Laser induced air-plasma is a simple and convenient source to generate intense terahertz (THz) waves through the ionization of gaseous molecules. However, the lack of enhancement and efficient frequency modulation methods for this source hinders its widespread application. Recently, we have shown that the two crossed two-color air-plasmas can significantly increase the THz wave generation or select the required THz emissions under the suitable time delay between the two identical ones (the same intensities of the two-color lasers and the same relative phase between them). In the latest research, we change one of the input laser pulses to generate the two crossed air-plasmas from the twocolors (800 nm + 400 nm) to only the single 800 nm. Interestingly, unlike the results we reported in our previous paper, no spectral interference fringe is observed in the THz spectrum, suggesting that it exhibits no interference between THz waves radiated from single-color and two-color laser generated filaments. For THz waves radiated by single-color laser generated plasma, both the absorption and radiation are related to the plasma frequency. However, the THz waves generated from the two-color laser fields appear to be quite different from the single-color one, thus leading to the absence of spectral interference. Research into the underlying mechanisms will reveal more on the plasma-based THz technology, promising to increase the THz intensity yield and deepening our understanding of the ultrafast dynamics.
Terahertz wave can be produced from two-color laser induced filaments, whose length is one of the critical factors that determine the output yield. Here, we propose a new method to generate the axial-length controllable filament using a liquid crystal spatial light modulator (LC-SLM). Previous studies have employed optical elements such as phase plates and deformable mirrors to optimize and control the filamentation process, but these methods lack flexibility. Our approach enables the programmable modulation of the phase distribution and spatial distribution using SLM. By loading a diffraction phase pattern, it enables convenient manipulation of the length, diameter, electron density of the plasma filament, while also providing advantages in flexibility and feedback optimization for improving terahertz output. Moreover, we demonstrate how the fan-shaped segmentation phase method can be applied to generate air plasma and control the length of the filament to enhance terahertz output. Our method can overcome the saturation effect induced by plasma clamping, therefore improve the terahertz output. To verify its feasibility, we establish a far-field distribution model for terahertz yield based on the photo-current model and investigate the effect of filament length on terahertz output under different conditions. The simulation results show that our method can significantly improve terahertz output at different frequencies and initial phase of the two-color field. Overall, our approach offers a relative simple and effective way to control the length of plasma filaments and enhance terahertz output using SLM technology, it has great potential applications in terahertz biophysics.
Two-color laser induced filament can emit coherent broadband terahertz (THz) radiation. Surprisingly, when using a relative long focal length of lens to generate a long plasma filament, the THz wave frequency of the radiation will peak at around 1 THz. This can be found in the measurements by using different methods, such as the electro-optic sampling, fluorescence, air-biased coherent detection and interferometer method, however this phenomenon and its underlying reason have never been discussed before. In this paper, the observed THz peak frequency from the long filament can be interpreted by the phase match between the femtosecond laser and the generated THz emissions. These findings gain more insights into the generation mechanism of THz wave, and can help to boost the THz emission yield and tune the central frequency of THz emissions by manipulating the plasma.
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