Destruction of the Polarisation of Resonance Radiation by weak Magnetic Fields

A. Ellett

M. Jasperse

This thesis presents a novel magnetic resonance imaging (MRI) technique for spinor Bose-Einstein condensates (BEC) with the paramagnetic Faraday effect. This quantum-photon interface couples the spin of the BEC to the polarisation of an off-resonant laser, which in a magnetic field gradient allows a 1D density profile of the BEC to be reconstructed. As opposed to conventional imaging techniques that are destructive or diffraction limited, this technique is minimally destructive to the BEC and not diffraction limited. Multishot in situ imaging of a single condensate is therefore possible, paving the way for time-resolved studies and 2D/3D reconstructions. A theoretical model for Faraday imaging is developed using the tensor polarisability formalism for the atom-light interaction, and the signal-to-noise ratio is derived. Resolution limits induced by Stern-Gerlach separation are considered, encouraging rapid measurements with strong magnetic field gradients. Analysing the trapping potential reveals that the `magic-zero' wavelength 790nm produces no dipole force and enables a tightly focused probe laser to be used without perturbing the trap. Polarisation-maintaining fibers were seen to impart large fluctuations depending on the incident polarisation, which were minimised. The broad diode laser emission background is seen to reduce the lifetime of the BEC, which was corrected with an interference filter. Short-time Fourier transforms are used to process the photodetected signal, showing the evolution of the polarisation rotation frequency and amplitude. Frequency-modulation is observed at the power line frequency, corresponding to magnetic field fluctuations induced by nearby equipment. Birefringence of optical elements results in elliptical polarisation of the probe and an effective magnetic field, which is cancelled using a quarter-waveplate. The quadratic Zeeman effect causes rapid amplitude modulation, and is eliminated by applying a microwave dressing field. Radiative spin echo is achieved in a magnetic field gradient, proving that dephasing is coherent, and that multishot imaging possible. Magnetic resonance imaging (MRI) is performed on a partially-evaporated atom cloud, distinguishing the thermal and condensed fractions. A split dipole trap is used to create two spatially separated BECs, which are individually resolved during MRI. The resolution is only limited by the strength of the applied magnetic field gradient, and is not subject to the diffraction limit. Custom coils will enable MRI of condensates at the sub-micron scale.

Michel Rousseau, J.P. MATHIEU

Robert Williams Wood, A. Ellett

Abstract The emission of polarised light by fluorescent sodium and iodine vapours was discovered by one of the present writers many years ago, and a similar phenomenon was looked for repeatedly, but without success, in the case of the much simpler (presumably) type of emission which occurs when a gas or vapour re-emits without change of wave-length the energy which it abstracts from the incident beam, as when the monochromatic radiation 2536 from a mercury arc traverse as exhausted quartz bulb containing a drop of mercury at room temperature, or the light of a sodium flame falls upon a glass bulb containing highly rarefied sodium vapour. Lord Rayleigh has recently discovered, however, that the resonance radiation of mercury vapour shows fairly strong polarisation, and reported results which apparently indicated that the polarisation becomes stronger as the exciting radiation penetrates to greater and greater depths of the vapour, being practically absent at the point where the radiation enters the bulb or tube. This would mean that polarisation of the light results only when the excitation is, by frequencies, a little out of tune with the natural frequency of the mercury atom, a matter of great theoretical interest. In a report of some work done last year by one of us this last observation of Lord Rayleigh was not confirmed, the polarisation appearing of full strength right up to the entrance window of the exciting beam. A possible explanation of Lord Rayleigh's results will be offered in the present paper in which it will be shown that a very weak magnetic field, properly oriented, destroys completely all traces of polarisation.

А. Н. Ораевский

J. Kourlas

Resonant fluorescent spectra are calculated in cases where the incident laser radiation is linearly polarised and the fluorescent radiation circularly polarised. The incident radiation is coherent, monochromatic and sufficiently intense to enable AC Stark splitting of the atomic levels. The long-time limit spectra are calculated when the atom consists of two magnetically degenerate levels. In addition, the spectra are derived when the degeneracy is removed by a constant magnetic field.