter coupled to metal nanostructures. The second part involved a study of collective spontaneous emission from multiple emitters coupled to metal nanoparticles; the study was possible due to a computational method developed earlier for solving such problems. This established that collective modes of emission from many emitters are not the source of this divergence of theory from the observations. Later, we proposed a theory for a modified partition of optical states into the radiative and non-radiative (absorbing) parts, which is also valid for the strong-coupling regime of an emitter and absorbing matter. Note that the effects of a weak coupling, also known as the Purcell effect, can be recast as the quantum interference of the classical paths of a photon. We invoked the quantum interference of additional paths involved in the strong coupling regime of the emitter and a metal nanostructure. These additional non-classical paths of the photon arise due to the possible re-absorption of the photon by the emitter, from the excited metal nanostructure. This modified partition of optical states was shown to predict the experimental observations well. Finally, the proposed theory was also incorporated into the models of collective emission, and this allowed us to elucidate the coherence of these classical and non-classical paths in bulk materials dispersed with extremely small metal nanoparticles. To conclude this work, we also studied the decoherence of this effect with variations in the number of emitters and metal particles, and the role of finite sizes of emitters on the strengths of coupling and this resulting effect. Our work that further establishes the proposed theory using a first principles microscopic model of non-local interactions will be reported elsewhere.