A New High-efficiency Regime for Gas Phase Terahertz Lasers

Intellectual Merit
Optically pumped far-infrared (OPFIR) lasers are one of the most powerful continuous-wave THz sources. However, such lasers have long been thought to have intrinsically low efficiency and large sizes. Moreover, all previous theoretical models failed to predict even qualitatively the experimental performance at high pressures.

MIT MRSEC researchers have developed a new model that captures nearly the full physics of the lasing process and correctly predicts the behavior in the high-pressure regime. Validated against experiments, the model shows that nearly all previous OPFIR lasers were operating in the wrong regime, and that 10x greater efficiency is possible by redesigning the THz cavity with 1000x smaller volume.

Schematics of the compact OPFIR laser cavity. The cavity is a copper tube with a movable back wall used to tune the cavity frequency to match the laser gain, pumped with an IR laser through a pinhole in the front window. 13CH3F molecule gas is filled in the cavity.

Broader Impact
With the ability to model the full physics of OPFIR lasers accurately, many further discoveries await the extension of these approaches to new gases and new cavity designs. For example, even more compact OPFIR lasers can be built by replacing the large-size CO2 pump laser with quantum cascade laser. In addition, the frequency tunability of the quantum cascade laser makes it possible to achieve multiple laser lines within one compact OPFIR laser.

Total quantum efficiency (QE) of commercial OPFIR lasers and our Manley—Rowe compact OPFIR laser, normalized by the (MR) limit on QE. The experimentally demonstrated laser achieves a QE that is 29% of the MR limit which improves to 39% after cavity optimization. Both are 10x better than the best commercial laser at the same frequency, while being 1000x smaller.