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| Credit: Yuxin Leng |
Applications for ultra-intense ultrashort lasers are numerous and include industrial service, health care, national security, and basic physics. These kinds of lasers are now widely used in fundamental physics to study strong-field laser physics, with particular applications in vacuum quantum electrodynamics, laser-driven radiation sources, and laser particle acceleration.
A change in the gain medium for large-aperture lasers (from neodymium-doped glass to titanium:sapphire crystal) has resulted in a dramatic increase in peak laser power, from the 1-petawatt "Nova" in 1996 to the 10-petawatt "Shanghai Super-intense Ultrafast Laser Facility" (SULF) and the 10-petawatt "Extreme Light Infrastructure—Nuclear Physics" (ELI-NP) in 2019. That adjustment reduced the pulse duration of high-energy lasers from roughly 500 femtoseconds (fs) to around 25 fs.
Nonetheless, 10-petawatt lasers seem to be the maximum power for ultra-intense, ultrashort titanium-sapphire lasers. Currently, researchers typically give up on titanium:sapphire chirped pulse amplification in favor of optical parametric chirped pulse amplification, which is based on deuterated potassium dihydrogen phosphate nonlinear crystals, for 10- to 100-petawatt development planning. That technique will be extremely difficult to realize and use for the next 10–100 petawatt lasers because of its low pump–to–signal conversion efficiency and poor spatiotemporal–spectral–energy stability.
However, as a well-developed technology that has successfully achieved two 10-petawatt lasers in China and Europe, titanium:sapphire chirped pulse amplification still has a lot of potential for the next stage of development of ultra-intense ultrashort lasers.
A broadband laser gain medium of the energy-level type is titanium:sapphire crystal. The energy storage process is completed when the pump pulse is absorbed and builds up a population inversion between the upper and lower energy levels. The energy stored in the titanium:sapphire crystal is retrieved for laser signal amplification after the signal pulse passes through it multiple times. On the other hand, with transverse parasitic lasing, the signal laser amplification is diminished and the stored energy is consumed by an enhanced spontaneous emission noise along the crystal diameter.
At the moment, titanium:sapphire crystals' maximum aperture is limited to 10-petawatt lasers. Because strong transverse parasitic lasing develops exponentially with titanium:sapphire crystal size, laser amplification is still not achievable even with bigger titanium:sapphire crystals.
Researchers have developed a novel method for addressing this problem that entails cogently tiling many titanium:sapphire crystals together. This technique, as described in Advanced Photonics Nexus, effectively increases the aperture diameter of the entire tiled titanium:sapphire crystal and truncates the transverse parasitic lasing within each tiling crystal, breaking through the current 10-petawatt limit on the ultra-intense ultrashort lasers.
As stated by the corresponding author Yuxin Leng of the Shanghai Institute of Optics and Fine Mechanics, "Our 100-terawatt (i.e., 0.1-petawatt) laser system effectively demonstrated the tiled titanium:sapphire laser amplification. With this method, we were able to achieve near-ideal laser amplification, which included short pulses, small focal spots, broadband spectra, constant energies, and high conversion efficiencies."
Coherently tiled titanium:sapphire laser amplification, according to Leng's group, offers a reasonably simple and affordable means of exceeding the existing 10-petawatt limit.
"By adding a 2×2 coherently tiled titanium:sapphire high-energy laser amplifier in China's SULF or EU's ELI-NP, the current 10-petawatt can be further increased to 40-petawatt and the focused peak intensity can be increased by nearly 10 times or more," Leng explains.
The technique is expected to improve ultra-intense
ultrashort lasers' experimental potential for strong-field laser physics.
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