Objective Broadband terahertz (THz) wave has important applications in many fields, such as material identification, security inspection, optical communication, etc. It has attracted worldwide researchers' attentions in recent years. Two-color femtosecond laser filamentation can produce THz waves with electric field as high as 21 MV/cm and broad bandwidth up to 200 THz, which show significant advantages over other THz wave generation methods. In addition, THz source produced by femtosecond laser filamentation can achieve the long-distance delivery of THz wave. Due to the serious diffraction phenomenon of THz waves and the strong absorption of THz waves by water vapor in the atmosphere, THz wave cannot propagate long distance in air, while by long-distance delivery of femtosecond laser filament, THz wave can be produced in the remote locations, providing a feasible way for the related application of THz waves in the atmosphere.

For the two-color femtosecond laser filamentation, the time delay, spatial walk-off, and polarization state of the fundamental and second harmonic laser beams strongly affect the generation efficiency of terahertz wave. In this paper, we demonstrate experimentally and numerically that when the polarization directions of the fundamental and the second harmonic beams are adjusted to be identical by using the dual-wavelength wave plate, obvious pulse separation will be generated due to the group delay dispersion and birefringence of the dual-wavelength wave plate. Therefore, simply adding a dual-wavelength wave plate will decrease the generation efficiency of THz wave, even a zero-order dual-wavelength wave plate is employed. By using a zero-order dual-wavelength wave plate and a dispersion compensating plate, the effect of pulse separation can be reduced, thereby increasing the terahertz generation efficiency. This work demonstrates the role of dual-wavelength wave plate and dispersion compensating plate in increasing the THz wave generation efficiency when two-color femtosecond laser filamentation is employed.

Methods When femtosecond laser pulse passes through a dual-wavelength wave plate, the laser pulse splits into two pulses in the temporal domain due to the polarization dependent group velocity in the wave plate. This paper investigates the influence of the pulse splitting on the generation of terahertz waves in two-color femtosecond laser filamentation both experimentally and numerically. In experiments, we use a β-BBO with a thickness of 100 μm to excite the second harmonic beam of the 800 nm fundamental laser beam. α-BBO crystals with different thicknesses are used to compensate the time delay between the fundamental and second harmonic laser beams. The polarization directions of the fundamental and second harmonic laser beams are adjusted by the dual-wavelength wave plates with different thicknesses. The power of the produced THz wave is obtained by the Golay cell detector. The photocurrent model is used in the simulation process, and the effect of pulse separation on the generation of terahertz wave is investigated by calculating the ionizing radiation process.

Results and Discussions We investigate the THz wave generation by the two-color femtosecond laser filamentation when the multi-order, zero-order or no dual-wavelength waveplate is employed both experimentally and numerically. It is found that the terahertz radiation efficiency using a multi-order dual-wavelength wave plate is lower than that using no dual-wavelength wave plate or a zero-order dual-wavelength wave plate.

According to the simulation results, it concludes that:

1. The THz generation efficiencyincreases by about 120 times when the multi-order dual-wavelength wave plate is replaced by the zero-order dual-wavelength wave plate (Fig.9).

2. The THz generation efficiency in the case using the zero-order dual-wavelength wave plate is about 5.4 times larger than thatwithout any dual-wavelength wave plate (Fig.6 and Fig.9).

According to the experimental results, we can conclude that: 1) The THz generation efficiencyincreases by about 45 times when the multi-order dual-wavelength wave plate is replaced by the zero-order dual-wavelength wave plate. 2) The THz generation efficiency in the case using the zero-order dual-wavelength wave plate is about 2.5 times larger than thatwithout any dual-wavelength wave plate (Fig.10).

Both experimental and simulation results show that the use of a relatively thin zero-order dual-wavelength wave plate can effectively suppress the pulse separation phenomenon (Fig.7) and improve the THz generation efficiency. Therefore, the zero-order dual-wavelength wave plate should be implemented in the THz wave generation by the two-color femtosecond laser filamentation.

Conclusions When femtosecond laser pulse passes through a dual-wavelength wave plate, the laser pulse splits into two pulses in the temporal domain due to the polarization dependent group velocity in the wave plate. This paper investigates the influence of the pulse splitting on the generation efficiency of terahertz waves by two-color femtosecond laser filamentation both experimentally and numerically. It is found that the use of dual-wavelength wave plates to control the polarization states of the fundamental and the second harmonic beams will cause the phenomenon of pulse separation and decrease the generation efficiency of terahertz waves. Replacing the multiple-order dual-wavelength wave plate by the zero-order one together with a dispersion compensator can weaken the effect of the pulse splitting and enhance the terahertz generation efficiency.