Ultrafast Spatiotemporal Dynamics of Terahertz Generation by Ionizing Two-Color Femtosecond Pulses in Gases

Abstract

We present a combined theoretical and experimental study of spatiotemporal propagation effects in terahertz (THz) generation in gases using two-color ionizing laser pulses. The observed strong broadening of the THz spectra with increasing gas pressure reveals the prominent role of spatiotemporal reshaping and of a plasma-induced blueshift of the pump pulses in the generation process. Results obtained from (3+1)-dimensional simulations are in good agreement with experimental findings and clarify the mechanisms responsible for THz emission.

Theory of terahertz emission from femtosecond-laser-induced microplasmas

“…(21), where E 2 →Ẽ 1 and ι 2 → ι 1 as defined in Eq. (16). Then, by analyzing symmetry properties completely analogous to above, it turns out that for driving pulses with a Gaussian beam profile, ι 1 is symmetric, and thus cannot excite a non-radiative solution E 1,d of the form in Eq.…”

Intense ultrashort terahertz pulses: generation and applications

“…Scattering of the ionized electrons by the atoms of the medium disrupts their original trajectories, which results in a coherent bremsstrahlung echo at THz frequencies. Generally, the full treatment of spatio-temporal propagation in the plasma is quite complex and needs to take into account effects such as plasma-induced phase modulation and defocusing [61]. For a complete description of THz generation in gas plasmas highly sophisticated simulations will be required.…”

Boosting Terahertz Generation in Laser-Field Ionized Gases Using a Sawtooth Wave Shape

“…This 3D model was successfully tested against experimental data for THz generation from two-color pulses [22]. We use an adapted version of the unidirectional pulse propagation equation [31] for linearly polarized pulses…”

“…In such two-color pump setup, free electrons produced by tunnel ionization acquire a nonzero drift velocity and generate a quasi-dc current which is responsible for THz emission [19]. The mechanism underlying THz generation in gases is intrinsically related to the optically induced stepwise increase of the free electron density near the extrema of the ionizing optical field [15,16,22]. An easy explanation is that the Fourier transform of a steplike function in time has its maximum near the zero frequency.…”

“…Recently, large-sized organic crystals were used to deliver THz pulses with GV=m electric field strength, and a conversion efficiency of about 1% was demonstrated [10]. The second method is based on focusing a femtosecond pulse together with its second harmonic into a gas cell and create a plasma [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In such two-color pump setup, free electrons produced by tunnel ionization acquire a nonzero drift velocity and generate a quasi-dc current which is responsible for THz emission [19].…”