Figure 1: Coherent injection seeding of a QCL.

(a) A THz pulse injected into the QCL before the gain is turned on initiates lasing. The carrier phase of the QCL emission is then set by the injected THz pulse. (The dashed line represents the value of the gain at laser threshold.) (b) The carrier phase of the emission from a QCL is synchronized with respect to a femtosecond (fs) sampling beam. (c) The carrier phase of the emission from a free-running QCL is not synchronized with respect to a femtosecond sampling beam. The femtosecond sampling beam will measure an average electric field of zero. (d) The phase of the THz pulses is fixed from pulse to pulse even if the carrier phase of the femtosecond laser pulses varies. (e) THz pulses are generated in response to the envelope function (green) of the intensity of the femtosecond laser pulse, which does not depend on the carrier phase . Only the second harmonic response (blue) of the instantaneous intensity depends on the carrier phase Δφ. However, it has no role in the generation of terahertz radiation.