ABSTRACT
The light force exerted on an atom can be of two types: a dissipative, spontaneous force and a conservative, dipole force. The spontaneous force arises from the impulse experienced by an atom when it absorbs or emits a quantum of photon momentum. When an atom scatters light, the resonant scattering cross section can be
written as σ0 = λ20/2π where λ0 is the on-resonant wavelength. In the optical region of the electromagnetic spectrum the wavelengths of light are of the order of several hundreds of nanometres, so resonant scattering cross sections become quite large,∼10−9 cm2. Each photon absorbed transfers a quantum of momentum h¯k to the atom in the direction of propagation (h¯ is the Planck constant divided by 2π , and k = 2π/λ is the magnitude of the wave vector associated with the optical field). The spontaneous emission following the absorption occurs in random directions; and, over many absorption-emission cycles, it averages to zero. As a result, the net spontaneous force acts on the atom in the direction of the light propagation, as shown schematically in the diagram of figure C1.4.2. The saturated rate of photon scattering by spontaneous emission (the reciprocal of the excited-state lifetime) fixes the upper limit to the force magnitude. This force is sometimes called radiation pressure.
