Laser detection of optical signals

A system of unexcited three-level atoms, located in a nonresonant laser field, was examined. One of the excited levels in the system becomes populated under the influence of a weak optical signal, as a result of which the atoms resonate with the laser field. A consequence of this is intense scattering of the laser radiation and particularly excitation of the hitherto unexcited modes of the cavity in which the system of atoms is located. The system considered thus possesses states which are specially sensitive to external agencies and therefore sensitive optical-radiation detectors can be based on these states.