In recent years, the technology of terahertz (THz) source has developed rapidly. Due to the advantages of long-distance generation and non-damage thresholds, THz wave generation from the air plasma excited by femtosecond laser attracted global attention. Since the first time the method was proposed, it has experienced many tremendous developments and leaps. In this letter, we propose a new method to generate THz wave with higher intensity. We demonstrate that the energy of THz wave can be effectively enhanced when the air plasma excited by a vortex beam. The energy of THz wave induced by two-color laser field generated through a 100-mm-thick BBO crystal is recorded by using a Golay detector. By comparing the intensity and spatial distribution of THz wave generated by different orders of vortex beams, we find that the energy of THz wave generated from low-order vortex beams is stronger than that from Gaussian beams with the invariant laser energy. To understand the effect of the vortex beam on the generation of THz wave, we compared the distribution of the focal plane between the Gaussian beam and the vortex beam. We believe this method can pave the way for finding the type of laser beam which can produce a stronger THz wave.
Terahertz (THz) wave has attracted considerable attention in recent years because of its potential applications. The intense THz waves generated from air plasma induced by two-color femtosecond laser are widely used due to its high generation efficiency and broad frequency bandwidth. The parameters of the laser change the distribution of the air plasma, and then affect the generation of THz wave. In this research, we investigate the THz wave generation from air plasma induced by quasi-square Airy beam. Unlike the common Gauss beam, the quasi-square Airy beam has ability to autofocus and to increase the maximum intensity at the focus. By using the spatial light modulator (SLM), we can change the parameters of phase map to control the shape of the Airy beam. We obtain the two-color laser field by a 100-um-thick BBO crystal, then use a Golay detector to record THz wave energy. By comparing terahertz generation at different modulation depths, we find that terahertz energy produced by quasi-square Airy beam is up to 3.1 times stronger than that of Gauss beam with identical laser energy. In order to understand the influence of quasi-square Airy beam on the BBO crystal, we record THz wave energy by changing the azimuthal angle of BBO crystal with Gauss beam and Airy beam at different modulation depths. We find that the trend of terahertz energy with respect to the azimuthal angle of the BBO crystal keeps the same for different laser beams. We believe that the quasi-square Airy beam or other auto focusing beam can significantly improve the efficiency of terahertz wave generation and pave the way for its applications.
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