We consider collective emission of a single photon from a cloud of
N two-level atoms (one excited, $N\ensuremath{-}1$ ground state). For a dense cloud the problem is reduced to finding eigenfunctions and eigenvalues of an integral equation. We discuss an exact analytical solution of this many-atom problem for a spherically symmetric atomic cloud. Some eigenstates decay much faster then the single atom decay rate, while the others undergo very slow decay. We show that virtual processes yield a small effect on the evolution of rapidly decaying states. However, they change the long time dynamics from exponential decay into a power-law behavior which can be observed experimentally. For trapped states virtual processes are much more important yielding additional decay channels which results in a slow decay of the otherwise trapped states. We also show that quantum mechanical treatment of spontaneous emission of weakly excited atomic ensemble is analogous to emission of
N classical harmonic oscillators.
Matter coupled to light can emit photons. In the absence of an external field this process is known as spontaneous emission. The radiation properties depends in detail the nature of the emitter. In this thesis we consider a two level point-source emitter, and examine how the emission is affected by engineering the local environment. We begin by studying analytically how embedding an emitter in a multi-layer cylindrical structure with radius much smaller than the emission wavelength alters the intensity of the emitted light. We find that for carefully chosen metal-dielectric interfaces, the emission can be strongly enhanced by the plasmonic effect. With this intuition, we experimentally study an InGaN semiconductor quantum dot-in-wire structure as our two level emitter, and manipulate the local field environment by coupling to a lossy plasmonic cavity. We find that the strong enhancement of the field around the quantum dot due to the metal more than compensates for the non-radiative losses, leading to order-of-magnitude increases in the radiative spontaneous emission rate, as well as overall brightness. We then examine how the emission of light can be affected by other nearby emitters. The cooperative effects are strongly dependent on the dimension of the system which controls the electromagnetic mode overlap of the emitters. We present a unified formalism capturing how these cooperative effects change from one dimension to the next. Cooperative light scattering between emitters underpins collective effects such as super- and sub-radiance, and we numerically investigate how our previous results give rise to the key experimental signatures that can be used to identify those phenomena. Specifically, we study how the superadiant decay rate scales with the size of a
2-dimensional atomic cloud, and how cooperative effects between a small number of emitters alter the emission spectrum of strongly driven resonance fluorescence.