| PREVIOUS PRESENTATION | BACK TO PROGRAM OVERVIEW | NEXT PRESENTATION |
THz radiation coherent accumulation along the two-color laser filament in Air
Zeliang Zhang, Xingyou Li, Pengfei Qi, Lu Sun and Weiwei Liu*
Institute of Modern optics, Nankai University
*liuweiwei@nankai.edu.cn
Terahertz (THz) radiation generated by the two-color femtosecond laser filamentation is a promising high-intensity THz source. The filament’s intrinsic characteristics, especially the filament length, determine the THz radiation strength. However, a detailed analysis of the quantitative relationship and physical mechanism between the laser filament length and the THz radiation intensity under high peak power driving laser is still lacking. This paper investigates the effect of filament length on the THz radiation by modulating the basic characteristics of the two-color laser field and changing the focal length to control the laser filament length. Experiment results show that the long filament length is beneficial for improving THz radiation intensity. The theoretical simulation indicates that THz radiation generated by the long filament arises from the coherent accumulation of THz wave at each cross-section along the filament. This result suggests that extending the filament length is an effective scheme to enhance the intensity of THz radiation generated by long two-color femtosecond laser filament.
Introduction
It has been demonstrated experimentally that the THz conversion efficiency of the two-color femtosecond laser filament is determined by multiple laser parameters, including the two-color laser peak power, polarization, and spatiotemporal walk-off [1,2]. The polarization of the FW and SH determine the two-color laser electric field intensity and THz conversion efficiency obtains maximum when the polarization of FW and SH is parallel. During the generation of the two-color laser in β-BBO, the Poynting vector direction of the FW and SH in the β-BBO has a spatial walk-off and the group velocity of the FW and SH in the β-BBO has a temporal walk-off. The spatiotemporal walk-off effect also influences the THz conversion efficiency [2]. The above analysis mainly describes the basic characteristics of the two-color laser field [3–6] and its impact on THz radiation, but does not pay attention to the relationship between the intrinsic properties of the filament and THz radiation intensity. As the cause of THz radiation, the intrinsic properties of two-color laser filaments greatly affect the conversion efficiency of THz radiation. The laser filament can be seen as a gain channel for THz generation and the modulation of the filament characteristics (especially the filament length) can further improve the THz conversion efficiency.
We have further controlled the polarization and compensated the time delay of FW and SH pulses through double wavelength waveplate (DWP) and α-BBO crystal to precisely study the influence of the filament length on THz radiation to quantitatively study the relationship between the THz radiation and the filament length, filament diameter, and electron density.
Experiment setup
We controlled the polarization and compensated the time delay of FW and SH pulses through double wavelength waveplate (DWP) and α-BBO crystal in Fig.1 (a) to quantitatively study the relationship between the THz radiation and the filament length, filament diameter, and electron density. Fig.1(b) and (c) show the experiment setup of the side fluorescence imaging and the time-resolved shadowgraphs, respectively. In this letter, polarization status and spatiotemporal walk-off have been adjusted to the optimal status. The peak power of the driving laser exceeds the critical power needed for self-focusing in air which means that the filament produced by all focal length reach a saturation of the peak intensity inside the filament.
Figure 1: Schematic diagram of the experiment. (a) The spatiotemporal walk-off compensating by α-BBO and polarization modulation by double wavelength waveplate (DWP). Experiment Setup of the side fluorescence imaging (b) and time-resolved shadowgraphs (c).
Result and discussion
Three kinds of lenses (f = 300mm, 400mm, and 500mm) were used to produce different filament lengths to further explain the principles of filament length modulation. As shown in Fig.2 (a), (b) and (c), the diameter of the filament gradually increases while the electron density inside the filament gradually decreases. The difference is that the filament length is significantly extended. By characterizing the plasma density, filament diameter, and filament length and precisely calculating the THz radiation along the whole filament length (Fig.2 (d)), we can conclude that the filament length is the core parameter that influences the THz radiation intensity. When the filament length is lengthened, the THz signal increases obviously. The simulation curve shows a good coincidence with the measured THz signal in the experiment. The coherent emission of the THz wave from the whole filament in the far field can be shown as,
$$E_{THz}(z,t)\propto\frac{dJ(t)}{dt}e^{i\Theta(Z_1)}$$
$$I_{THz}\propto\int_{\tau_0}^\tau \int_{z_0}^z dE_{THz}^2(z,t)$$
\(E_{THz}\) and \(J(t)\) are the THz wave electric field and the net photocurrent intensity at \(Z_1\) inside the filament. \(\Theta(Z_1)=\Theta(Z_0)+k_{THz}\eta(z)dz\) is the THz wave phase. \(\tau_0\) and \(\tau\) are the start and end time of ionization process. \(z_0\) and \(z\) are the start and end position of the filament.
Figure 2: The two-color filament evolution as the focal length changed. (a) and (b) are the filament diameter and length, respectively. (c) Plasma density inside the filament under focal length modulation. (d) Comparison of the experiment and simulation result of the THz energy.
For the photocurrent model, THz radiation intensity gradually decreases with the focal length extension. The reason is that decreasing of the peak power density inside the filament because the long focal length leads to a larger filament diameter. This phenomenon can also lead to a decrease in the electron density inside the filament. The decreasing of the two-color peak power density and the electron density both lead to the attenuation of the THz radiation from the filament. However, this conclusion is contrary to our experimental results (Fig.2). The reason is due to the photocurrent model only calculates the transient current produced at the cross-section of the focal position. This limitation results in the photocurrent model not being able to describe the effect of the filament length on THz radiation. The physics model in this letter can support the experimental results, where the THz radiation from the total two-color laser filament is considered as a THz signal accumulation from each filament cross-section. This practical THz modulation mechanism can be used for promising THz sources.
References
[1] A. D. Koulouklidis, C. Gollner, V. Shumakova, V. Yu. Fedorov, A. Pugžlys, A. Baltuška, and S. Tzortzakis, Observation of Extremely Efficient Terahertz Generation from Mid-Infrared Two-Color Laser Filaments, Nat. Commun. 11, 292 (2020)
[2] V. Yu. Fedorov and S. Tzortzakis, Powerful Terahertz Waves from Long-Wavelength Infrared Laser Filaments, Light Sci. Appl. 9, 186 (2020)
[3] Z. Yu et al., 0.35% THz Pulse Conversion Efficiency Achieved by Ti:Sapphire Femtosecond Laser Filamentation in Argon at 1 kHz Repetition Rate, OEA 5, 210065 (2022)
[4] Z. Yu, L. Sun, N. Zhang, J. Wang, P. Qi, L. Guo, Q. Sun, W. Liu, and H. Misawa, Anti-Correlated Plasma and THz Pulse Generation during Two-Color Laser Filamentation in Air, Ultrafast Science 2022, 1 (2022)
[5] J. Dai, N. Karpowicz, and X.-C. Zhang, Coherent Polarization Control of Terahertz Waves Generated from Two-Color Laser-Induced Gas Plasma, Phys. Rev. Lett. 103, 023001 (2009)
[6] Q. Su, W. Liu, D. Lu, P. Qi, O. Kosareva, and N. Zhang, Influence of the Tilting Angle of a BBO Crystal on the Terahertz Radiation Produced by a Dual-Color Femtosecond Laser, IEEE Trans. THz Sci. Technol. 9, 669 (2019)