Terahertz lasers may soon experience their moment of glory. These lasers produce radiation that sits somewhere between infrared light and microwaves along the electromagnetic spectrum.
Terahertz lasers are capable of penetrating standard packaging materials such as cardboard, fabrics, and plastics, and can be used for identifying and detecting a variety of biomolecular species and chemicals. Therefore, they have been the focus of extreme research. Terahertz lasers are even used for imaging certain types of biological tissues without affecting them.
The potential application of terahertz lasers can be fulfilled by enhancing their brightness and intensity, and this can be accomplished by improving the beam quality and power output.
Sushil Kumar, an associate professor in the Department of Electrical and Computer Engineering in Lehigh University, and his research group are working at the frontline of terahertz semiconductor “quantum-cascade” laser, or QCL, technology. Kumar is also affiliated with the Center for Photonics and Nanoelectronics in Lehigh University.
Earlier in 2018, he had reported an easy yet effective method through which the power output of single-mode lasers can be improved based on a novel type of “distributed-feedback” mechanism.
The outcomes were published in the Nature Communications journal and attracted a great deal of attention as a significant advancement in terahertz QCL technology. The study was carried out by graduate students, including Yuan Jin, overseen by Kumar, and in association with Sandia National Laboratories.
Kumar, Jin, and John L. Reno from Sandia National Laboratories have now reported another breakthrough in terahertz technology: the team has devised an innovative phase-locking method for plasmonic lasers and, through this technique, have accomplished a maximum high power output for terahertz lasers.
The laser developed by the researchers emitted maximum radiative efficiency for any single-wavelength semiconductor QCL. The study results have been described in a paper titled “Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode” published recently in the Optica journal.
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. 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.
Sushil Kumar, Associate Professor, Department of Electrical and Computer Engineering, Lehigh University
Researchers generally use phase-locking to improve the beam quality and optical power output of semiconductor lasers. Phase-locking is an electromagnetic control system that drives a range of optical cavities to produce radiation in lockstep.
Terahertz QCLs, which typically use optical cavities with metal coatings or claddings to confine light, are a group of lasers called plasmonic lasers. These lasers are known for their poor radiative characteristics.
It is said that only a limited number of methods are available in previous literature that could possibly be used to enhance the power output and radiative efficiency of such plasmonic lasers by considerable 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. The demonstrated method makes use of traveling surface waves of electromagnetic radiation as a tool for phase-locking of plasmonic optical cavities.
Yuan Jin, Department of Electrical and Computer Engineering, Lehigh University
Jin continued “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.”
In the recent past, Kumar’s team developed a unique technique in which traveling surface waves spread along the metal layer of the cavities but propagate beyond the surrounding medium of the cavities rather than inside. This technique continues to present new opportunities for more breakthroughs.
The researchers believe that the capacity of the output power of their lasers may forge partnerships between application scientists and laser researchers toward the development of sensing platforms and terahertz spectroscopy based on these kinds of lasers.
This breakthrough in QCL technology is the culmination of a long-term research effort made by Kumar’s laboratory at Lehigh University. Kumar and Jin mutually devised the finally implemented concept through experimentation and design across a period of about two years.
Thanks to the association with Dr Reno from the Sandia National Laboratories, Kumar and his team were able to get semiconductor material to develop the quantum cascade optical medium for these lasers.
According to the scientists, a major breakthrough in this study is the layout of the optical cavities, which is slightly independent of the semiconductor material properties. The inductively coupled plasma (ICP) etching tool, which was recently procured at the Center for Photonics and Nanoelectronics of Lehigh University, played a major role in driving the performance limits of these lasers, added the researchers.
This latest study denotes a paradigm shift in the development of single-wavelength terahertz lasers with narrow beams and, going forward, will be developed in the days to come, stated Kumar, adding: “I think the future of terahertz lasers is looking very bright.”
The semiconductor lasers were produced at the nanofabrication facility of the Center for Photonics and Nanoelectronics based at Lehigh University.
The study was partly carried out at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. It was partly funded by grants from the National Science Foundation (ECCS 1351142 and ECCS 1609168).
Jin, Y., et al. (2020) Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode Optica. doi.org/10.1364/OPTICA.390852.