| Studies of three different coherent effects in laser spectroscopy are presented. (1) The Raman heterodyne effect is a new stimulated Raman process, induced by an optical and a radio frequency field, and is used for detecting steady state or pulsed nuclear magnetic resonance (NMR) in both ground and excited electronic levels. It is demonstrated in the impurity ion praseodymium in a lanthanum trifluoride single crystal, where level splittings, inhomogeneous and homogeneous linewidths and shapes of the hyperfine transitions in ground and excited states are measured with high precision and sensitivity. Shot noise limited performance of the technique makes possible the first observation of coherent spin transients in NMR in an excited electronic level, thus allowing a comparison with optical dephasing measurements and current line broadening theory. (2) Optical rotary echoes, the optical analog of rotary spin echoes in NMR, are observed in iodine vapor. Rotary echoes in a two-level system are produced when the coherent driving field undergoes a sudden phase shift. The initial nutation dephases, and then rephases to form an echo following the phase-shifting pulse. A detailed perturbative calculation is performed and predictions are confirmed. The results are found to be identical with those in NMR despite their differences in transition frequencies, the importance of spontaneous emission, and energy level structure. (3) The phenomenon of oscillatory free induction decay is predicted for a two-level system that is coherently prepared by an intense square wave excitation pulse. The effect applies to quantum optics and NMR when the transition is inhomogeneously broadened and the applied field is intense enough. Numerical results for a finite inhomogeneous width and analytic results when it is infinite are obtained. A general theorem on coherent transients in quantum optics and NMR is derived for a two-level system subject to inhomogeneous broadening and coherent pulsed preparation where the pulse shape is arbitrary. The theorem states that the coherent emission which follows is confined to an interval which precisely equals the duration of the applied pulse. It is extended to a multi-level system where the energy spacing, the number of applied fields, and their frequencies are arbitrary. These predictions have now been fully confirmed by experiments in the rf and optical regions. |
