Enhancement Of Third-order Harmonic Generated By Noncollinear Focused Dual Intense Laser Pulses In N₂

The propagation of intense laser pulse in transparent matter causes strong nonlinear effects, such as four-wave mixing, self-focusing, self-phase modulation (SPM), third-order harmonic generation(THG) etc.. An intense femto-second laser pulse can propagate many Rayleigh lengths in gas. Propagation of high-power pulse over such long distance is known to be the result of the dynamic balance between the Kerr self-focusing due to the nonlinear effects in gaseous media, diffraction, and the defocusing effect of plasma formation due to multiphoton ionization. The dynamic equilibrium between the self-focusing and defocusing in the channel can support a long plasma channel in the interaction of the intense laser pulse and gaseous media. This is beneficial to get high harmonic conversion efficiency in the interaction, beause the conversion efficiency between the fundamental wave and harmonic wave does not only depend on laser intensity, but also on interaction length. Extending of interaction length owned to the plasma channel is a promising way to achieved high conversion efficiency. THG is believed a promising method for achieving high brightness, coherent ultraviolet and x-ray light sources. Therefore, much attention has been paid to THG by the self-guided femto-second pulses in gaseous media.Recently, THG has been observed and studied extensively, but these researches were mainly on THG of plasma channel produced by single focused femto-second pulse and THG drived by dual collinear pulses in gaseous media.In this letter, we focus our attention on THG of a plasma channel generated by two noncollinear focused high-power femto-second laser pulses in N₂ cell. The laser was operated at 10HZ and delivered pulses of a 10mJ maximum energy, 60fs FWHM (full width at half maximum) minimum duration, with spectrum centred at 820nm. In the experiment a interesting phenomena, sharp enhancement of TH emission is observed. The conditions which affect the enhancement of TH, such as gas pressure, relative delay of two pulses, chirp of fundamental wave, respective energy of two pulses are investigated. An optimized condition of enhancement of TH is found, under which 200 times enhancement of TH is accomplished. And the condition is 1.4atm of gas pressure, 1.7×10⁴ fs² of the fundamental wave chirp and less than 33fs of the two pulses relative delay.TH emission, as the lowest harmonic generation in gaseous media, results on such aspect achieved in this paper is availabe to understand dynamic process of high-order harmonic. And it's beneficial to high-order harmonic generation of gas jet in vacuum in future, as well.