A far-infrared laser or terahertz laser (FIR laser, THz laser) is a laser having an output wavelength in the far-infrared or terahertz frequency band of the electromagnetic spectrum ranging from 30-1000 µm (frequency 0.3-10 THz). FIR lasers are used in terahertz spectroscopy, terahertz imaging, and diagnostics for fusion plasma physics. They can identify explosives and chemical warfare substances using infrared spectroscopy or measure plasma densities using interferometry techniques.
Researchers have made a significant step toward bringing terahertz frequencies out of their difficult-to-access section of the electromagnetic spectrum and into common applications. The researchers describe in a recent publication a first-of-its-kind terahertz laser that is tiny, works at room temperature, and can create 120 unique frequencies spanning the 0.25 — 1.3 THz range, significantly more than prior terahertz sources.
The laser has the potential to be employed in a variety of applications, including skin and breast cancer imaging, drug detection, airport security, and ultrahigh-capacity optical wireless communications.
The study was published in APL Photonics by a team from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) in partnership with the DEVCOM Army Research Lab and DRS Daylight Solutions.
This is a game-changing method for producing terahertz radiation. This laser has the potential to become a crucial technology to bridge the terahertz gap for applications in imaging, security, or communications due to its compactness, efficiency, wide tuning range, and room temperature operation.
Professor Federico Capasso
“This is a game-changing method for producing terahertz radiation,” said Federico Capasso, the paper’s senior author and the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “This laser has the potential to become a crucial technology to bridge the terahertz gap for applications in imaging, security, or communications due to its compactness, efficiency, wide tuning range, and room temperature operation.”
Because most terahertz sources are either very bulky, inefficient, or rely on low-temperature devices to produce these elusive frequencies with limited tuning, 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.
In 2019, the Capasso Group, in partnership with MIT and the US Army, created a prototype that demonstrated terahertz frequency sources could be compact, room temperature, and broadly programmable by combining a quantum cascade laser pump with a nitrous oxide molecular laser.
Terahertz radiation is often believed to be electromagnetic radiation with frequencies ranging from around 0.1 THz to 10 THz, corresponding to wavelengths ranging from 3 mm to 30 m. These frequencies are higher than those of radio waves and microwaves, but lower than those of infrared light. Terahertz radiation is sometimes known as submillimeter radiation since its wavelengths are in the range of 0.03 mm to 3 mm, and are frequently less than 1 mm. Also, at least the higher-frequency part of the terahertz region may also be called the far infrared.
The new study more than triples the prototype’s tuning range. The new laser, among other things, replaces nitrous oxide with methyl fluoride, a chemical that reacts strongly with optical fields.
“This molecule is particularly good at absorbing infrared and generating terahertz,” said Arman Amirzhan, the paper’s first author and a graduate student at SEAS. “We enhanced the efficiency and tuning range of the laser by utilizing non-toxic methyl fluoride.”
“Methyl fluoride has been utilized as a terahertz laser for nearly 50 years, but when driven by a bulky carbon dioxide laser, it only generates a couple of laser frequencies,” said Henry Everitt, senior technologist for optical sciences at the US Army and co-author of the paper. “The two advances we disclose, a tiny laser cavity pumped by a quantum cascade laser, combine to enable methyl fluoride to lase on hundreds of lines.”
This laser already has the potential to be one of the smallest terahertz lasers ever created, and the researchers hope to make it even smaller. “With a device that is less than a cubic foot in size, we will be able 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 the team’s principal researcher.
“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 ranging 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 paper co-author.
