Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal, and R. Morandotti

https://journals.aps.org/pra/accepted/8007bY2dMa71016661fa2ff421161a92659e17711

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission can be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed new light on the role of intense transient fields in the coherent emission from photoexcited gas molecules. One of the key phenomena accompanying the focusing of an intense laser pulse in air is the fluorescence from gas molecules.()()()..~[1–3]. Under appropriate excitation conditions, those molecules produce coherent radiation, which is appealing for standoff spectroscopy applications, especially when emitted in the opposite direction of the ionizing laser pulse..().()()..~[4,5]. Owing to nitrogen’s abundance in our atmosphere, one of the most investigated effects has been the ultraviolet (UV) forward emission from photoexcited molecular nitrogen ions, first described as lasing by Luo and co-workers in 2003..()…….()..~[6]. A number of experiments report narrowband coherent emission at 391 nm and 428 nm, corresponding to the transitions ${N_2^{+}(B}^2{\Sigma }_u^{+}(\nu =0))\to {N_2^{+}(X}^2{\Sigma }_g^{+}({\nu }^{\prime }=0,1\left)\right)$ (see also Fig. 1(c) for potential energy diagrams). Such observations show narrowband amplification at the frequencies corresponding to molecular transitions. Those are seeded, for instance, by harmonics of the near-infrared (NIR) pump pulse…()….(..)(()())(..)…….()..~[7–9], white light…..()…()..~[10], or self-seeded by the pump’s supercontinuum itself…..()()()..~[10–12]. Such reports have reinforced the idea that a lasing process is at the origin of the coherent emission. However, the mechanism responsible for the gain is still not fully understood, although it is likely that laser-driven couplings between electronic states in the ion..().().()……..(..)…..()..~[13,14] and rotational effects()..~[15 –21] play a role Here, we report our experimental observation and analysis of the effect of a strong terahertz (THz) electric field on the coherent emission from photoexcited nitrogen ion molecules, as outlined in Figs. 1(a) and 1(b). We show a correlation between the THz field amplitude and the coherent emission at both 391 nm and 428 nm wavelengths. We interpret our results as consequence of the THz electric field induced symmetry-breaking of the gas molecules. This is turn leads to a THz-controlled seeding of the coherent emission by means of the THz induced second-harmonic of the near-infrared pump pulse. In our investigations we used a single-cycle pulse at THz frequencies as a strong electric field. The THz transient was generated by the transverse photocurrents induced through gas ionization via a laser pulse at 1.8 $$m carrier combined with its second harmonic.