Color switching of a terahertz quantum cascade laser

Abstract

The mode formation of a terahertz Quantum Cascade laser with two optical transitions is studied experimentally. The emission spectrum shows two well separated frequency regions at 3.4 THz and 3.8 THz corresponding to two different upper laser states | 3 » and | 4 ». From the relative strength of the two colors, population and electron scattering effects for the two states are investigated at different operating temperatures and in the presence of a strong magnetic field. At elevated temperatures, the population of state | 3 » is continuously reduced, resulting in only the 3.8 THz transition lasing at 140 K. For an applied magnetic field, the elastic scattering channel from | 4 » → | 3 » is progressively suppressed, resulting in laser emission switching from 3.4 THz to 3.8 THz.