The objective of this program is to develop actively mode-locked quantum cascade lasers as a practical source of ultrashort pulses and frequency combs in the mid-infrared range. The intellectual merit is to explore a novel and highly promising approach to produce ultrashort pulses and frequency combs in the mid-infrared range. There are currently no mode-locked semiconductor lasers operating in this spectral region. The project will enable generation of stable, powerful mode-locked pulses in a robust and compact setup. This work will also lead to significant advancement in laser physics and nonlinear optics of semiconductor nanostructures. A new operation regime and configuration of a quantum cascade laser source will be demonstrated, studied and utilized. Therefore, the proposed research is truly transformative and will greatly advance the state-of-the art. The broader impacts are wide-ranging and substantial. Mid-infrared mode-locked semiconductor lasers will be key elements for many important applications ranging from mid-infrared remote sensing and imaging to laser metrology, material processing, laser therapy and laser surgery. Mode-locked quantum cascade lasers will provide a compact, robust solution for the above applications. Educational and outreach plan is aimed at enhancing the education and training opportunities in applied physics and engineering. The proposed research includes both theoretical and experimental components and is really interdisciplinary. This combination of disciplines offers a unique educational environment for students. Knowledge and techniques developed during research will be incorporated into courses and disseminated through publications, technology transfer, and the research groups? websites. Substantial outreach activity is planned.