A collaboration between the research groups of Henry O. Everitt of the US Army and Federico Capasso of Harvard University has demonstrated a tunable, bright, continuous-wave terahertz source. As reported recently in Science, this high-power, continuous-wave source could be used in several spectroscopic applications.

Until recently, there have been few sources of tunable, coherent terahertz radiation. One of the first methods for producing high-power, continuous-wave terahertz radiation was by converting mid-infrared light from a carbon dioxide (CO₂) laser to the terahertz region. The mid-infrared light from the CO₂ laser was used to excite rotational-vibrational transitions in gas molecules, which emitted terahertz light upon relaxation. Because the tunability of the CO₂ laser was limited, other methods for producing terahertz radiation became more popular.

Despite the shift away from molecular terahertz lasers, Everitt still saw an opportunity for this technology if a tunable pump laser could be found. The transitions between vibrational levels within a molecule typically lie within the infrared region of the electromagnetic spectrum. Each molecular vibrational level contains many rotational sublevels; transitions between rotational sublevels fall within the terahertz spectral region. Previous work by Everitt and other collaborators predicted that if a powerful, continuous-wave source could target a single transition between different vibrational levels, the resulting population inversion would lase terahertz radiation corresponding to the smaller difference between neighboring rotational sublevels. Tuning to different transitions between vibrational levels would generate different frequencies of terahertz radiation.

Simultaneously, quantum cascade lasers (QCLs) have matured to be able to produce high-power, continuous-wave, mid-infrared light. Quantum cascade lasers are semiconductor lasers that produce mid- to far-infrared light with high degrees of tunability. QCLs have been a “huge deal,” says Capasso, who pioneered this technology in the 1990s at Bell Laboratory. “QCLs, now commercially available, opened up the infrared region of the spectrum to many applications.”

In the current study, Everitt and Capasso combined their previous work to create a QCL-pumped molecular laser (QPML) that produces continuous-wave terahertz radiation using nitrous oxide (N₂O) gas. Using their novel set-up, light from the QCL was tuned to a specific N₂O resonance and then amplified in a laser cavity filled with N₂O. The laser cavity was just over 10 cm in length, significantly shorter than previous gas-cavities used in the earlier carbon-dioxide-pumped molecular laser terahertz sources.

“They introduced a frequency-tunable, bright source of terahertz radiation based on QCL-pumped molecular lasers,” says Manijeh Razeghi of Northwestern University, who was not associated with this study.

Everitt and Capasso have many ideas for further developing the QPML beyond this first experimental demonstration. “One of the next steps is to design an appropriate cavity that maximizes this performance,” says Everitt. Both researchers would also like to explore some of the other gases mentioned in their recent publication. Capasso says that it should be possible to move up to higher frequencies—to a few terahertz. Razeghi anticipates that “laser lines spanning more than one terahertz with output power expected to reach up to one milliwatt are possible from many molecular gases pumped by QCLs.” 

Several spectroscopic applications may benefit from this new terahertz source. While some terahertz spectroscopic studies rely on short pulses of terahertz radiation, the high-power, continuous-wave radiation from a QPML could be preferable in certain applications. “One exciting spectroscopic application for the QPML is probing the interstellar medium,” says Everitt, “to detect faint signals from cold gases there.” The QPML could also be used to characterize the terahertz spectrum of different materials. Databases of spectral signatures for different materials exist in other regions of the electromagnetic spectrum. According to Capasso, “The molecular laser—which until now was limited in scope and application because of power and very limited tunability and low efficiency—may have the same impact in the terahertz as the QCL had in the mid-infrared.”  

Read the abstract in Science.