Negative photoconductivity due to coherent trapping of electrons in n-GaAs

A planar antenna on epitaxial n-GaAs, illuminated by an external THz source, has been used to create intense THz fields to study donor and cyclotron resonance transitions in a magnetic field at low temperature. When the THz field is resonant with such a transition, the resulting extremely strong coupling between the two levels leads to coherent population trapping by single frequency excitation. This effect is visible through the observed negative photoconductivity.