Spatio-temporal theory of lasing action in optically-pumped rotationally excited molecular gases

Abstract

We investigate laser emission from optically-pumped rotationally excited molecular gases confined in a metallic cavity. To this end, we have developed a theoretical framework able to accurately describe, both in the spatial and temporal domains, the molecular collisional and diffusion processes characterizing the operation of this class of lasers. The effect on the main lasing features of the spatial variation of the electric field intensity and the ohmic losses associated to each cavity mode are also included in our analysis. Our simulations show that, for the exemplary case of methyl fluoride gas confined in a cylindrical copper cavity, the region of maximum population inversion is located near the cavity walls. Based on this fact, our calculations show that the lowest lasing threshold intensity corresponds to the cavity mode that, while maximizing the spatial overlap between the corresponding population inversion and electric-field intensity distributions, simultaneously minimizes the absorption losses occurring at the cavity walls. The dependence of the lasing threshold intensity on both the gas pressure and the cavity radius is also analyzed and compared with experiment. We find that as the cavity size is varied, the interplay between the overall gain of the system and the corresponding ohmic losses allows for the existence of an optimal cavity radius which minimizes the intensity threshold for a large range of gas pressures. The theoretical analysis presented in this work expands the current understanding of lasing action in optically-pumped far-infrared lasers and, thus, could contribute to the development of a new class of compact far-infrared and terahertz sources able to operate efficiently at room temperature.

High-resolution THz gain measurements in optically pumped ammonia

“…The transparency when the stimulated emission is equal to the absorption is about 1 mW for both lines. The gain dependence on the pump power is linear up to approximately 13 mW where probably depletion of the ground states starts, probably due to the slow vibrational relaxation of ν 2 = 1(s) states after stimulated emission [22]. The slope of the linear part of the gain for the (3,3) line is 0.08 mW -1 and 0.05 mW -1 for the (4,4) line.…”

“…Figure 5 (right) shows the THz gain as a function of the pressure. The gain increases linearly with the pressure due to the higher number of molecules in the volume up to a certain point where collisions between molecules and/or cavity walls cause increased nonradiative relaxation [22] and exponentially decrease of population inversion. The optimal pressure is between 20 -30 µbar and 25 -45 µbar for (3,3) and (4,4) line respectively.…”

Scalable numerical approach for the steady-stateab initiolaser theory

“…More complicated gain profiles, line shapes, and other material properties can easily be incorporated into our approach as well. SALT can, e.g., be coupled to a diffusion equation in order to model the migration of excited atoms in molecular-gas lasers [58,59]. Based on the mathematical relation of the multimode lasing equations to incoherent vector solitons (Appendix E), we believe that numerical methods commonly used in soliton theory can also be adopted to efficiently solve the multimode SALT equations.…”

Design and Signature Analysis of Remote Trace-Gas Identification Methodology Based on Infrared-Terahertz Double-Resonance Spectroscopy

“…Indeed, the specificity of our DR technique depends on the rarity of such coincidences and the associated idiosyncrasies of a specific molecule's rotational and vibrational spectra. Although additional coincidences emerge at atmospheric pressure because of the increased collisional linewidths, this can be an advantage if some of the additional coincidences produce DR signatures within more favorable THz propagation windows [15][16][17][18].…”