THz generation by filamentation of two-color femtosecond laser pulses

Summary form only given. Terahertz (THz) sources have attracted an increasing interest for applications in time-domain spectroscopy and security screening for many years [1]. Recently, two-color laser filaments were reported to be efficient tools for the remote generation of THz radiation over long distances in gases [2]. Despite numerous experimental results, there is, however, no clear understanding of the dominant physical mechanism driving THz generation in filamentation regime, i.e., either rectification by four-wave mixing (FWM) or plasma currents generated by tunnel ionization. Here, we examine THz generation by two-color filamentation in argon by means of fully three-dimensional (3D) numerical simulations for different pump pulse energies, durations and central wavelengths. Evaluating the THz spectra from either Kerr or plasma sources reveals that photocurrents are dominant in THz generation during filamentation at clamping intensity. Our numerical results reproduce several experimental features, such as the growth of the THz yield with the pump pulse energy or with the pulse duration.Using the unidirectional pulse propagation equation (UPPE) model [3] with a tunnel ionization rate, we simulate the Ilamentation of various pump pulses with 12 % of their energy converted to the second harmonic (SH). Figure 1(a,b) shows the evolution of a 800-nm Gaussian pump pulse (red curves), a perturbed Supergaussian one (green curves), both with 20 fs duration and ~ 7 mJ energy, and a Gaussian pump pulse with doubled duration and energy (blue curves). Using a noisy Supergaussian proIle favors the emergence of multiple Ilaments, which spread out the pulse energy and lower the plasma response. The THz Ield amplitude reaches 2 GV/m [see Fig. 1(b)´s inset] and the THz yield attains ~ 1.5 μJ for the 20-fs pulses, which corresponds to 25 times the THz energy of a small Ilament created with a 270 μJ, 20-fs pump (not shown). This conIrms the quasi-linear growth o- THz energy with the pump-pulse power reported in [2]. In addition, the 40-fs Gaussian pulse develops several temporal peaks and thus triggers more ionization events associated with higher peak density. These features lead to an almost fourfold increase of the THz signal compatible with [2]. To understand the origin of the THz signal, we plug the on-axis temporal proIles of the propagated pulses into either the Kerr term or the plasma term of the UPPE model, which are potential sources for THz generation. The resulting spectra, shown in Fig. 1(c), evidence that, for the Gaussian pulses, plasma currents prevail over FWM contributions at clamping intensity.In conclusion, 3D numerical simulations conIrm the efIcient generation of THz radiation over long distances by two-color laser Ilaments for various pump parameters. Local THz spectra computed from Kerr and plasma sources show that the photocurrent mechanism prevails in producing THz emission at clamping intensity. Experimental features are reproduced and we identify typical spectral signatures of Kerr-driven and plasma-driven THz generation, which could be used as diagnostics in further experiments [4].