A new method is presented for the deflection of atomic and molecular beams by momentum transfer from resonantly absorbed and reemitted electromagnetic radiation. The method, which uses adiabatic rapid passage by chirped optical pulses, is capable of high energy efficiency. It can be used for isotope separation.
The application of counterpropagating, short, chirped laser pulses for the deflection and splitting of an atomic beam is investigated. A simple model is proposed to describe the case in which the pulses induce population transfer in the adiabatic-passage regime and in which the effects of spontaneous emission are negligible during the action of a single laser pulse. Spontaneous emission between the pulses is not neglected, however, and it is shown that it has a significant effect on the evolution of the average transversal velocity of the atoms, as well as the dispersion of the transversal velocity.
Limitations of existing methods of isotope separation, using atomic beam deflection induced by radiation pressure, are discussed. A new method, based on selective momentum transfer by intense unchirped reflected laser pulses, is proposed. The pulses produce population inversion in one isotope leaving the other unaffected. This population inversion is obtained using isotope selective optical precession. The method is supported by the full solution of the equations of motion of a two-level atom interacting with realistic light pulses of arbitrary strength.
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
The exchange of momentum between atoms and photons in a deflection experiment is usually described by different formalisms depending on whether the interaction time T is shown or long compared with the radiative lifetime tau R . The authors present a new approach to this problem leading to a single theoretical expression valid in both limits and therefore allowing the transition between them to be studied. They interpret in this way the resonant Kapitza-Dirac effect and the optical Stern and Gerlach effect appearing in the short-time limit (T<< tau R ) as well as the deflection profiles usually deduced from a Fokker-Planck equation in the long-time limit (T>> tau R ). The transition between these two regimes is interpreted in terms of momentum transfer due to absorption and stimulated emission of laser photons, convoluted by the distribution of recoil due to spontaneously emitted photons.
Periodic adiabatic rapid passage from counterpropagating light pulses is used in a parameter domain outside of the conventionally expected successful range to produce large optical forces on atoms by coherent control of the momentum exchange between the light and atoms. Both the magnitude and velocity range are much larger than those of the usual radiative force. We observed the force on metastable He atoms by the deflection of the atomic beam with periodic pulse sequences. We have also been able to map out the force magnitude in the two-dimensional parameter space of peak Rabi frequency and sweep range, and we find qualitative agreement with our model.