Development of THz Laser Frequency Combs

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Title: Terahertz (THz) Laser Frequency Combs Non-Technical: Frequency combs, which can be viewed as rulers in frequency domain, have revolutionized high-precision metrology and spectroscopy, and for which half of the 2005 physics Nobel prize was awarded. At terahertz (or trillion hertz, in a nontechnical unit), many species exhibit strong and distinctive spectral fingerprints, that is, they absorb or emit strongly at several specific frequencies at THz. THz frequency combs will be the idea tool to study those species. Prior to our breakthrough development in 2014, THz frequency combs have been generated using bulky pump lasers with nonlinear generation. As such, they are very inefficient and can produce only low levels of power (~microwatts). In our recent breakthrough, we developed laser frequency combs at THz which generated much higher power levels (~5 mW). The principal investigator's work has been reported in media aimed at broad communities such as Technology Review, Laser Focus, NPR Market Place, Discovery, Popular Science, and Boston Museum of Science. Through collaborations, the THz lasers developed in the PI's group have helped to enhance infrastructures at other institutions in THz-related activities by adding a crucial enabling component. The principal investigator has incorporated elements in the research project into a graduate course in solid-state physics and a undergraduate foundational course Signals and Systems. Furthermore, effort has been made to involve members of underrepresented minorities, women, and undergraduate students in the research activities. In 2010, based on the technologies developed in the PI's group, a former student co-founded a start-up company LongWave Photonics. Since then, the company has sold many turn-key systems of THz lasers (Easy QCL) to customers worldwide. We fully expect that the proposed work will enable the development of new commercial products which will be of interest to many customers in biochemical communities. Such a development will secure the American leadership in this technical area, and create well-paid jobs in the U. S. Technical: Following the breakthrough, the proposed research seeks to develop high-performance THz frequency combs based on quantum cascade lasers, and to demonstrate their applications in THz optical coherent tomography (OCT) and dual-comb spectroscopy. Recently, by clever dispersion engineering we have successfully developed THz frequency combs spanning over a ~500 GHz bandwidth. As fundamental oscillators, these laser combs generate almost four orders of magnitude greater power than other THz frequency combs based on nonlinear frequency conversion. We plan to develop THz laser frequency combs uniformly spanning over an octave bandwidth (2 THz) with 10 mW output power, and demonstrate them in real applications. Towards this objective, we will pursue four interrelated tasks: developing heterogeneous THz quantum cascade gain media with flat gain curves spanning over an octave bandwidth (2 THz), developing broadband dispersion compensators with high index contrast to compensate the dispersion over such a broad bandwidth, achieving more robust comb operations by utilizing the internally generated beatnote to actively enforce comb formation and performing f-2f frequency stabilization, and demonstrating comb applications in THz optical coherent tomography and high-resolution dual-comb spectroscopy. This development will have a significant impact on the science and technologies in THz frequencies, which have great potentials in biochemical sensing and spectroscopy, imaging for medical and security applications, astrophysics, remote atmospheric sensing and monitoring, and high-bandwidth free-space communications.