We experimentally and theoretically study two different tripod configurations using metastable helium (${}^4$He*), with the probe field polarization perpendicular and parallel to the quantization axis, defined by an applied weak magnetic field. In the first case, the two dark resonances interact incoherently and merge together into a single electromagnetically induced transparency peak with increasing coupling power. In the second case, we observe destructive interference between the two dark resonances inducing an extra absorption peak at the line center.

• Received 9 February 2011

DOI:https://doi.org/10.1103/PhysRevA.84.023811