Research in ultrafast phenomena and ultra-high-speed communications could be enabled by a new infrared laser engineered by CnrNano scientists and co-workers in UK, France and Germany. Ultra-short light pulses with high intensities, at Terahertz frequencies, can probe light-matter interaction phenomena, capture snapshots of molecular dynamics, and drive high-speed communications. In semiconductor lasers, the so-called mode-locking is the principal set of techniques for generating ultra-short pulses. However, engineering sources that emit short pulses in the far-infrared electromagnetic spectrum, that of Theraherz waves, has always been prevented by limitations of fundamental nature. Now a team of researchers led by Miriam Serena Vitiello from Istituto Nanoscienze and Scuola Normale Superiore has demonstrated that this fundamental limit can be overcome. The study, a collaboration between Istituto Nanoscienze, University of Leeds, University of Cambridge, Laboratoire de Physique de l’Ecole Normale Supérieure, Technical University of Munich, appears online April 20 in the journal Nature Photonics. Istituto Nanoscienze's Miriam Serena Vitello, led author of the study, said: "Generating short pulses in the Terahertz range through passive mode-locking was one of the longest standing goals over the last two decades, since it has long been assumed to be inherently hindered by the fast recovery times associated with the intersubband gain of such lasers. We innovatively combined the groundbreaking quantum cascade laser technology with graphene optical properties, to develop a new generation of passive mode-locked Terahertz photonic wire lasers". Researchers engineered a miniaturised Terahertz laser capable of spontaneously emitting pulses of only 4ps duration. Scientists exploited an innovative device architecture that includes the integration of localised strips of graphene, operating as saturable absorbers, distributed along the entire cavity of a semiconductor quantum cascade laser. The resulting configuration is compact, fully electronic, and extremely economical. The heterogeneous gain medium employed in the device has been developed at the University of Leeds by the group of Professor Giles Davies, and Professor Edmund Linfield; while the cavity architecture that integrates a saturable graphene absorber, grown at the Cambridge Graphene Center, was designed and built at Nest laboratory of Istituto Nanoscienze and Scuola Normale Superiore in Pisa, by Vitiello, Elisa Riccardi and Valentino Pistore. The researchers note that this is the first ever demonstration of  the possibility of generating short pulses passively, i.e. spontaneously, in a quantum cascade laser. "We take advantage of the high transparency modulation and fast recovery time of about 2-3 picoseconds of graphene saturable absorption, which favors pulsed emission over naturally occurring continuous wave emission", scientists explain. Such an ultrafast technology in underexploited areas of the Terahertz frequency range, with unprecedented sensitivity and spectral resolution opens opportunities for novel long-term research. For example the investigations of ultrafast dynamics, real-time image reconstructions, tomographic measurements, and time-resolved spectroscopy of gases or complex molecules. It also envisages important perspectives in quantum science, as the coherent control of quantum systems, and in quantum optics, exploiting fast pulses that can bring molecular samples out of equilibrium, as well as in metrology and very high-speed communications. Furthermore, the developed Terahertz micro-lasers can become an innovative and compact alternative to bulky systems currently in use, promising an impact in areas such as biomedical image reconstruction, security scanning, process and quality control and cultural heritage. The research at Cnr Nano was funded by the ERC Grant SPRINT and by the Graphene Flagship.

Further information

Miriam Serena Vitiello's research group in Terahertz Photonics webpage. Scientific article: Elisa Riccardi, Valentino Pistore, Seonggil Kang, Lukas Seitner, Anna De Vetter, Christian Jirauschek, Juliette Mangeney, Lianhe Li, A. Giles Davies, Edmund H. Linfield, Andrea C. Ferrari, Sukhdeep S. Dhillon, and Miriam S. Vitiello, Short pulse generation from a graphene-coupled passively mode-locked terahertz laser, Nature Photonics (2023), https://www.nature.com/articles/s41566-023-01195-z