Anti-correlated plasma and THz pulse generation during two-color laser filamentation in air

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Scheme of the experimental setup. Credit: Ultra-fast science

The strong terahertz (THz) waves generated by femtosecond laser pulse-induced gas plasma have attracted much attention due to its ultra-wide spectral bandwidth, high electric field strength, and no property damage threshold. However, the abundant and multidimensional cross-scale light-matter interactions during filamentation intertwine, interact and constrain each other, which not only questions the physical mechanism of THz radiation, but also limits the optimization techniques of THz wave generation.

Although the THz wave generated by the two-color laser field filamentation is most often cited as positively correlated with the air plasma density, the research conducted by the group of Prof. Weiwei Liu of Nankai University and the group of Prof. Hiroaki Misawa of the university has van Hokkaido demonstrated a negative correlation between the radiated THz intensity and the plasma density during 1600 nm + 800 nm two-color laser filamentation. The electron capture of the excited nitrogen gas molecule in its excited state is believed to be the cause of the decreased plasma density, while the increased THz radiation is attributed to the higher electron drift rate.

By tuning the time delay between 1600 nm and 800 nm lasers, the plasma density is measured and a minimum value is found that is almost zero delay. The negative correlation between the plasma density and the THz wave radiation intensity further reveals that the THz radiation intensity shows maximum at the minimum plasma density.

The electronic energy level of the nitrogen molecule is modeled using the DFT method. Since the photon energy of a 1600 nm laser is 0.78 eV and the vibrational energy of a nitrogen molecule is 0.2 eV, a 1600 nm laser can cause resonance when the electron energy gap is about 0.78 ± 0.2 eV. When nitrogen gas is excited simultaneously by a 1600 nm and 800 nm two-color field, the electron is pumped to the LUMO+7 energy level.

  • Anti-correlated plasma and THz pulse generation during two-color laser filamentation in air

    (a) The relationship between the plasma density of the filament and the time delay of the two-color field (Δt1); (b) The generated THz efficiency as a function of Δt1 in the experiment is shown as the black solid line, while the simulated relative THz intensities from the empirical model is shown as a blue dashed line. The density of free electrons with different delays was measured on the axis of the filament at z = 2.7 mm and shown as a red dotted line. Credit: Ultra-fast science

  • Anti-correlated plasma and THz pulse generation during two-color laser filamentation in air

    (a) Calculated electronic energy level of nitrogen molecule; (b) Variation of the net current Jnet as a function of Δt1. Credit: Ultra-fast science

Moreover, the energy difference between LUMO+6 and LUMO+7 corresponds to the energy of 1600 nm photon. Therefore, a 1600 nm laser can induce resonance between these two energy levels to capture electrons, leading to a decrease in plasma density without delay. It is also noted that although the free electron density in the plasma has a minimum value when Δt1 is small, it is still possible for Jnet- to reach the peak, emitting the highest THz pulse energy. It has been confirmed that the drift speed accelerated by the two-color laser field plays a dominant role during the generation of THz pulses.

The research results not only clarify the relative importance of electron drift velocity and plasma density in THz radiation from filaments, but also point out the limitations of the traditional photocurrent model. The results are of great importance for optimizing the two-color laser filamentation to generate strong THz waves. In addition, new questions are raised about the optical ionization mechanism in filaments.

The research was published in Ultra-fast science.

Sapphire femtosecond laser filamentation in argon with a repetition rate of 1 kHz

More information:
Zhiqiang Yu et al, Anti-correlated plasma and THz pulse generation during two-color laser filament in air, Ultra-fast science (2022). DOI: 10.34133/2022/9853053

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