Researchers have been challenged for decades to produce sources of powerful, compact terahertz sources that can operate at room temperature. Coherent lasing at terahertz frequencies between 0.3 to 10 THz offer the potential to pass safely through materials like plastic and textiles without damaging them, with the ability to detect metal or liquid water, ideal for use in spectroscopy, imaging, security, and radar applications.
A few terahertz sources exist, enabling some types of stand-off detection and imaging applications, but they are bulky, lacking in power, and require low temperature operation that prohibit their use in terahertz wireless communications or tunable radar. Nor are they tunable.
In 2019, researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS; Cambridge, MA) found that excitation of molecular gases with a quantum-cascade laser (QCL) is an efficient way to overcome the long-standing tradeoff between power and room-temperature operation while providing tunability across a broad range of terahertz wavelengths.1 The mechanism of this new class of terahertz sources is excitation of the rotational and vibrational transitions of various gaseous molecules, such as nitrous oxide and methyl fluoride.2
In their latest work, a multi-institution team led by the Capasso Group at Harvard SEAS predicted and demonstrated the lasing of ammonia via a QCL pump. The ammonia QPML offers two lasing mechanisms that make these findings possible: pure inversion (PI) and rotation inversion (RI) molecular transitions.
The experimental setup involves a long and narrow copper tube cavity measuring 50 cm × 4.8 mm, with detectors at rear and front pinholes (see Fig. 1). Harvard research associate Paul Chevalier and colleagues used a tunable external cavity (EC)-QCL from DRS Daylight Solutions (San Diego, CA), tunable in the mid-IR from 920 to 1194 cm-1, to pump the ammonia cavity. A rear tuning mirror was used to adjust the cavity length, while a Golay cell detector collected the entire range of terahertz frequencies through the rear pinhole. A separate gas absorption cell with ammonia in the front of the apparatus monitored the tuning of the desired rotational-vibrational transition.The experiment confirmed the expected lasing frequencies of ammonia due to RI and PI transitions, all exhibiting reasonable power efficiency at powers up to 0.45 mW. But as an added bonus, a Schottky diode detector at the front of the cavity enabled detection of numerous additional emission signals. These surprise simultaneous lasing peaks arose from cascaded transitions below the pumped rotational state, which means ammonia has the potential to generate up to three or four significantly different frequencies at once in QCL pumped molecular lasers (QPMLs). In all, the team observed 34 different lasing transitions spanning 0.762 to 4.46 THz, seven of which are attributed to multiple excitation pathways. The multi-line nature of the ammonia QPML may permit dozens of tunable frequencies from 0.140 to 9.6 THz, some of them simultaneous (see Fig. 2).
Partners on the project included DRS Daylight Solutions, Duke University (Durham, NC), and the DEVCOM Army Research Laboratory (Houston, TX). What’s the U.S. Army’s interest in the advance? The ammonia QPML could be used for variable range radar of the type that can operate at low frequencies at distances up to 1 km or higher frequencies for shorter distances of around 100 m. Such a variable frequency source could enable, for example, a radar that would be both impossible to detect and impossible for bystanders to jam, a useful technology for soldiers on the battlefield.
As a rule of thumb, Chevalier says frequencies below or close to 1 THz are better suited for longer atmospheric propagation than frequencies above 1 THz. So high-frequency QPML operation is particularly useful for short-range propagation.
“The ammonia QPML is not only a promising gain medium,” says Chevalier, “it shows the universality of the QPML concept, in particular to reach frequencies beyond 1 THz for short-range communications, radar, biomedicine, and imaging.” The fact that it can be made compact, perhaps as small as a breadbox, means the ammonia QPML lends itself portability and hopefully down the line, commercial viability.
To reach the market, says Chevalier, the next steps for the technology include further improvement of the optical conversion efficiency, and development of a compact, turnkey system with ease of operation.
“There is no other widely tunable terahertz laser that is compact and works at room temperature,” says Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, also the co-inventor of the QCL. “Many groups, including mine, have been trying for years to use commercial QCLs to create widely tunable lasers in the mid-infrared that operate at room temperature. Somewhat ironically, we have used the same mid-IR QCLs to make a new class of terahertz lasers that are widely tunable, compact, and operate at room temperature. So these developments are extremely significant.”
1. P. Chevalier et al., Appl. Phys. Lett., 120, 081108 (2022); https://doi.org/10.1063/5.0079219.
2. S. Cole Johnson, Laser Focus World, 58, 3, 14-17 (Mar. 2022); see www.laserfocusworld.com/14232942.