Enhanced and frequency-selectable terahertz emission from two crossed air-plasmas

Air-plasma is a convenient and table-top source for the broadband and coherent terahertz (THz) wave generation. However, the lack of enhancement and frequency-selecting methods for this source impedes its wide applications. Dealing with this issue, here we demonstrate a unique methodology using two spatially crossed air-plasmas. THz yields can be significantly enhanced by a factor of 7 when the two temporally overlapped THz waves are in-phase or nearly eliminated when in antiphase. This enhancement could be interpreted by using the four-wave mixing model. Meanwhile, the spectral interference of the THz amplitudes is observed and reconstructed. It enables the frequency-selectable narrow-band THz output under the suitable time delays between the two air-plasmas. These results overcome the saturation and suppression of the plasma-based THz technology and emphasize that the coherence between the THz emissions from the two filaments is a prerequisite of our methodology. Our research provides an unusual approach in enhancing and engineering the THz wave and can help to gain more insight into the mechanism of plasma-based THz generation.