Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have taken a step toward bringing terahertz frequencies out of their hard-to-reach region of the electromagnetic spectrum and into everyday applications. A team demonstrated a terahertz laser that is compact, operates at room temperature, and can produce 120 individual frequencies spanning the 0.25- to 1.3-THz range, far broader than previous terahertz sources. The team reports the device is the first of its kind. “This is a leap-ahead technology for generating terahertz radiation,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper. “Thanks to its compactness, efficiency, wide tuning range, and room temperature operation, this laser has the potential to become a key technology to bridge the terahertz gap for applications in imaging, security, or communications.” Schematic of the experimental setup. A gold-plated silicon wafer was used as a beamsplitter to reflect a small portion of the pump beam into the reference gas cell, while the rest entered the THz cavity. Courtesy of the Capasso Lab/Harvard SEAS. The terahertz frequency range — which sits in the middle of the electromagnetic spectrum between microwaves and infrared light — has remained difficult to reach for applications because most terahertz sources are either very bulky, are inefficient, or rely on low-temperature devices to produce these elusive frequencies with limited tuning. In 2019, the Capasso Group, in collaboration with MIT and the U.S. Army, developed a prototype that proved that terahertz frequency sources could be compact, room temperature, and widely tunable. The collaboration combined by combing a QCL pump with a nitrous oxide molecular laser for the capability. The current work more than triples the tuning range of that prototype. Among other advances, the new laser replaces nitrous oxide with methyl fluoride, a molecule that reacts strongly with optical fields. “This compound is really good at absorbing infrared and emitting terahertz,” said Arman Amirzhan, a graduate student at SEAS and first author of the paper. “By using methyl fluoride, which is nontoxic, we increased the efficiency and tuning range of laser.” “Methyl fluoride has been used as a terahertz laser for almost 50 years, but it only generates a couple of laser frequencies when pumped by a bulky carbon dioxide laser,” said Henry Everitt, the U.S. Army’s senior technologist for optical sciences, and co-author of the paper. “The two innovations we report, a compact laser cavity pumped by a quantum cascade laser, combine to give methyl fluoride the ability to lase on hundreds of lines." The laser, the researchers said, has the potential to be one of the most compact terahertz lasers ever designed. The team aims to make it even more compact. “A less-than cubic-foot device will enable us to target this frequency range for even more applications in short-range communications, short-range radar, biomedicine, and imaging,” said Paul Chevalier, a research associate at SEAS and lead researcher of the team. “The combination of mature, compact, quantum cascade lasers with molecular laser gain media has resulted in a very robust THz laser platform with a wide range of applications from fundamental research, to THz molecular detection and imaging, THz communications and security, and beyond,” said Timothy Day, senior vice president and general manager of DRS Daylight Solutions and co-author of the paper. The research was published in APL Photonics (www.doi.org/10.1063/5.0076310).
