Ultrafast response of harmonic modelocked THz lasers

Harmonic help boosts Terahertz laser Ultrafast terahertz and sub-terahertz communications could be enabled by a new infrared laser developed by researchers in France and Germany. Sukhdeep Dhillon at the Ecole Normale Superieure in Paris and co-workers built a quantum cascade laser that emits rapid pulses of far-infrared radiation, close to the microwave region. Their laser undergoes so-called harmonic mode locking, meaning that ultra-short pulses are generated, separated by intervals even shorter than the time it takes a photon to do a round trip of the laser resonator cavity. The researchers observed very stable mode locking at the second harmonic, with hints of other frequencies up to a 15th harmonic, enabling the laser to emit pulses at previously unseen rates. This was a result of considering the electronic as well as photonic properties of the laser, which operates at the frontier between these methods to generate electromagnetic radiation. As well as enabling high-bit-rate data transfer, such lasers could detect gases or probe ultrafast dynamics and phenomena across the physical, chemical and biological sciences. The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. However, unlike classic laser diodes, no demonstrations of harmonic modelocking, active or passive, have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity's fundamental round-trip frequency. Here, using time-resolved THz techniques, we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. We demonstrate the generation of the harmonic electrical beatnote within a QCL, its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach. Finally, we show indications of self-starting harmonic emission, i.e., without external modulation, where the QCL operates exclusively on a harmonic (up to its 15th harmonic) of the round-trip frequency. This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic, as well as photonic, nature of QCLs. These results open up the prospect of passive harmonic modelocking and THz pulse generation, as well as the generation of low-noise microwave generation in the hundreds of GHz region.