Terahertz Quantum Cascade Laser Light Propogates Beyond Laboratory Walls

Researchers at MIT and the University of Waterloo (Canada) created a high-power quantum cascade laser (QCL) capable of generating terahertz radiation outside of a laboratory environment. The laser has applications for skin cancer diagnostics and treatments, security, and gas detection, among other areas and industries.

Previously, terahertz radiation powerful enough for real-time imaging and quick spectral measurement was only possible at temperatures far below 200 K (−100° F), which requires bulky refrigeration equipment that precludes practical applications. The new device is operational at 250 K (−10° F), requiring only a compact portable cooler to achieve full functionality.

Scanning electron microscope image of a terahertz quantum cascade laser. Courtesy of Khalatpour, A., Paulsen, A.K., Deimert, C. et al.

The work stems from previous efforts by MIT electrical engineering and computer science professor Qing Hu, who began studying terahertz frequencies in 1991, creating a terahertz QCL in 2002.

A major difference between this design and Hu’s earlier designs is the height of the barriers within the laser designed to prevent leakage of electrons, a phenomenon that increases at high temperatures.

“We understood that over-the-barrier electron leakage was the killer,” Hu said. “So we put a higher barrier to prevent the leakage, and this turned out to be key to the breakthrough.”

Higher barriers had been explored previously but yielded inferior results, Hu said. The researchers had reasoned that increased electron scattering, associated with the higher barriers, was detrimental to a functional design. As a result, the researchers had previously avoided incorporating, or working with higher barriers.

In the current design, the researchers developed the correct parameters for the band structure for taller barriers, and an optimization scheme. They paired the structure with a “direct phonon scheme” that keeps the laser operating through configuration(s) in which lower lasing levels of each module, or steps of the structure’s “staircase” are depopulated of electrons through phonon scattering into a ground state. That state serves to inject electrons into the next step’s “upper level.” The process then repeats.

“These are very complex structures with close to 15,000 interfaces between quantum wells and barriers, half of which are not even seven atomic layers thick,” said co-author Zbig Wasilewski, professor of electrical and computer engineering and University of Waterloo Endowed Chair in Nanotechnology. “The quality and reproducibility of these interfaces are of critical importance to the performance of terahertz lasers. It took the best in molecular beam epitaxial growth capabilities — our research team’s key contribution — together with our MIT collaborators’ expertise in quantum device modeling and fabrication, to make such important progress in this challenging sector of terahertz photonics.”

The system further includes a compact camera and detector, and it can operate anywhere with an electrical outlet. This renders it capable of real-time imaging during skin cancer screenings or during surgical procedures to excise cancerous tissues. The cancer cells show up “very dramatically in terahertz” due to their higher water and blood concentration than normal cells, Hu said.

The device also has applications in quality control, as well as detection of gases, drugs, and explosives, such as detection of hydroxide, methamphetamine, and heroin, and explosives including TNT.

“Using the direct phonon scheme and taller barriers is the way to go forward,” Hu said. “I can finally see the light at the end of the tunnel when we will reach room temperature.”

The research was funded by NASA and the Natural Sciences and Engineering Research Council of Canada.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-00707-5).