  The development of the semiconductor quantum cascade laser (QCL) [1] has
enabled bright coherent sources operating at frequencies between the optical
(>100 THz) and electronic (<0.5 THz) ranges opening this frequency region for
fundamental science investigations [2-5] as well as for applications [6].
However, detailed information about the internal processes in QCLs and their
ultrafast dynamics are needed for their further development towards high output
power, longer wavelengths and stable pulsed operation. We introduce a novel
approach to study the gain and dynamics of a terahertz (THz) QCL [7] using
phase resolved measurements of the stimulated emission. This is enabled by
direct recording of the emitted electric field with <100 fs time resolution.
For the case of the THz QCL we demonstrate the validity of the quantum
mechanical description of the laser. We show for the first time the real
bandwidth of the terahertz gain medium, the current dependence of this gain,
and the losses associated with the wave propagation in the laser waveguide.
