We report on the observation of a strong optical dipole force which is exerted on atoms in a bichromatic standing light wave as a result of nonlinear wave-mixing processes. This force can substantially exceed the upper limit of the spontaneous light force and, in contrast to the dipole force in monochromatic standing-wave fields, acts with constant sign on a macroscopic spatial scale. PACS numbers: 32.80.Pj, 42.50.VkThe induced dipole force can strongly affect the motion of atoms in inhomogeneous light fields. 1,2 In standing-wave laser fields, which have been the subject of extensive studies, 3 " 6 a very strong dipole force can occur, greatly exceeding the principal upper limit of the spontaneous force. ] In a single monochromatic standing wave, however, the application of this possibly strong force to manipulate the motion of atoms is restricted by the fact that the force spatially oscillates on the optical wavelength scale: Here the dipole force cannot effectively act on a macroscopic spatial scale as it vanishes in a wavelength average.Recently, very interesting effects were considered in the works of Kazantsev and Krasnov, 7 Voitsekhovich et 0A, 8 and Javanainen: 9 Long-wavelength dipole forces are predicted to occur in bichromatic standing-wave fields as a result of a "rectification," being due to nonlinear wave-mixing processes. Different rectification schemes were investigated for two-level atoms 7,8 and a A-type three-level system. 9 A relatively weak rectified dipole force may have already been observed in an experiment; 10 here, however, it at least remains unclear which rectification scheme was realized.It is the aim of this Letter to present the first clear experimental demonstration of a strong rectified dipole force for the elementary case of two-level atoms in a bichromatic standing light wave (BSLW). Before discussing our experiment, let us explain the basic physics of the rectification effect in a simple comprehensible picture; a detailed theoretical description has already been given 7 but an illustrative explanation has not been provided up to now.We consider the dipole force exerted on two-level atoms in a bichromatic light field under the following condition, 7 allowing for a relatively easy theoretical description. We assume that one frequency component (co\) of the field is strongly detuned from resonance, so that the corresponding detuning A| greatly exceeds all other relevant parameters of all points of the field: |A,|»ai(r),a 0 (r),|Ao|,/;(1) here n 0 (r) and Cl\(r) denote the space-dependent optical Rabi frequencies of the two field components, AQ represents the detuning of the other field component (coo), and y is the natural transition linewidth (HWHM). We furthermore neglect all effects induced by the atomic motion, like, e.g., friction forces. 4,5 This approximation is justified as long as all Doppler shifts occurring in the bichromatic field remain small compared with the natural transition linewidth. We use a Fourier expansion of the optical Bloch equations to calculate the indu...
