New Approach for Multi-Watt Terahertz Lasers

A phase-locking scheme for plasmonic lasers is developed in which traveling surface-waves longitudinally couple several metallic microcavities in a surface-emitting laser array. (Source: Y. Jin, Lehigh U.)

Terahertz lasers have been the focus of intense study due to their ability to pene­trate common packaging materials such as plastics, fabrics, and cardboard and be used for identi­fication and detection of various chemicals and biomo­lecular species, and even for imaging of some types of biological tissue without causing damage. Fulfilling terahertz lasers’ potential for use hinges on improving their intensity and bright­ness, achieved by enhancing power output and beam quality. Sushil Kumar, associate professor in Lehigh University’s Depart­ment of Electrical and Computer Engi­neering, and his research team are working at the forefront of terahertz semi­conductor quantum-cascade laser (QCL) technology. Kumar, Jin and John L. Reno of Sandia have developed a new phase-locking technique for plasmonic lasers and, through its use, achieved a record-high power output for terahertz lasers. Their laser produced the highest radiative effi­ciency for any single-wavelength semi­conductor quantum cascade laser.

“To the best of our knowledge, the radiative effi­ciency of our terahertz lasers is the highest demons­trated for any single-wavelength QCL to-date and is the first report of a radiative efficiency of greater than 50% achieved in such QCLs,” said Kumar. “Such a high radiative effi­ciency beat our expectations, and it is also one of the reasons why the output power from our laser is signi­ficantly greater than what has been achieved pre­viously.” To enhance the optical power output and beam quality of semi­conductor lasers, scientists often utilize phase-locking, an electromagnetic control system that forces an array of optical cavities to emit radiation in lock step. Terahertz QCLs, which utilize optical cavities with metal coatings for light confine­ment, are a class of plasmonic lasers that are notorious for their poor radiative pro­perties. There are only a limited number of techniques available in prior literature, they say, that could be utilized to improve radiative effi­ciency and output power of such plasmonic lasers by significant margins.

“Our paper describes a new phase-locking scheme for plasmonic lasers that is distinctly different from prior research on phase-locked lasers in the vast literature on semi­conductor lasers,” says Jin. “The demons­trated method makes use of traveling surface waves of electro­magnetic radiation as a tool for phase-locking of plasmonic optical cavities. The efficacy of the method is demons­trated by achieving record-high output power for terahertz lasers that has been increased by an order of magnitude compared to prior work.”

Traveling surface waves that propa­gate along the metal layer of the cavities, but outside in the surrounding medium of the cavities rather than inside, is a unique method that has been developed in Kumar’s group in recent years and one that continues to open new avenues for further innovation. The team expects that the output power level of their lasers could lead to colla­borations between laser researchers and appli­cation scientists toward development of terahertz spectro­scopy and sensing platforms based on these lasers.

This inno­vation in QCL technology is the result of a long term research effort by Kumar’s lab at Lehigh. Kumar and Jin jointly developed the finally-imple­mented idea through design and experi­mentation over a period of approximately two years. The colla­boration with Reno from the Sandia National Laboratories allowed Kumar and his team to receive semi­conductor material to form the quantum cascade optical medium for these lasers. This research represents a paradigm shift in how such single-wavelength terahertz lasers with narrow beams are developed and will be developed going forward in future, says Kumar, adding: “I think the future of terahertz lasers is looking very bright.” (Source: Lehigh U.)

Reference: Y. Jin et al.: Phase-locked terahertz plasmonic laser array with 2  W output power in a single spectral mode, Optica 7, 708 (2020); DOI: https://doi.org/10.1364/OPTICA.390852

Link: Center for Photonics and Nanoelectronics, Dept. of Electrical and Computer Engineering, Lehigh University, Bethlehem, USA

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