Theoretical and experimental studies on terahertz radiation from laser-driven air plasma

Abstract

Strong terahertz (THz) radiation of MV/cm can be generated from air via two-color laser scheme. In this paper, we introduce three recent theoretical and experimental researches conducted by Wang et al., in which they explored the long-standing problem of THz generation mechanism and extended the scheme with uncommon frequency ratio. In the widely-studied two-color laser scheme, the frequency ratio of the two lasers is usually fixed at 2/1=1:2. In 2013 they predicted according to the plasma current model, for the first time, that the two-color scheme can be extended to a new frequency ratio 1:2n, where n is an positive integer. In 2017 they found that the frequency ratio can be further extended to much broader values. In that year, their experiments showed, for the first time, efficient THz generation with new ratios of 2/1=1:4 and 2:3. They observed that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization, but the polarization adjustment becomes inefficient by rotating the other laser polarization, which is inconsistent with the symmetric nature in the susceptibility tensor required by the multi-wave mixing theory; the THz energy shows similar scaling laws with different frequency ratios, which is inconsistent with the scaling predicted according to the multi-wave mixing theory. These experimental results are in agreement with the plasma current model and particle-in-cell simulations. Therefore, their studies not only push the development of the two-color scheme, but also show that the THz generation mechanism should be mainly attributed to the plasma current model.