Directional intense terahertz emission by abruptly autofocusing lasers
Phys. Rev. A 112, 013503 – Published 2 July, 2025
DOI: https://doi.org/10.1103/b57m-txqz
Abstract
Two-color laser induced plasma filamentation in air could serve as tabletop sources of broadband terahertz (THz) pulses. Ubiquitous air in the earth facilitates its widespread utilities, particularly, in wireless communication and remote sensing, exploiting the unique advantage of the air-based scheme that the THz sources can be delivered over standoff distances via the pump laser propagation in air. However, the THz emission pattern inevitably has a conical angular profile with a dip in the propagation axis. Therefore, THz energy concentration, propagation directionality, and accuracy of signal demodulation are significantly impaired, greatly limiting its direct applications. Here, we successfully eliminate the unfavorable conical profile by experiments and meanwhile enhance THz directionality and intensity by 17 folds at the focus by use of abruptly autofocusing laser beams. Our simulations with a far-field model well reproduce the experimental observations, when the air filament length is read from the experimentally recorded fluorescence images. More importantly, we find that because the Gouy phase of Gaussian beam is ignored, the widely adopted dephasing length is significantly overestimated by 30 folds, which cannot explain the observed conical THz emission even when the filament length is shorter than the dephasing length. While the abruptly autofocusing beams are employed, the dephasing length is increased by five folds and the dephasing effect can be well overcome, which causes elimination of the unfavorable conical profile and enhancement of the THz intensity. This scheme is easily accessible since the abruptly autofocusing beam can be achieved by imposing a spatial light modulator on the input Gaussian beam.
