@article {Hillbrand2020, title = {Mode-locked short pulses from an 8 um wavelength semiconductor laser}, journal = {Nature Communications}, volume = {11}, number = {1}, year = {2020}, month = {dec}, pages = {5788}, abstract = {Quantum cascade lasers (QCL) have revolutionized the generation of mid-infrared light. Yet, the ultrafast carrier transport in mid-infrared QCLs has so far constituted a seemingly insurmountable obstacle for the formation of ultrashort light pulses. Here, we demonstrate that careful quantum design of the gain medium and control over the intermode beat synchronization enable transform-limited picosecond pulses from QCL frequency combs. Both an interferometric radio-frequency technique and second-order autocorrelation shed light on the pulse dynamics and confirm that mode-locked operation is achieved from threshold to rollover current. Furthermore, we show that both anti-phase and in-phase synchronized states exist in QCLs. Being electrically pumped and compact, mode-locked QCLs pave the way towards monolithically integrated non-linear photonics in the molecular fingerprint region beyond 6 $μ$m wavelength.}, isbn = {4146702019592}, issn = {2041-1723}, doi = {10.1038/s41467-020-19592-1}, url = {http://www.nature.com/articles/s41467-020-19592-1}, author = {Hillbrand, Johannes and Opa{\v c}ak, Nikola and Piccardo, Marco and Schneider, Harald and Gottfried Strasser and Federico Capasso and Schwarz, Benedikt} }