Figure 4: Seed pulse saturation measurements.

(a) Electro-optically measured fields of the QCL output for various input THz pulse amplitudes. The THz pulse amplitude is varied by changing the antenna voltage. (The THz pulse amplitude is proportional to the antenna voltage.) Output fields are shown for input THz pulses generated with an antenna bias of 1 V (green curve), 0.25 V (blue curve) and 0.06 V (grey curve). The QCL DC bias (66 A cm−2) is below threshold. (b) The peak-to-peak (p-p) amplitude and root mean square (RMS) of the THz fields in part a for time periods >500 ps versus the antenna voltage (logarithmic scale). Inset—part b, on a linear voltage scale.