Change the pulse speed of the super-strong ultrashort laser

Super ultra-short lasers generally refer to laser pulses with pulse widths of tens and hundreds of femtoseconds, peak power of terawatts and petawatts, and their focused light intensity exceeds 1018 W/cm2. Super ultra-short laser and its generated super radiation source and high energy particle source have a wide range of application value in many basic research directions such as high energy physics, particle physics, plasma physics, nuclear physics and astrophysics, and the output of scientific research results can then serve the relevant high-tech industries, medical health, environmental energy and national defense security. Since the invention of chirped pulse amplification technology in 1985, the emergence of the world's first beat watt laser in 1996 and the completion of the world's first 10-beat watt laser in 2017, the focus of super ultra-short laser in the past has mainly been to achieve the "most intense light". In recent years, studies have shown that under the condition of maintaining super laser pulses, if the pulse transmission speed of super ultra-short laser can be controlled, it may bring twice the result with half the effort in some physical applications, which is expected to reduce the scale of super ultra-short laser devices, but improve its effect in high-field laser physics experiments.

Distortion of pulse front of ultra-strong ultrashort laser

In order to obtain the peak power under limited energy, the pulse width is reduced to 20~30 femtoseconds by enlarging the gain bandwidth. The pulse energy of the current 10-beak-watt ultra-short laser is about 300 joules, and the low damage threshold of the compressor grating makes the beam aperture generally greater than 300 mm. The pulse beam with 20~30 femtosecond pulse width and 300 mm aperture is easy to carry the spatiotemporal coupling distortion, especially the distortion of the pulse front. Figure 1 (a) shows the spatio-temporal separation of the pulse front and the phase front caused by the beam role dispersion, and the former shows a "spatio-temporal tilt" relative to the latter. The other is the more complex "curvature of space-time" caused by the lens system. FIG. 1 (b) shows the effects of ideal pulse front, inclined pulse front and bent pulse front on the spatio-temporal distortion of the light field on the target. As a result, the focused light intensity is greatly reduced, which is not conducive to the strong field application of super ultra-short laser.

FIG. 1 (a) the tilt of the pulse front caused by the prism and the grating, and (b) the effect of the distortion of the pulse front on the space-time light field on the target

Pulse speed control of ultra-strong ultrashort laser
At present, Bessel beams produced by conical superposition of plane waves have shown application value in high field laser physics. If a conically superimposed pulsed beam has an axisymmetric pulse front distribution, then the geometric center intensity of the generated X-ray wave packet as shown in Figure 2 can be constant superluminal, constant subluminal, accelerated superluminal, and decelerated subluminal. Even the combination of deformable mirror and phase type spatial light modulator can produce arbitrary spatio-temporal shape of the pulse front, and then produce arbitrary controllable transmission speed. The above physical effect and its modulation technology can transform the "distortion" of the pulse front into "control" of the pulse front, and then realize the purpose of modulating the transmission speed of ultra-strong ultra-short laser.

FIG. 2 The (a) constant faster-than-light, (b) constant sublight, (c) accelerated faster-than-light, and (d) decelerated sublight light pulses generated by superposition are located in the geometric center of the superposition region

Although the discovery of pulse front distortion is earlier than super ultra-short laser, it has been widely concerned along with the development of super ultra-short laser. For a long time, it is not conducive to the realization of the core goal of super ultra-short laser - ultra-high focusing light intensity, and researchers have been working to suppress or eliminate various pulse front distortion. Today, when the "pulse front distortion" has developed into "pulse front control", it has achieved the regulation of the transmission speed of super ultra-short laser, providing new means and new opportunities for the application of super ultra-short laser in high-field laser physics.