Enhancement of terahertz wave emission from air plasma excited by harmonic three-color laser fields

Here we use the two-dimensional photocurrent model to analyze the terahertz (THz) wave generation from the air plasma excited by the harmonic three-color femtosecond lasers with wavelengths of 1200 nm, 600 nm, and 400 nm, respectively. This three-color harmonic frequency combination is readily-accessible under laboratory condition by using the suitable optical parametric amplifier and frequency doubling crystals, without the need of complex optical components for third harmonic wave. Through theoretical simulations, we find that the addition of 400 nm ultrafast laser with extremely low energy to the two-color laser fields of 1200 nm and 600 nm could effectively enhance the THz output. The THz wave energy emitted from the plasma point source could be easily enhanced up to 5.8 times. By either replacing the 400 nm laser with 800 nm laser or changing the near-infrared fundamental wavelength from 1200 nm to 1600 nm, the enhancement effect all noticeably reduces. These results reveal that the three-color laser fields which satisfy the harmonic frequency relation could achieve the utmost THz yield compared with the common two-color fields or non-harmonic three-color fields. This is of great significance for obtaining more powerful THz sources. The optimal conditions for the strongest THz wave generation are also calculated as a guidance for application.