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.
The Optical Parametric Oscillator (OPO) is a simple and effective method to generate desirable and specific wavelength. The OPO laser is small, portable and has high efficiency, low threshold and can be operated at room temperature. In this paper, we report an electrically controlled tunable optical parametric oscillator operating from wavelength of 3.0 to 6.7 μm. The nanosecond OPO is based on BaGa4Se7 (BGSe) crystal and pumped by a 10 ns 1.064 μm electro-optic Q-switched Nd:YAG laser. The output mid infrared idler wave can be tuned from 3.0 μm to 6.7 μ with pulse energy as high as 2.55 mJ at 3.8 μm and can be even increased with more pump power. The output idler wavelength can be tuned by turning the rotation angle of the crystal, which is controlled by an electronic motor. The motorized continuous rotation stage is compact and highly precise with minimum rotation step of 0.0025°. This enables the fine tuning of the phase-matching angel and the resulting output wavelength. The central idler wavelength is 4.6 μm (rotation angle is 0°).
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