Cooperative interactions in dense thermal Rb vapour confined in nm-scale cells

Abstract

This thesis presents an investigation of the fundamental interaction between light and matter, realised with a rubidium vapour confined in a cell whose thickness (in the propagation direction) is less than the optical wavelength. This confinement allows observation of spectroscopic features not found in longer cells, such as Dicke narrowing. These effects are measured experimentally and a theoretical model is developed, which allows the characterisation of the medium in terms of the atomic electric susceptibility. Interactions between the atoms and their surroundings, whether this be the walls of the cell or other nearby atoms, are explored. In the frequency domain we ob- serve broadening and shifts of the spectral features due to these interactions. The atom-surface interaction shifts the spectral lines, following the expected 1/r^3 van-der-Waals behaviour. The interatomic dipole-dipole interactions are more complex, and we find cooperative effects play an important role. We present an experimental verification of the full spatial dependence of the cooperative Lamb shift, which follows the theoretical prediction made 40 years ago, an important demonstration of coherent interactions in a thermal ensemble.
The interactions also play a role in determining the refractive index of the medium, limiting the maximum near-resonant index to n = 1.31. Using heterodyne interferometry, we experimentally measure an index of n = 1.26± 0.02. This index enhancement leads to large bandwidth regions where a significant slow- or fast-light effect is present. We verify the fast-light effect in the time domain by observing the superluminal propagation of a sub- nanosecond optical pulse, and measure the group index of the medium to be ng = −1.0 ± 0.1 × 10^5, the largest negative group index measured to date.
We investigate the radiative decay rate using time-domain fluorescence, and we observe radiation trapping effects in a millimetre-thickness vapour. Fi- nally, we present results on sub-nanosecond coherent dynamics in the system which are achieved by pumping the medium with a strong optical pulse.