Unified Traveling-Wave Antenna Model for THz Wave Generation during Single- and Dual-Color Laser Filamentation

Significance 

Terahertz (THz) technology, which reside in electromagnetic spectrum between microwaves and infrared light, has caught considerable attention over the past two decades due to its wide range of potential applications, including imaging, spectroscopy, and communication. Laser-induced plasma filaments created by high-intensity femtosecond lasers is considered one of the most interesting methods to generate THz radiation. The process of generating THz radiation using laser-induced plasma filaments has typically been studied under two schemes: single-color and dual-color photoionization. In single-color filamentation, a single laser wavelength is used to ionize the medium, whereas dual-color filamentation involves the simultaneous use of a fundamental laser frequency and its second harmonic. Historically, these two approaches have been treated as distinct, with separate physical models developed to explain the underlying mechanisms in each case. However, this compartmentalized understanding presents several challenges. First, it limits the ability to fully understand the fundamental processes driving THz generation in laser-induced plasma filaments. Second, the lack of a unified theoretical framework complicates efforts to compare and contrast the two methods, thereby hindering the optimization of THz source characteristics such as intensity, bandwidth, and angular distribution. Moreover, the relative phase difference between the fundamental laser and its second harmonic in dual-color filamentation significantly influences the properties of the generated THz waves, a factor that has not been adequately addressed in existing models. Given these challenges, there is a need for a comprehensive theory that can bridge the gap between single- and dual-color filamentation schemes. Such a unified framework would provide a systematic understanding of the commonalities and differences between the two approaches, thereby facilitating more effective optimization and application of THz sources. To this end, researchers from the University of Shanghai for Science and Technology, led by Feifan Zhu, Dr. Jiayu Zhao, Dr. Li Lao, Dr. Yan Peng, and Dr. Yiming Zhu, developed a new model to describe THz wave generation in both single- and dual-color laser fields based on the concept of a traveling-wave antenna (TWA). The research work is now published in Optics Letters.

The authors investigated the THz yield under different initial phases (ϕ0). Using a 1 THz, they varied the initial phase values (0, π/4, and π/2) for a length-varied filament with a constant dephasing length (ld) of approximately 22 mm, generated by an 800 nm and 400 nm laser within a plasma density (𝑁𝑒) of about 1016 cm−3 and found that the THz yield increased almost linearly with 𝜙0=𝜋/4, whereas the yields for 𝜙0=0 and 𝜙0=𝜋/2 exhibited step-like growth around the yield at 𝜋/4. This demonstrated that while the initial phase influences periodic fluctuations, it does not alter the overall trend of THz enhancement with increasing filament length. These observations validated the TWA model’s capability to account for phase effects in THz generation. They studied the frequency-dependent THz angular distributions by calculating far-field THz angular dispersion at 1, 3, and 5 THz, maintaining a plasma density (𝑁𝑒) of 1×1016 cm−3, a dephasing length 𝑙𝑑 of 22 mm, and a filament length (𝑙) of 2𝑙𝑑. They showed typical conical THz emissions, with the angular dispersion decreasing from 7° to 3° as the frequency increased from 1 to 5 THz. These findings closely matched previously reported data and confirmed that the TWA model could accurately reproduce the angular distribution even with filaments shorter than the dephasing length. This indicated the model’s robustness in describing THz emission patterns across a range of frequencies. The authors also investigated the effect of the dephasing length (𝑙𝑑) on THz radiation by modifying the plasma density (𝑁𝑒), which directly affects 𝑙𝑑 and the findings showed that the conical angles of THz emissions decreased as the filament length increased, consistent with the behavior of a metal antenna whose radiation angle decreases with increasing length. In another scenario, 𝑁𝑒 was varied between 5×1016  and 1×1018 cm−3, resulting in 𝑙𝑑 ranging from 16 to 3 mm. Here, the conical angles remained roughly constant despite changes in filament length. This phenomenon was explained by the interplay between filament length and 𝑁𝑒, which affects the phase matching conditions. The TWA model successfully accounted for these variations, demonstrating its versatility in handling different plasma densities and filament lengths.

The corresponding author, Jiayu Zhao, stated that a feasible method to elucidate the underlying connections behind contradictory phenomena is to conduct a commonality analysis. In this regard, the unified TWA model study of single- and dual-color laser filament radiation of THz waves has set a good example. The previously incompatible physical mechanisms of single- and dual-color fields have been effectively integrated. For instance, addressing the similar far-field THz characteristics in both fields (such as higher frequencies being internal, lower frequencies external, and smaller divergence angles with longer filaments), our TWA model can comprehensively describe the far-field THz radiation behavior of filaments. Simultaneously, the classical “transition-Cherenkov radiation model” has been incorporated into the antenna model, enabling scholars from different fields to quickly find common ground within this theoretical framework, thereby gaining a clear understanding of the overall physical picture. Our next step will be to continue expanding the TWA model to explain the THz radiation of more complex configurations of filament arrays.

In conclusion, University of Shanghai for Science and Technology scientists developed a unified theoretical framework that bridges the gap between single- and dual-color laser filamentation schemes. The TWA model’s capacity to incorporate both approaches into a single framework allows for a more comprehensive understanding of the fundamental processes driving THz generation. The unified theory can also facilitate further research and experimentation, and provide a solid foundation for investigating new phenomena and optimizing existing technologies. Additionally, the authors incorporated the phase factor into the TWA model to account for the relative phase difference between the fundamental laser and its second harmonic which addressed an important gap in previous models. Another advantage of the study is enhanced understanding the role of phase differences which can lead to more precise control over THz wave generation, improving the quality and consistency of the output. Indeed, the ability to accurately describe and predict the behavior of THz waves generated through laser-induced plasma filamentation can significantly enhance the optimization of THz sources. Researchers and engineers can now fine-tune the parameters of their laser systems to maximize the intensity, bandwidth, and angular distribution of the generated THz waves which can lead to more efficient and effective THz sources for various applications. The improved control over THz wave generation enabled by the TWA model can enhance the resolution and sensitivity of THz imaging and spectroscopy systems. For example, in medical diagnostics, higher resolution and sensitivity can lead to earlier detection of diseases and more accurate assessments of tissue properties. THz waves is also considered for use as potential carriers for next-generation wireless communication systems due to their high-frequency capabilities and large bandwidth.

About the author

Jiayu Zhao is an associate professor and doctoral supervisor at the Terahertz Technology Innovation Research Institute of the University of Shanghai for Science and Technology. He has been recognized as a “Dong fang ying cai” Scholar, a “Qi ming xing” Scholar, and a “Chen guang” Scholar by the Shanghai Municipality. His primary research focus is on the intense terahertz sources and spectroscopy detection systems. He has published over 20 high-impact SCI papers in areas such as laser plasma-based terahertz sources. He has also led projects funded by the National Natural Science Foundation and sub-projects under the National Key R&D Program of the Ministry of Science and Technology. He has received several prestigious awards, including the First Prize of the Tianjin Natural Science Award in 2017, the First Prize of the Shanghai Technological Invention Award in 2020, and was recognized among the Top Ten Advances in Chinese Optics in 2018.

Reference

Zhu F, Zhao J, Lao L, Peng Y, Zhu Y. Unified framework for terahertz radiation from a single- or two-color plasma filament. Opt Lett. 2024 Jan 1;49(1):41-44. doi: 10.1364/OL.498603.

Go to Opt Lett.

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