Three-step model for third-harmonic generation in air by nanosecond lasers

Abstract

Understanding the phenomena involved in harmonic generation in plasmas by high-power pulsed lasers is a paramount task for developing new techniques for generation of coherent radiation in ultrashort bursts. Although first experiments on harmonic generation involved nanosecond lasers and inspired further interest in the subject, numerical simulations on harmonic generation are currently mainly oriented toward ultrashort fs lasers. This paper presents a combined theoretical–experimental approach to the generation mechanisms and the properties of third-harmonic (TH) radiation generated by infrared nanosecond laser pulses in air-breakdown plasma. The paper indicates that, at the microscopic level, the generation of TH can be described by a three-step model, which involves breakdown of nitrogen molecules in the air. First, the nitrogen molecules undergo cascade-impact ionization; then, the ionized molecules are quasi-resonantly excited through three-photon absorption; in the third step, the nitrogen molecules de-excite to the fundamental level with associated emission of TH radiation. At the macroscopic level, the three-step model is implemented considering that the breakdown plasma is a conductive nonlinear medium whose third-order susceptibility and complex conductivity depend upon the cubic root of the driving laser intensity. The 2D numerical simulations performed in the frame of this model are in good agreement with the experimental data in terms of TH generation efficiency, collimation, and polarization of TH radiation, indicating the validity of the theoretical model presented here. The model enables realistic calculations with affordable computing power for prediction and control of the TH generation process driven by nanosecond laser pulses. The results are important from the fundamental and practical points of view, thus providing an efficient tool for prediction of nonlinear optical phenomena in laser-produced plasmas and for noncontact diagnosis of harmonic-generating plasmas.