Abstract
1 Introduction
Terahertz (THz) waves[1], which are significantly less developed and utilized than microwaves and light waves, used to be regarded as the ‘THz gap’[2] in the electromagnetic spectrum due to the limitations of traditional electronic or optical methods in producing terahertz radiation during the early years. Such THz pulses have attracted significant interest for their potential applications in fields such as biology, medicine, material science, optical communication and the military[3–8]. To generate THz waves, several routine methods, such as cascade quantum laser[9,10], optical rectification[11,12], photoconductive[13] and vacuum electronic[14,15] methods, have been widely demonstrated. Due to the material damage threshold, these methods are incapable of generating extremely powerful THz pulses. Such pulses can be used as a powerfully driven pulse for probing and controlling material properties[16,17], biological macromolecules[4], electron beam detection[18] and charged particle acceleration[19,20]. For instance, the mechanisms of four-wave mixing and photoionization become saturated in under-dense plasma just around the power of
During the past two decades, the quick development of relativistic laser systems, whose peak intensity exceeds
Here, we propose a novel plasma wiggler that uses a femtosecond (fs) laser pulse to produce frequency-tunable and extremely powerful terahertz radiations, improving the flexibility and accuracy of terahertz applications. Figure 1(a) draws the concept of the plasma wiggler. A laser pulse is used to irradiate a block-shaped near-critical density plasma, producing hot electrons[39]. The accelerated electrons move in two different trajectories. One kind of high-energy electrons gets rid of the potential barrier of the surface electric field and goes away from the plasma surface. The other group of electrons is trapped by the barrier acting as a wiggler. Figure 1(b) illustrates the laser–plasma interaction snapshot at around 273 fs. When this electron beam passes through the transverse interfaces of the plasma, THz radiation[38] and sheath fields
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Figure 1.(a) Schematic for the generation of a high-power, collimated, narrow-band and center-frequency-tunable THz pulse. An intense femtosecond laser pulse irradiates on the left-hand side of a block-shaped near-critical density plasma. Hot electrons generated by laser ponderomotive force can be separated into two groups: the electrons in group A moving forward leaving the plasma and the electrons in group B reciprocating under the sheath fields ; here the transverse sheath fields are induced when electrons pass through the plasma transverse interfaces. Under the action of , electrons in group B could be pulled back into the plasma and pass through the transverse interface on the other side. Such wiggler-like motions of these electrons can emit the desired THz pulse. (b) The electron accelerating in the plasma. (c) The trajectories of the two groups of electrons (blue and red) in the surface charge separation field.
Based on the theoretical model and PIC simulations, we find that the center frequency of the THz pulse can be regulated by changing the thickness of the plasma. In simulations using a laser pulse with energy of approximately 430 mJ, the generated THz pulse has a divergence angle of approximately 20°, an ultra-strong-field strength of over 80 GV/m, a laser–THz conversion efficiency of over 2.0% and a center frequency tunable from 4.4 to 1.5 THz by varying the plasma thickness from 20 to
2 Results
2.1 Theoretical model and simulation setup
When an intense laser pulse irradiates on a plasma, as shown in Figure 1(b), hot electrons whose beam length is close to the laser duration can be generated by laser ponderomotive force[39]. The electron beam transports in the plasma and passes through its transverse interfaces. As a result, transverse sheath fields
Figure 2.In the case of the electron penetrating with different (the angle at which the electron enters ), the relation between the electron threshold kinetic energy and the transverse location where the electron could be pulled back into the plasma.
Here,
2.2 Electron kinetics
To generate electrons in group B, one should use a plasma with limited transverse size to ensure the occurrence of reciprocating motion. Simulations, with plasma of different lengths
When
Figure 3.The angular-spectra distribution of the hot electrons. The electrons from a plasma length of (a) collected by a screen with a radius of in the first 270 fs of the simulation could be classified to group A, and (b) the electrons behind group A can be assigned to group B. The electrons from a plasma length of (c) in group A collected by a screen whose radius is in the first 770 fs and (d) the electrons in group B.
We used a semicircular receiving screen, whose radius was 250 μm with the center locating at the midpoint of the plasma right-hand interface, to collect the radiation field. Then, the angular-spectra distribution of the THz source could be obtained from the radiation field through Fourier transform[47]. In this work, the angular spectrum method[47–49] was employed to remove the near-field radiation, and then all the results of the THz source could be recognized as far-field radiation.
In the simulation with
2.3 Spectrum of terahertz pulses
Figures 4(a)–(c) show the angular-spectra distribution of the THz pulses from the plasma lengths
Figure 4.Simulation results from the plasma of different lengths , while the thickness was fixed to . The angular-spectra distribution of the THz pulses from (a) , (b) and (c) . (d) The radiation field before filtering (blue line) and the field of the THz pulse (red line) collected at from the simulation of .
Every time the electron passes through the transverse interfaces of the plasma, THz radiation can be emitted and becomes stronger in the case of the electrons in group B oscillating for several periods. When
2.4 Tunable frequency
From Figures 3(a) and 3(c), one can see that the electrons in group A had a large divergence angle. The electron density
Figure 5.(a) The center frequency of the THz source and the laser–THz energy conversion efficiency from the simulations with plasmas of different lengths from 50 to , while was fixed to . (b) The center frequency of the THz source from the simulations (blue line) and the theoretical model of
Equation (4) indicates that we could regulate the center frequency of the THz pulse by changing the thickness of the plasma
3 Discussion
To validate the physical results, we conducted 3D PIC simulations using a box size of
Parametric simulations were also carried out to see the effects of laser intensity, plasma density and density gradient at the boundary (more details in the
4 Conclusion
In summary, we propose a laser-driven plasma wiggler for efficiently generating a high-power, collimated, narrow-band and center-frequency-tunable THz pulse, by manipulating the electron reciprocating motion. A theoretical model is developed to describe the physical principle of realizing the center-frequency-tunable THz pulse. According to the model and PIC simulations, the center frequency of the THz pulse corresponds strictly to the reciprocating motion period of the electron beam. Simulations indicate that the center frequency of the THz pulses can be tuned from 4.4 to 1.5 THz as the plasma thickness changes from 20 to
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