Optics Communications 443 (2019) 63–68 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom Third harmonic generation in double-pulse laser induced air plasma Lev Nagli ∗ , Michael Gaft, Yosef Raichlin Ariel University, Physics Department, Ariel 40700, Israel ARTICLE INFO Keywords: Laser induced plasma Third harmonic generation Air laser breakdown ABSTRACT We found a fast hydrodynamic process in air double pulse Laser Induced Plasma (LIP) during the first 50 ns of plasma life. This process leads to strong inhomogeneity in the refractive index of air LIP, disturbance in the Gouy phase shift, and an increase in Third Harmonic Generation (THG). The efficiency of THG strongly depends on the confocal parameter of the focusing beam. The decrease in the focus of the pumping lens, despite the increase in the pump intensity, leads to a decrease in TH intensity. This can be attributed to the change from weak to tight focusing conditions in THG. 1. Introduction Optical harmonic generation in gas plasmas has long been stud- ied [1]. A sufficiently strong focused laser pulse leads to air breakdown and air plasma creation, causing pumping beam harmonics in the air plasma. These harmonics are polarized, spectrally narrow, and collinear with the incident pumping beam. In media exhibiting inversion symme- try, such as gases and laser ablation plasmas, only odd harmonics are generated. The intensity of the third harmonic (TH) is proportional to the cubic pumping pulse intensity and the square of the phase-matching integral J. The phase matching integral depends on the wavevector mismatch Δ and on focusing beam colinear parameter b. For weakly focused beams when  ≫ |  |, || (where |  |, || are the starting and ending points of the harmonic generation, respectively), the phase matching integral J is proportional to the interaction length L = ( −  ). In the opposite case of tight focusing ( ≪ ) in gases with normal dispersion (Δ ≤ 0), J = 0, the TH vanishes in an infinitely long homogeneous medium. Qualitatively, this is attributed to the Gouy  phase shift, which leads to destructive interference of TH generated before and after the focus and disappearance of the THG [1–8]. The inter-pulse delayed double pulse produced plasma with differ- ent optical parameters along a preliminary created plasma plume. As a result, an inhomogeneous optical medium is created in which the phase matching integral no longer vanishes and harmonic generation becomes possible in any b–L ratio [3]. To enhance TH intensity, a different method is proposed and realized by further increasing gas media inhomogeneity [7–10]. The TH intensity can also be significantly enhanced, if there is a multiphoton resonance between the pumping source and the gas’s atoms or molecules [11–15]. It is important to note that THG theories are mostly related to fast laser pumping beam pulses in the ps and fs time scale. Still, harmonic generation is observed in even ns laser pulse durations [1,16–18]. In ∗ Corresponding author. E-mail address: levna@ariel.ac.il (L. Nagli). most of these experiments, LIP is created by DP pumping. One laser pulse produces a LIP due to laser-induced breakdown (LIB) on the sample surface. The other laser pulse, after some delay, crosses the pre- liminary created plasma and generates a TH inside the initially created plasma plume. The detailed mechanism of the pre-plasma effect on THG is discussed in the literature, mostly for fs laser pulses [3,10,11,19]. In this research, we report the results of investigations on THG in ambient air under collinear DP excitation. We found a new maximum at about 50 ns in the dependence of the TH intensity on the inter-pulse delay. In order to understand the existence of this fast maximum, time resolved shadowgraphy [20] was used. Shadow imaging of the DP air excited plasma reveals strong inhomogeneity of the LIP at a shot time delay of ∼50 ns. In our opinion, this inhomogeneity is the main reason for the fast THG maximum in DP air LIP. It should be noted that at the time of publication of our manuscript an article was published with similar results [19]. In this paper, the authors investigated the effect of a previously created ns plasma on THG by fs laser pulse. These results will be discussed later in our paper. 2. Experimental setup Experimental setup is shown in Fig. 1. 2.1. LIP and THG setup For plasma creation, we used a 1064 nm DP Nd: YAG laser (model LQ215-D), with energy from both laser pulses at 100 mJ, an operation repetition rate up to 20 Hz, a pulse duration at FWHM of 7 ns, beam diameter of 5 mm and laser pulse jitter at about 0.1 ns. The first laser is horizontally polarized ( ∥ ), while the second laser is vertically https://doi.org/10.1016/j.optcom.2019.03.033 Received 20 December 2018; Received in revised form 11 February 2019; Accepted 11 March 2019 Available online 14 March 2019 0030-4018/© 2019 Published by Elsevier B.V.