Hanle effect

The Hanle effect (also known as zero-field level crossing) is a reduction in the polarization of light emitted by atoms excited by polarized light, when subjected to a magnetic field. Typically, when atoms are excited by linearly polarized light, the emitted fluorescence is also polarized. If a weak transverse magnetic field is applied, the emitted light becomes depolarized (or its polarization rotates).

This effect can be utilized in experiments to measure the lifetime of excited states and detect the presence of magnetic fields.

Unlike Faraday effect which causes polarisation rotation proportional to the length of travel within the magnetic field, instead for Hanle effect, the rotation attenuation reaches its maximum as soon as the sample experiences the longitudinal magnetic field and there is no dependence upon the length of path traveled in the magnetic field.