Strong Spatial Confinement of Terahertz Waves Along Laser Plasma Filaments

Significance Femtosecond laser filamentation in air refers to the technical approach of using the femtosecond laser to ionize air near its focal point, forming a plasma channel (also known as the optical filament) that emits terahertz (THz) waves. Due to the remote generation of plasma filaments and the broadband and high intensity characteristics of the emitted THz waves, femtosecond laser filamentation overcomes water vapor absorption losses in free -space THz transmission; thus, it is advantageous for applications such as remote sensing and communication in THz band. Therefore, the study of physical mechanisms of THz wave radiation by femtosecond laser filaments has become an important branch of THz science. However, there currently remains controversy surrounding the mainstream models for THz wave generation, and important experimental observations such as THz polarization and angular dispersion distribution remain in disagreement. For example, the two mainstream models, i.e., four -wave mixing (4WM) and photocurrent (PC), have fundamental differences: when the THz wave yield is significant, the former assumes a relative phase difference of 0 between the fundamental and second harmonic laser fields, while the latter assumes a pi /2 phase difference. Furthermore, regarding the dependence of the far -field divergence angle of THz waves on frequency, the mainstream viewpoint represented by the off -axis phase matching model suggests that only dual -color filaments longer than the dephasing length can radiate THz waves with high -frequency components distributed inside and low -frequency components distributed outside. However, this explanation fails to account for the similar radiation behavior observed in short filaments, and contradicts experimental findings in some literature, which report high -frequency outside, low -frequency inside, or frequency -independent angular distribution.Progress These unresolved contradictions pose significant challenges in the study of THz wave radiation from dual -color filaments. The reason for these contradictions could be that femtosecond laser filamentation is a complex optical phenomenon involving multiple nonlinear processes such as optical Kerr self -focusing and plasma defocusing. Therefore, a single physical model is likely insufficient to encompass the entire dynamic mechanism of filamentation-induced THz wave radiation. Accordingly, we propose a three -process theory that incorporates mainstream models and the recent experimental observation of spatial confinement of THz waves inside the laser filament. We first divided the filamentation process into Kerr self -focusing and plasma defocusing before and after the laser intensity breaking the ionization threshold. Then, in the first process, when neutral gas molecules are not yet ionized, we primarily consider the 4WM effect of the pump laser and its harmonic in air, which generates THz waves. The second process occurs when the laser intensity exceeds the ionization threshold, resulting in the ionization of air and the formation of a plasma (free electrons). Under the drive of the time -asymmetric electric field of the dual -color laser pulse, the plasma oscillates and gives rise to a nonzero trailing current (also known as drift current or residual current) and the emission of THz waves. Finally, considering the time scale of THz waves (ps), which is much larger than the establishment time of the plasma filament (tens or hundreds of fs) and much smaller than the plasma lifetime (ns), the filament can be treated as a quasistatic waveguide for THz waves. During the transmission of THz wave, its interaction with the plasma free electrons leads to the spatial confinement of THz waves at the radial edge of the filament. This constitutes the third process. Based on the analysis above, we explain the THz radiation regarding the three processes using the mainstream models of 4WM and PC, as well as the proposed one-dimensional negative dielectric waveguide model (1DND). As shown in Fig. 1, if we consider only the first or second process, the predictions of the THz orthogonal polarization components by the two mainstream models alone are unsatisfactory. If we incorporate the THz spatial confinement effect as the crucial third process, it is necessary to consider the spatial mode distribution, energy loss, and spectral changes of THz waves after interaction with the plasma. This consideration leads to the best fit to experimental results. The THz spatial confinement effect has also facilitated several novel applications and addressed key challenges in the field (Fig. 4), including super -resolution THz imaging, high conversion efficiency of THz strong electric fields, and flexible manipulation of broadband THz polarization states.Conclusions and Prospects In conclusion, the proposed 4WM+PC+1DND model presents a new mechanism that overcomes the limitations of a single physical model, bridges the connections between mainstream models, and provides a unified framework for fundamental theories that were previously incompatible. This model could comprehensively and reasonably explain the unresolved evolution of THz polarization states with the rotation of the frequency -doubling crystal in dual -color field radiation, as well as experimental results such as near -field THz modes and far -field THz spatial chirping. It provides a fresh opportunity to reevaluate past experimental findings and resolve related contradictions in the field. Future prospects of the THz spatial confinement effect can be focused on the following aspects. 1) Investigation of the physical mechanisms behind single -color and dual -color filamentation for THz wave generation: due to the universality of the THz spatial confinement effect, it can be incorporated into both single -color and dual -color THz generation processes. This might establish a foundation for a unified understanding of the common mechanisms in these two important research directions. 2) Exploration of new technologies: compressing THz waves into subwavelength spatial scales is expected to lead to technological innovations. For example, broadband THz all -optical computation techniques relying on spatial compression in subwavelength narrow channels can be developed utilizing THz wave spatial confinement for guidance and transmission, which has the potential for breakthroughs in this field. 3) Interdisciplinary exploration: the integration of the 1DND model with principles from micro and nanooptics can be explored. Concepts such as surface waves and epsilon near zero (ENZ) can be utilized to investigate THz spatial confinement across scales ranging from submillimeter to nanometer. This approach can provide new insights into laser -induced plasma filaments using the concept of surface plasmon polariton waveguides. 4) Laser filaments as self -balanced physical systems: based on the THz spatial confinement effect, laser filaments in the THz regime can become a new platform for self -balanced physical systems. Similar to the action of silicon -based fiber waveguides on light, this filament platform can greatly facilitate the study of new laser ionization mechanisms, THz wave transmission principles, and modulation methods from a new perspective.