An optical polarization rotator is implemented without any magnetic field by using dispersion due to two-photon transitions. The polarization rotator is useful for powerful coherent linearly-polarized optical beams. A second powerful circularly-polarized coherent optical beam provides control for the polarization rotation of the linearly-polarized beam which interacts more strongly with the oppositely circularly-polarized component of the first beam than with the other component of the first beam. The difference in interaction occurs because the gaseous medium atoms in the cell in which the interaction occurs begin and end a nearby resonance transition that determines the two-photon dispersion in the same angular momentum state; and, because of this quantum state selection rule, the result is a relative delay between the two circularly-polarized components of the linearly-polarized beam and a consequent rotation of the orientation of linear polarization.

The invention relates to an ellipsometer for measuring the degree of elliptical polarization of a beam of coherent optical radiation. Applications of the invention include a device for measuring birefringence and a material detector for measuring small concentrations of substances in a gaseous medium.

A class of elements is dependent upon induced absorption for transmitted energy. Absorption is introduced by first pumping an active medium to produce a first excited state--generally an electronically excited state--whereafter the energy level of such excited state is reduced to a lower "metastable" excited state or by pumping directly to such metastable state. Increased absorption for radiation of a quantum energy corresponding with the difference between the metastable and some higher excited state occurs during the time interval that population of the metastable state is maintained. Elements may operate as extremely rapid shutters, switches, modulators, pulse sharpeners, etc.

Selective optical resonator comprising at least two mirrors and a selective means disposed between two of these mirrors, said means being constituted by: an at least partial linear polarization means for the light in a direction P, a first birefringent plate having two neutral lines, one forming an angle .alpha..sub.1 =0 with direction P, an optically active substance having a birefringence with dispersion and which rotates the light polarization direction by an angle .theta. dependent on the wavelength of the .lambda. light, a second plate which is identical to the first having two neutral lines, one forming the angle .alpha..sub.2 =.theta. with the direction P, whereby the resonator is then selective for said wavelength .lambda..