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Broadband terahertz (THz) emission generated from laser induced gas plasma provides an effective tool for studying nonlinear spectrum, imaging and remote sensing. Recently, the contribution of plasma oscillation to the THz emission was revealed from the nitrogen molecules pumped by intense two-color laser pulses. Plasma oscillation contributes only to the THz emission at relatively low plasma density due to negligible plasma absorption. More generally, with the THz emission generated from the ionizing gaseous medium, the surrounding plasma is expected to play an important role in the generation process. For the THz radiation from laser filament, the plasma region is extended in the laser propagation direction, and the effect of surrounding plasma on the emitted THz spectrum needs studying. In this work, we investigate the relation between pump power and filament length from THz spectrum emitted by air filament driven by two-color laser pulse. The time domain spectrum of THz field is recorded by an electro-optic (EO) sampling technique. In our experiments, significant frequency shifts are observed as the pump power and the filament length increase, and we find that the center frequency of the THz radiation is shifted towards longer wavelength, which is the so called red-shift of the THz spectrum. This red-shift is independent of THz radiation angle. The observations are explained by the plasma absorption inside the air filament. Our theoretical model is based on three mechanisms: the ionization-induced photocurrent, the plasma current oscillation and the plasma absorption. We coherently add up all the local THz fields inside the air filament, and simultaneously consider the plasma absorption induced correction of the THz spectrum. The simulation well reproduces the experimental observation. The skin depth decreases as the plasma density increases, thus the plasma absorption dominates the red-shift process. If the skin depth is larger than the filament length, the plasma oscillation contributes to the THz spectrum dominantly, and thus leading to the blue-shift of THz spectrum. Our results indicate that for the extended filament length or higher plasma density, the combining effect of photocurrent, plasma oscillation and absorption, results in the observed low-frequency broadband THz spectrum. Our study offers a method of coherently controlling the broadband THz spectrum.
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Keywords:
- terahertz radiation /
- laser plasma /
- plasma frequency /
- plasma absorption
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图 2 当驱动光功率分别为25 GW (红色实线)及75 GW(蓝色点线)时实验测量的(a)时域光谱及(b)归一化频谱; (c) THz光谱的中心频率随着驱动光功率的变化(其中蓝色点图为实验结果图, 红色曲线为模拟结果)
Fig. 2. Measured (a) THz temporal waveforms and (b) normalized THz spectra at different pump power; (c) central frequencies as a function of the pump energy (The blue dots are the experimental results and the red solid line is from the simulation).
图 3 (a) 驱动光功率为45 GW时, 频率为1 THz与3 THz的辐射角分布; (b)不同锥形辐射角下, THz光谱的中心频率随着驱动光功率的变化 (虚线连接的实心点为实验结果, 实线为计算结果)
Fig. 3. (a) Far-field THz profiles at different frequencies at the pump power of 45 GW; (b) THz central frequencies as a function of the pump energy at various emission angles (Dashed line with solid dots is the experimental results and the solid line is the simulation results)
图 4 驱动光功率为25 GW 时 (a)沿激光传输方向中点处径向和(b)沿驱动光传输方向z的等离子体频率(蓝色实线)及趋肤深度(红色点线)
Fig. 4. Plasma frequency (blue solid line) and skin depth (red dot line) as a function of the (a) radial axis and (b) pro-pagation direction at the pump power of 25 GW.
图 5 当驱动光功率分别为25 GW和75 GW时模拟计算的归一化频谱
Fig. 5. The simulated THz spectra at 25 and 75 GW pump power.
图 6 THz光谱的中心频率随着等离子体光丝长度的变化(蓝色点图为实验结果图, 红色曲线为模拟结果)
Fig. 6. THz central frequencies as a function of the plasma length (The blue dots are the experimental results and the red solid line is the simulation results).
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[1] 周胜利, 张存林 2009 航天返回与遥感 30 32
Google Scholar
Zhou S L, Zhang C L 2009 Spacecraft Recovery & Remote Sensing 30 32
Google Scholar
[2] Grischkowsky D, Keiding S, van Exter M, Fattinger C 1990 J. Opt. Soc. Am. B 7 2006
Google Scholar
[3] Ferguson B, Zhang X C 2002 Nat. Mater. 1 26
Google Scholar
[4] Liu W, Luo Q, Chin S L 2003 Chin. Opt. Lett. 1 56
[5] Li Y T, Wang W M, Li C, Sheng Z M 2012 Chin.Phys. B 21 095203
Google Scholar
[6] Pickwell E, Wallace V P 2006 J. Phy. D Appl. Phys. 39 301
Google Scholar
[7] 何志红, 姚建铨, 时华锋, 黄晓, 罗锡璋, 江绍基, 王鹏 2007 56 5802
Google Scholar
He Z H, Yao J Q, Shi H F, Huang X, Luo X Z, Jiang S J, Wang P 2007 Acta Phys. Sin. 56 5802
Google Scholar
[8] Clery D 2002 Science 297 761
Google Scholar
[9] 张显斌, 施卫 2006 55 5237
Google Scholar
Zhang X B, Shi W 2006 Acta Phys. Sin. 55 5237
Google Scholar
[10] You D, Jones R R, Bucksbaum P H, Dykaar D R 1993 Opt. Lett. 18 290
Google Scholar
[11] Dreyhaupt A, Winnerl S, Dekorsy T, Helm M 2005 Appl. Phys. Lett. 86 121114
Google Scholar
[12] Stepanov A G, Bonacina L, Chekalin S V, Wolf J P 2008 Opt. Lett. 33 2497
Google Scholar
[13] Fülöp J A, Pálfalvi L, Klingebiel S, Almási G, Krausz F, Karsch S, Hebling J 2012 Opt. Lett. 37 557
Google Scholar
[14] Hamster H, Sullivan A, Gordon S, White W, Falcone R W 1993 Phys. Rev. Lett. 71 2725
Google Scholar
[15] Matsubara E, Nagai M, Ashida M 2012 Appl. Phys. Lett. 101 011105
Google Scholar
[16] Kim K Y, Glownia J H, Taylor A J, Rodriguez G 2012 IEEE. J. Quantum. Elect. 48 797
Google Scholar
[17] Cook D J, Hochstrasser R M 2000 Opt. Lett. 25 1210
Google Scholar
[18] Oh T I, Yoo Y J, You Y S, Kim K Y 2014 Appl. Phys. Lett. 105 041103
Google Scholar
[19] Kim K Y, Glownia J H, Taylor A J, Rodriguez G 2007 Opt. Express 15 4577
Google Scholar
[20] Debayle A, Gremillet L, BergéL, Köhler C 2014 Opt. Express 22 13691
Google Scholar
[21] Li N, Bai Y, Miao T S, Liu P, Li R X, Xu Z Z 2016 Opt. Express 24 23009
Google Scholar
[22] Li X L, Y Bai, Li N, Liu P 2018 Opt. Lett. 43 114
Google Scholar
[23] You Y S, Oh T I, Kim K Y 2012 Phys. Rev. Lett. 109 183902
Google Scholar
[24] Gorodetsky A, Koulouklidis A D, Massaouti M, Tzortzakis S 2014 Phys. Rev. A 89 033838
Google Scholar
[25] Kim K Y, Taylor A J, Glownia J H, Rodriguez G 2008 Nat. Photonics 2 605
Google Scholar
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