Cascaded amplification of air lasing

Air lasing initiated by femtosecond laser filamentation holds exciting prospects for remote sensing. However, low output energies restrict its practical applications. We demonstrate a cascaded amplification strategy that breaks this critical barrier. By implementing quadruple-pass focusing of 5.7-millijoule, 800-nanometer femtosecond pulses in nitrogen, we achieve a 40-fold enhancement in the energy of N 2 + lasing, yielding an output of approximately 4.4 microjoules. This multipass configuration also reduces the lasing threshold by nearly 70%, facilitates spectral broadening of pump pulses, and compresses the pulse duration of N 2 + lasing. Capitalizing on these combined effects, we developed a single-beam coherent Raman spectroscopy, which enables simultaneous detection of multiple species and exhibits high sensitivity at the parts-per-million level. This work successfully addresses the energy bottleneck of air lasing using only commercially available femtosecond lasers and provides a promising approach for standoff chemical analysis.