Optically Pumped Terahertz Molecular Laser: Gain Factor and Validation up to 5.5 THz

Abstract

Quantum cascade laser‐pumped terahertz (THz) gas lasers are at the edge of revolutionizing THz science where powerful yet tunable sources have long been lacking. Maybe one of the last remaining drawbacks to a wider use of these instruments lies in the lack of available databases of potentially lasing transitions for users. A new figure of merit, the molecular gain factor is proposed, that allows to discriminate transitions by their lasing potential. Using this factor, catalogs of THz laser lines of ammonia, both 14NH3 and 15NH3, up to 10 THz are reported. Demonstration of the use of these two catalogs, and of the pertinence of the molecular gain factor, is made by experimentally observing 32 laser lines of 14NH3 and 5 lines of 15NH3 up to 5.5 THz. Prospects to generalize the use of this molecular gain factor to a wide range of molecules are discussed.

THz gain measurements in optically pumped ammonia

“…In order to design a laser, experimental values of the gain for each transition are needed even if the gain factor can be estimated using databases [4]. Gain values were previously measured for two transitions [5] and showed that the gains were the highest measured in a continuous wave pumped gas.…”

“…Figure 4 shows a summary of the measurement of the gain of six pure inversion transitions of NH3 for the same pump power density Pp and pressure (20 µbar) versus the molecular gain factor Gm defined in [4] and calculated using the HITRAN database. The red line shows a fit considering a relaxation rate Γ of 2.5x10 -5 s -1 .…”

“…NH3 has several advantages for optical pumping: strong static dipolar moment, small molecular mass, high vapor pressure and strong absorption near 10 µm. By identifying the MIR/THz transitions pairs we have recently shown that optical pumping of NH3 will potentially generate a huge quantity of new THz frequencies [4].…”

Generation of high repetition rate THz radiation at the mill-watt-level via optical rectification in an enhancement cavity

“…However, generating and detecting THz radiation is still a challenging task due to the lack of efficient and compact sources and detectors. At present, different approaches for THz generation have been explored, such as optically pumped molecular gas terahertz laser [4], quantum cascade laser [5], photoconduction [6], and free-electron lasers [7]. Each of those methods presents limitations mainly related to costs, dimensions, environmental working parameters, and thus in general low applicability.…”