Giant oscillator strength of excitons in bulk and nanostructured systems

Coherence phenomena in atomic vapors and crystals can have remarkable effects on emission and absorption of light, such as superradiance and “giant oscillator strength.” An exciton has intrinsic spatial coherence due to its existence in a periodic crystal and so exhibits giant oscillator strength, which may be described as the scaling of its optical transition rate with the number N of lattice sites in the coherence volume, at least for diameter ⪡λ/2. If one can realize sufficient coherence volume, then ultrafast (e.g. picosecond) spontaneous radiative lifetimes are achievable, whereas no single atom can have a spontaneous radiative lifetime this short for visible light. We have been intrigued by the question of what sets the upper limit of oscillator strength as the coherent source size increases beyond a half wavelength of light in the medium. This paper discusses a semi-classical diffraction-based treatment which reduces to the Rashba-Gurgenishvili/Henry-Nassau r3-dependence for a very small source volume and includes the development toward forward-wave emission for a large coherent source volume in which the initial state is a plane-wave exciton.