Dynamics of the Optical Piston

Abstract

Recently we have demonstrated a semipermeable optical piston [1] using the principle of light-induced drift (LID) [2]. In the present work,new experimental results on the dynamics of the optical piston are reported and compared with theory. The basic idea of LID and the optical piston is as follows. Na atoms are present as trace impurity in a buffer gas. A laser beam, travelling along the z-axis and tuned near the Na D resonance, produces velocity-selective saturation of the Na atoms due to the Doppler effect; a hole will occur in the 3S ground-state velocity distribution, whereas a peak will appear in the 3P excited-state distribution. This corresponds to opposing fluxes of excited and unexcited Na atoms. In the presence of a buffer gas (in our case Ar) both velocity distributions tend to thermalize,due to velocity-changing collisions, and, since the elastic collision cross-section of an excited Na atom is larger than that of a ground-state Na atom, these fluxes do not compensate each other. As a result,the Na atoms drift in the same direction as the laser beam when the latter is tuned into the red Doppler wing. Note that in LID the laser photons act as “Maxwell demons”, labelling a specific velocity class and transforming the random atomic motion (partly) into ordered motion, i.e. drift. Net momentum transfer, as in radiation pressure, does not occur: equal but opposite momenta are imparted to the Na vapor and to the buffer gas. In an optically dense system the laser intensity is strongly position-dependent; this leads to the drift velocity being spatially nonuniform and hence, in case that the laser beam completely fills the inner diameter of a tube, to the formation of a steep Na front, an “optical piston” [1,3].