Ultra-short pulses from quantum cascade lasers for terahertz time domain spectroscopy

Although the quantum cascade laser (QCL) is a promising compact semiconductor terahertz (THz) source, its success in creating ultra-short pulses is limited. THz short pulses have many applications, including in time domain spectroscopy. There have been demonstrations of short pulse (few picosecond duration) generation from THz QCLs based on active modelocking, although the stability of the pulses is limited. We show that THz QCLs can be modelocked passively using a two-section cavity, where the sections are in- dependently controlled by bias voltages. While one of the sections produces gain, the other produces quantum coherent saturable absorption and helps to create ultra-short pulses.