Quantum Cascade Laser Sources Of High-Power, Coherent Frequency Combs
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Abstract title: Compact, high-power semiconductor laser sources of coherent frequency combs for fast and efficient molecular detection Abstract: Non-technical: The objective of the project is the development of compact, practical, high-power semiconductor laser sources of coherent frequency combs in the mid-infrared spectral range. The combs work as rulers in frequency space, providing fast and accurate readings of frequencies of molecular vibrations. These frequencies can serve as unique identifiers of different molecules. Therefore, the proposed laser sources will enable ultra-broadband and fast molecular spectroscopy, which has a wide range of applications in trace gas environmental monitoring, pharmaceutical quality control and remote detection of biochemical agents. The project will explore a new mechanism of frequency comb generation, which is based on coherent pulsations in quantum cascade lasers. The project will be pursued as a collaborative effort between the team members at Harvard and Texas A&M University and in collaboration with world-leading spectroscopy experts from Europe. This will create unique inter-disciplinary and multi-cultural education, research, and outreach opportunities for all involved students and researchers. Technical: The objective of this collaborative research is to develop high-power semiconductor laser sources of coherent frequency combs in the mid-infrared based on continuously pumped quantum cascade lasers. The field of quantum cascade laser mode locking and frequency combs has seen rapid expansion over the last three years, due to contributions of the proposing team and several other groups. However, so far broadband phase-coherent combs have been demonstrated in lasers of special design in a narrow interval of currents near threshold. Ultrafast gain relaxation presents a fundamental obstacle to most mode locking techniques. The proposing team will focus on a new route to frequency combs through coherent Rabi oscillations, which result in parametric generation of a phase-coherent "supercomb" of modes separated by terahertz frequency intervals. The resulting harmonic mode locking is equivalent in time domain to amplitude modulation at terahertz frequencies comparable to the gain relaxation rate. This new mechanism of frequency comb generation is entirely phase-coherent and intrinsically operates high enough above threshold, yielding high power per mode. The project will pursue several strategies of utilizing the Rabi mechanism for generation of stable, high-power, and broadband combs using standard high-performance laser chips.