theory*

In this work, we present both a new theoretical description and experimental validations of molecular gas optically pumped far infrared (OPFIR) lasers that have much higher efficiency than traditional OPFIR lasers with much more compact volumes. First, we demonstrate a ¹³CH₃F OPFIR laser that achieves 10× greater efficiency and 1000× smaller volume than comparable commercial lasers. To fully understand this, we developed a new ab-initio theory that matches experiments quantitatively, within experimental uncertainties with no free parameters, by accurately capturing the interplay of millions of degrees of freedom in the laser—unlike previous OPFIR-laser models involving only a few energy levels that failed to even qualitatively match experiments at high pressures. Our model is general enough to capture the lasing behaviors of many other gases. By optically pumping N₂O with a QCL instead of a CO₂ laser, we predict a new regime of broad frequency-tunability for this traditionally narrow-band THz source. These results offer the possibility of a new generation of compact, frequency-tunable THz sources.