Isolating the gain in the nitrogen molecular cation

Abstract

The nitrogen molecular cation is a promising gain medium for the air laser, which could be a powerful tool for remote sensing applications. After nearly a decade of international scientific effort, there are still questions about the gain mechanisms. This thesis explores gain in the nitrogen molecular ion using an experimental configuration that isolates the generation of gain from uncontrolled effects. We use a narrow nitrogen gas jet in vacuum to minimize propagation distance and a probe pulse to measure gain.

We first test the role of inelastic scattering during electron recollision as a mechanism to populate the excited state, and we find that it has a small contribution. Then, we measure gain dynamics by varying the probe delay. We measure short- and long-term gain in different conditions. The long-term gain is consistent with the decay of population inversion due to collisions between electrons and ions in the plasma. The decay is modulated due to rotational wave packets in the states of the ion. Rotational coherence decays on the same timescale as the gain due to the incoherent mixing of states.

We then introduce an additional non-ionizing pulse that interacts with the ion after ionization. The additional pulse changes the experiment to pump-probe spectroscopy on the ion itself. It can manipulate the rotational wave packets. It can also halt or redirect the emission following the probe pulse by coupling the ground lasing state with a middle state. In both cases, it controls the characteristics of gain and emission at a distance. These interactions highlight the role of the middle state, which is an essential ingredient for gain.

Short-term gain appears at low pump intensity when the pump or control pulses overlap the probe pulse. It is consistent with wave mixing due to Raman gain in a V-system.