In this work quantum coherence effects in dense rubidium atomic vapour are investigated. Prepared in a laser driven atomic system, coherent effects, like the well-known electromagnetically induced transparency, have a remarkable ability to significantly alter optical properties of the used medium. Electromagnetically induced transparency and related phenomena like slow and stored light can be explained in terms of dark state polaritons, representing quasiparticles consisting of light and matter components. In this thesis, experiments aiming to the concept of quasiparticles are described. Furthermore, in this thesis the concept of resonance retrieval of a stored coherence is investigated, a concept that might prove useful in metrology. In this thesis, the focus is set on the experimental study of the particle-like nature of dark state polaritons. Therefore, a radiofrequency spectroscopy is performed on stationary and moving polaritons with an internal level structure. To this end, an experimental setup is described that allows for the existence of two state dark polaritons in a tripod level scheme. The internal level structure of these polaritons is then probed by a radiofrequency field and a transfer of population between two dark states is observed. The measured population transfer, though with a limited contrast, strongly indicates that dark state polaritons can be considered as quasiparticles with an internal level structure. Since the dark state polaritons exhibit features of an effective magnetic dipole moment, the existence of the Aharonov-Casher phase effect for these quasiparticles is investigated experimentally. It is predicted by a known Aharonov-Casher effect that taking two particles with a finite magnetic dipole moment around a line of charge causes an accumulated phase difference between them. In this work, a detection system with a phase sensitivity of 10-5 rad is described, which sets the upper boundary on the experimentally detectable Aharonov-Casher phase for the hot rubidium gas system. In the last part of this thesis experiments on storage and resonant retrieval of an atomic coherence are described. To this end, an RF field creates an atomic coherence which is stored and subsequently read out with an optical field. The retrieved field is shown to be frequency locked to the optical field. The difference frequency is, within the limits of experimental sensitivity, at the atomic resonance frequency independently on the amplitude and frequency variations of the radiofrequency excitation field.
A comparison between forward and diffusely scattered light propagating in a coherently prepared ultracold 87Rb atomic vapor is presented. This research is part of the ongoing effort to characterize the processes, such as diffusion, that contribute to coherence loss in atomic media under conditions of electromagnetically induced transparency, for applications in realistic systems. Toward this end, a magneto optical trap (MOT) of 87Rb has been built, and the atomic vapor sample characterized in terms of atomic density, shape and size, temperature, and optical depth. Next, two co-propagating beams were sent through the sample, to establish an electromagnetically induced transparency in a Λ-type system. In these measurements the two laser fields are not phase-locked, so ground state coherence will be affected by the lasers' phase fluctuations. Observation of the forward and diffusely scattered light were made for different conditions of probe pulse length, control field intensity, and probe field frequency detuning. Storage and retrieval of slow light pulses was achieved for the forward scattering (transmission) channel, but was not observed unambiguously in the diffusely scattered (fluorescence) channel.
The transport-of-intensity equation (TIE), namely the continuity equation associated with a coherent paraxial optical wavefield, is widely used for phase retrieval. It is a second-order partial differential equation which may be solved for the phase of a coherent paraxial field such as a monochromatic scalar optical beam, given the intensity and longitudinal intensity derivative in a plane perpendicular to the optical axis. We show how the coherent flow associated with the TIE may be augmented by a diffusive flow associated with a scalar or tensor diffusion field. Such diffusive flow can arise via scattering from unresolved spatially random microstructure in an illuminated sample, blurring effects of an extended chaotic source that illuminates the sample, the resolution-reducing effect of shot noise in detected intensity images of the sample, and the sharpening effect (negative diffusion) associated with scattering from sharp sample edges. Augmenting the TIE's modeling of coherent flow with a diffuse-flow channel leads to a Fokker-Planck extension to this equation. Two different augmentations are obtained, using several complementary derivations. The inverse problems of phase retrieval and diffusion-field retrieval are then considered, for defocus-based imaging and mask-based imaging. When symmetric overfocus and underfocus images are used for phase retrieval, the diffusive term drops out and our Fokker-Planck formalism implies that any ensuing TIE-based phase-retrieval method needs no modification in light of our formalism. However, the same focal-series dataset---typically an infocus image, a weakly overfocused image, and a weakly underfocused image---may also be employed to access the additional channel of information associated with the Fokker-Planck diffusion field. Our formalism is applicable to visible light, x-ray, electron, and neutron imaging.