Enhanced Population on Ionic Excited States by Synchronized Ionization and Multiphoton Resonance

TL;DR

This study reveals a new efficient mechanism for exciting N_2^+ ions using synchronized ionization and multiphoton resonance with femtosecond lasers, significantly enhancing air lasing efficiency at around 1000 nm.

Contribution

It introduces a novel ionic excitation mechanism involving dynamic three-photon resonance synchronization, improving air lasing performance and reducing energy thresholds.

Findings

Maximum N_2^+ lasing at ~1000 nm pump wavelength

Pump-energy threshold reduced fivefold at optimal wavelength

Dynamic resonance synchronization enhances ionic population transfer

Abstract

We study population distributions and lasing actions of N_2^+ driven by femtosecond lasers with various wavelengths, and uncover an efficient ionic excitation mechanism induced by synchronized ionization and multiphoton resonance. Our results show that the strongest N_2^+ lasing appears around 1000 nm pump wavelength. At the optimal wavelength, the pump-energy threshold for air lasing generation is reduced by five folds compared with that required by the previous 800 nm pump laser. Simulations based on the ionization-coupling model indicate that although the Stark-assisted three-photon resonance can be satisfied within a broad pump wavelength range, the optimal pump wavelength arises when the dynamic three-photon resonance temporally synchronizes with the ionization injection. In this case, the ionic dipoles created at each half optical cycle have the same phase. The dipole phase locking promotes the continuous population transfer from ionic ground state to the excited state, giving rise to a dramatic increase of excited-state population. This work provides new insight on the photoexcitation mechanism of ions in strong laser fields, and opens up a route for optimizing ionic radiations.