Phase-locked arrays of high-power terahertz lasers with ultra-narrow beams

Lehigh University, Bethlehem PA

Investigators

Abstract

Abstract title High-power terahertz lasers radiating in ultra-narrow beams based on a novel phase-locking scheme for applications in sensing and spectroscopy Non-technical description The terahertz region of electromagnetic spectrum is significantly underdeveloped due to lack of high-power sources of radiation. Existing monochromatic sources have low output power and/or highly divergent beams, which makes them unsuitable for important applications in terahertz sensing, imaging, and spectroscopy. This project aims to develop terahertz semiconductor lasers that could emit up to hundred milliwatts of average optical power in a narrow beam with less than five degrees of angular divergence. Such a performance is predicted for lasers operated in a compact cryocooler, and will be a significant improvement over present state-of-the-art. A new distributed-feedback scheme that allows phase-locking of multiple terahertz metallic cavities is proposed, which leads to significantly improved radiative efficiencies. The lasers will be developed with the semiconductor quantum-cascade technology that will allow integration of tens of different lasers, each emitting at a range of discrete frequencies, on a single semiconductor chip for broad spectral coverage. The availability of such integrated arrays of high-power terahertz lasers could lead to a transformative impact in the field of terahertz science and technology, by enabling development of cost-effective scientific-instruments for rapid non-invasive/remote sensing, detection, and analysis of a variety of chemicals and biomolecular species such as packaged drugs and explosives, pharmaceutical compounds, and biological samples. There is a strong probability that new business directions will be established in photonics industry if the goals of this project are met successfully. At educational level, the research will be made accessible to undergraduate students especially from diverse backgrounds, who will get hands-on training toward advanced concepts in laser design and characterization, plasmonics, terahertz science, and cryogenic electrical and optical measurements. Technical description Cryogenically cooled semiconductor quantum-cascade lasers (QCLs) are the most powerful solid-state sources of coherent terahertz radiation; however, presently, single-mode terahertz lasers operating at practically viable temperatures could only radiate average optical power in the order of a milliwatt, which is insufficient for most targeted applications. This proposal seeks to improve power output from such QCLs by two-orders of magnitude in the range of hundred milliwatts with significantly improved beam quality. A portable electrically operated Stirling cryocooler will provide the required cooling for the semiconductor laser chips that could have tens of such phase-locked QCL arrays emitting at a range of discrete terahertz frequencies. Such a development could lead to commercialization of terahertz QCLs similar to the highly successful mid-infrared QCLs that have spawned recent economic activity in tens of companies worldwide. Terahertz QCLs utilize parallel-plate metallic cavities with strong mode confinement, for which a unique modal coupling scheme for multiple cavities through surface via the surrounding medium (vacuum or air) is proposed. Owing to the very long wavelengths at terahertz frequencies, it is shown that in-phase surface-plasmon-polariton modes for coupled cavities can be excited with periodic photonic structures fabricated using conventional lithography. Radiation in an ultra-narrow beam is expected owing to the spatially extended radiated wavefront of the phased-locked optical mode. Development of high-power terahertz lasers has the potential to enable commercial activity in terahertz spectroscopy, sensing, and imaging. Laser based terahertz instruments could lead to new insights in fields as diverse as cell-biology for study of biomolecular structure and dynamics, non-destructive evaluation and detection of drugs, pharmaceutical products, explosives, non-invasive imaging of biological samples, and remote-sensing of Earth's atmosphere and outer space. A successful outcome of this project will potentially lead to collaborations with industry (specifically, semiconductor laser companies), some of which have shown recent interest for development of terahertz instruments. At the University level, special efforts will be made to involve undergraduate students especially from diverse backgrounds in the research activities during each year.