Optical Orientation

The optical orientation of paramagnetic atoms is the ordering, by means of optical radiation, of the directions of the magnetic moments and associated mechanical moments of the atoms of a gas. It was discovered in 1953 by A. Kastler. A distinction is made between optical orientation proper, in which an atomic gas acquires a macroscopic magnetic moment, and alignment, which is characterized by the appearance of anisotropy of the distribution of the moments of atoms while the total macroscopic moment of the gas remains equal to zero.

Optical orientation proper occurs during resonance absorption or scattering of circularly polarized light by atoms. The photons of such radiation have an angular momentum equal to ± ℏ, where ℏ is Planck’s constant; during interaction with an atom they impart the angular momentum to the atom. In a gas of paramagnetic atoms this results in the preferred orientation of the mechanical moments of the electrons and, consequently, of the magnetic moments of the atoms. Thus, the simplest explanation of optical orientation is that it follows from the law of conservation of angular momentum in a photon-atom system. Alignment, in contrast to optical orientation proper, is brought about not by circularly polarized light but by linearly polarized or unpolarized radiation.

The absorption of incident radiation by an oriented gas changes appreciably. Optical orientation is detected by this effect and also by the optical anisotropy accompanying optical orientation in the gas—dichroism, double refraction, and rotation of the plane of polarization of the transmitted light.

Optical orientation has been accomplished directly with vapors of alkali and alkaline-earth metals, with atoms of inert gases in metastable states, and with some ions. Paramagnetic atoms, the peculiarities of whose electron structure rule out direct optical orientation, can be oriented indirectly through collisions with other atoms that are already oriented, that is, through spin exchange. Charge carriers in semiconductors can also be optically oriented.

The “internal” magnetic field of oriented electron shells can bring about the orientation of the magnetic moments of atomic nuclei. This orientation is preserved much longer than electron orientation—its relaxation time is greater. This effect is made use of in the building of quantum gyroscopes. Oriented atoms are used to study weak interatomic interactions and the interactions of electromagnetic fields with atoms. Quantum magnetometers with optical orientation—usually electron orientation—permit the detection of extremely small (about 10^-8 oersted) changes in magnetic field intensity for a range from zero to several hundred oersteds.

Optical orientation is a special case of optical pumping—the disturbance of the energy equilibrium state of a substance in connection with light absorption.

E. B. ALEKSANDROV