A superradiative transient has been observed following rapid passage of an exciting signal through the absorption resonance of a molecular beam of ammonia.
Traditionally, the phenomenon of optical coherence has been observed by detecting coherent transients (i.e., optical nutation, FID, photon echoes) propagating along the laser beam following passage through an absorbing medium. Recently, we have demonstrated that detection of the incoherent (spontaneous) emission from selectively prepared states provides the optical coherence of the ensemble. Our observation of the nutation and the photon echo at right angles to the exciting beam gives directly optical T_1, T_2, and the moment of the laser excited transition. This technique is simple and not susceptible to some of the problems inherent in coherent absorption experiments. Details about this laser technique will be presented. We will also discuss the different dynamical information one obtains about excited states in gases, molecular beams and solids.
Over the past 30 years, optical manipulation of neutral atoms has been primarily performed with a monochromatic laser beam. The simplest tool for the control of atomic motion is the radiative force exerted by a monochromatic laser on a twolevel atom. The radiative force arises from absorption followed by spontaneous emission, and its magnitude is limited by the atom's excited state lifetime. The coherent momentum exchanges between light elds and atoms can be exploited to produce long-range optical forces much greater than the radiative force through the use of absorption-stimulated emission processes. Adiabatic Rapid Passage (ARP) is a long-existing method to invert the population of a two-level nuclear spin system. Its extension to the optical domain necessitates a frequency chirped light pulse to interact with a two-level atom via the dipole interaction. I will first present a numerical study of the properties of optical forces on moving atoms derived from purely stimulated processes produced by multiple ARP sequences. This will be followed by experimental observations of long-range ARP forces much larger than the radiative force in metastable helium. Sequences of properly timed laser pulses may be used for rapid deceleration of neutral atomic (or molecular) beams. | 171 pages
Rapid passage signals have been observed in which a delayed switch from absorption to emission is observed as the optical density of acetylene in a long path length cell is increased. The delayed switch results in the generation of a large, narrow, transient gain signal. Using numerical solutions of the Maxwell–Bloch equations we have demonstrated that this effect is due to constructive interference between the incident laser field and the field generated by the response of the gaseous medium. This occurs owing to the minimal collisional damping at the low gas pressures within the gas cell, so that the chirp rate of the laser is faster than the collisional reorientation time of the molecules.