Efficient Generation of Third Harmonic Radiation in Air Filaments: a Revisit

In this paper we revisit the third harmonic generation from a femtosecond laser filament in air and its significant enhancement (~ 100 times) with a intercepting pump pulse, which has been reported very recently. The enhanced third harmonic is studied as a function of the pulse polarization, time delay between the pump and signal pulses, laser pulse energy, etc. We provide an explanation for the enhancement of third harmonic based on a quenching of interference effects present near filamentation threshold. Simulations based on a two-color propagation code reproduce well the experimental observations and confirms our interpretation. Introduction High-harmonic generation in gases with intense laser pulses has stimulated many research interests because it provides a simple method for achieving laser radiation in UV ~ XUV domain [1-8]. In particular, several papers reported that an efficient conversion (~ 0.2%) from a femtosecond laser pulse to its third harmonic (TH) can be achieved in ambient air or noble gases, where the pulse undergoes filamentation [9-14]. The subject has been pioneered by Aközbek et al. [9]. By including cross phase modulation effects in a two color nonlinear propagation model, these authors showed that the fundamental wave and its third harmonic become quasi phase-locked over the filament length. For a comprehensive review of the filamentation of intense femtosecond laser pulses in air, see Refs. [15, 16]. Very recently, the subject has been revived by reports that third harmonic generation in air filaments could be increased by 2 orders of magnitude by using a second intense IR pulse (hereafter the pump pulse) to intercept the filament [17-19]. However, the experimental results reported by various groups differ in several aspects. The main discrepancy concerns the effect of time delay d between the filament forming pulse (hereafter the signal pulse) and the intercepting pulse. In Ref. [17] and [19], the authors report a large third harmonic enhancement subsisting for time delays d of several tens of picoseconds. By contrast, in Ref. [18] the authors find that the effect only occurs during the time overlap between pump and filament forming signal pulse. Another discrepancy concerns the influence of the polarizations of the two laser pulses. The authors of references [17, 19] observe an enhancement whether both lasers are parallel or  Corresponding author : [email protected] ha l-0 08 52 02 5, v er si on 1 4 Se p 20 13 Author manuscript, published in "Optics Communications 284, 19 (2011) 4706" DOI : 10.1016/j.optcom.2011.05.050