Low-threshold efficient N2+ lasing driven by sub-cycle soliton dynamics in a hollow waveguide

Here, we demonstrate population dynamics in nitrogen molecular ions (N2+) driven by sub-cycle soliton dynamics. High-order-soliton compression of a 12-fs, 10-μJ-level pump pulse in a N2-filled tapered hollow capillary forms a sub-cycle asymmetric transient that tunnel-ionizes N2 to N2+ and, through direct single-photon resonant excitation, creates population inversion between the ground X2Σg+ state and the excited B2Σu+ state—a process distinct from the widely adopted three-state coupling picture—at unexpectedly low pump energy.

Experimentally, we obtain 100-nJ-level N2+ lasing pulses at 391 nm with conversion efficiencies up to 3.3 × 10-3 and pump energies below 50 μJ, improving both efficiency and threshold by more than one order of magnitude over prevailing filamentation-based schemes. Our results establish sub-cycle manipulation of ionic populations as a powerful approach that bridges two largely separate fields (sub-cycle soliton dynamics and N2+ lasing), and paves the way for narrow-band, high-beam-quality lasing pulses that may find wide applications in advanced spectroscopy and nonlinear pump-probe experiments.