Anomalous absorption of light under nonresonance conditions

The absorption spectrum of a weak probe field in atomic vapours of rubidium, cesium and samarium in the presence of a strong field is studied theoretically. It is shown that away from resonance with an atomic transition, the probe radiation may have an anomalously high absorption coefficient (several times higher than the resonance value) in an anomalously narrow spectral range. The ultranarrow resonance in the probe-field absorption line wing is not related to any real transitions in the atom. Its position is determined by detuning from resonance frequency and by the strong-field intensity. It is shown that for identical frequencies of the strong and probe waves, the resonance can be recorded by using weak magnetic fields. The effect can be applied in the ultrahigh resolution spectroscopy and for precision measurements of the magnetic field strength.