Radiative Decay and Level Shift of an Atom in an Optical Resonator

This thesis describes a theoretical and experimental investigation of the intensities, linewidths, and frequencies of radiation by an atom in an optical resonator. It is found that the atom's spontaneous emission rate may be enhanced or inhibited, depending on the tuning of the resonator, and its transition frequencies shifted. A simple classical description of these effects is given in which the atom is modelled as a dipole oscillator whose radiated field is reflected back onto itself by the resonator's mirrors. A more rigorous and detailed quantum mechanical description is also given, and it is found that for the case of a weakly excited two-level atom, the quantum and classical calculations agree. Two separate experiments are carried out. In the first, an atomic beam of ytterbium passes between the mirrors of a confocal resonator and is excited by a single mode cw dye laser on the 1S0 P1 transition, and the intensity of radiation by the atoms into the cavity modes is recorded as a function of cavity tuning. The spontaneous emission rate of the atoms into the cavity modes is enhanced or inhibited by a factor of the order of the resonator's finesse when the resonator is tuned or detuned from the atomic resonance, respectively. This is the first observation of cavity-modified spontaneous emission by atoms in a resonator at visible wavelengths. The total spontaneous emission rate is modified by only about 2%, because the solid angle: subtended by the cavity mirrors is small. This change in total spontaneous emission rate is indirectly verified by a measurement of the intensity of fluorescence out the sides of the resonator. In the second experiment, barium atoms are excited near the center of a concentric optical resonator, and the center frequency and linewidth of the 1So P1 transition is studied as a function of resonator tuning. Shifts in the transition center frequency, due to radiative level shifts, and changes in linewidth, due to enhanced and suppressed spontaneous emission, are observed. The total spontaneous emission linewidth is increased by 20% and decreased by 8%. This is the first demonstration of both line narrowing and broadening and frequency shifts arising from vacuum radiative effects in a spectroscopic experiment. The experimental conditions are very precise: two-level atoms are excited by a single cw dye laser, and competing decay channels and nonradiative effects are absent. Therefore a direct comparison between theory and experiment can be made, and it is found that good agreement is obtained. Thesis Supervisor: Michael S. Feld Title: Professor of Physics