Extreme THz fields from two-color filamentation of midinfrared laser pulses

Nonlinear THz photonics is probably the last frontier of nonlinear optics. The strength of both the electric and the magnetic fields of these ultrashort low-frequency light bunches opens the way to exciting science and applications. Progress in the field though is slow because of the deficiency in suitable sources. Here we show that two-color filamentation of midinfrared $3.9\mu \mathrm{m}$ laser pulses allows one to generate single-cycle THz pulses with multimillijoule energies and record conversion efficiencies. Moreover, the focused THz peak electric and magnetic fields reach values of GV/cm and kT, respectively, exceeding by far any available quasi-dc field source today. These fields enable extreme field science, including into other, relativistic phenomena. In addition, we elucidate the origin of this high efficiency, which is made up of several factors, including a mechanism where the harmonics produced by the midinfrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation.

Figure 1

Comparison of energy and electric field of our source with present state-of-the-art tabletop THz sources: Clerici2013 [17], Oh2014 [10], Shalaby2015 [8], Kuk2016 [11], Fülöp2014 [7], and Vicario2014 [6].

Figure 2

(a) Peak intensity $I_{\max }$, (b) peak fluence $F_{\max }$, (c) peak plasma concentration $n_{e\max }$, and (d) plasma concentration integrated over radius, $D_{e\max }$, vs propagation distance $z$ for the 3.9 $\mu \mathrm{m}$ [blue (dark-gray) line] and the 0.8 $\mu \mathrm{m}$ [orange (light-gray) line] two-color laser pulses.

Figure 3

Dependence of integrated pulse power spectrum $S$ on propagation distance $z$ and frequency $f$ for (a) 3.9 $\mu \mathrm{m}$ and (b) 0.8 $\mu \mathrm{m}$ two-color laser pulses. The black dashed line separates the harmonics of fourth order and higher.

Figure 4

(a) Energy of THz pulse $W$ vs propagation distance $z$. (b) THz electric field vs time $t$ at $z=190\mathrm{mm}$. THz pulses are generated during two-color filamentation of 3.9 $\mu \mathrm{m}$ pulses in the case of air with [orange (light-gray) line] or without [blue (dark-gray) line] high harmonics generation.

Figure 5

Dependence of pulse power spectrum $S$ on frequency $f$ and angle $\theta$ for (a) 3.9 $\mu \mathrm{m}$ and (b) 0.8 $\mu \mathrm{m}$ two-color laser pulses at distance $z=220\mathrm{mm}$.

Figure 6

(a) THz pulse energy $W_{\mathrm{THz}}$, (b) THz conversion efficiency $Q_{\mathrm{THz}}$, (c) estimated peak electric $E_{\mathrm{THz}}$, and (d) magnetic field $B_{\mathrm{THz}}$ of focused THz pulse vs input energy $W$ of 3.9 $\mu \mathrm{m}$ two-color laser pulses.

Figure 7

The real part $n^{\prime }$ of the refractive index $n$ [orange (light-gray) line] and the absorption coefficient [blue (dark-gray) line] vs frequency $f$ for air (a) with and (b) without ${\mathrm{CO}}_2$ gas. The spectrum of the initial 3.9 $\mu \mathrm{m}$ two-color laser pulse is marked by light-gray filling.