Breaking Barriers: Pushing Beyond the 10-Petawatt Limit with Latest Laser Amplification

A perfectly pieced-together titanium:sapphire laser amplification apparatus. Yuxin Leng is to credit for this achievement.

Ultra-intense ultrashort lasers, which have a vast range of uses including but not limited to national security, industrial utilization, healthcare, and fundamental physics research, have become an invaluable instrument in studying strong-field laser physics. Their applications include, but are not restricted to, laser-particle acceleration, laser-driven radiation sources, and vacuum quantum electrodynamics.

The surge in peak laser power from the 1996’s 1-petawatt “Nova” to the “Shanghai Super-intense Ultrafast Laser Facility” (SULF) of 2017 equipped with 10-petawatt and the “Extreme Light Infrastructure – Nuclear Physics” (ELI-NP) of 2019 having the same 10-petawatt, is mainly attributed to a change in gain medium for large-aperture lasers - shifting from “neodymium-doped glass” to “titanium:sapphire crystal” - which reduced the pulse duration of high-energy lasers from approximately 500 femtoseconds (fs) to about 25 fs.

Despite their mammoth potential, it appears that the titanium:sapphire ultra-intense ultrashort lasers have a maximum potential limit of 10-petawatt. Consequently, scientists are diverting their efforts from the titanium:sapphire chirped pulse amplification technology to optical parametric chirped pulse amplification technology for the development planning of lasers with a range from 10-petawatt to 100-petawatt. This new direction is primarily exploiting deuterated potassium dihydrogen phosphate nonlinear crystals.

While this new technology brings with it certain challenges in terms of its lower pump-to-signal conversion efficiency and unsatisfactory spatiotemporal-spectral-energy stability, it has sparked hope for the future applications and realization of 10–100 petawatt lasers. Meanwhile, the titanium:sapphire chirped pulse amplification technology, which has successfully led to the realization of two 10-petawatt lasers in China and Europe, still holds great promise for future advancements of ultra-intense ultrashort lasers.

The titanium:sapphire crystal functions on an energy-level-type broadband laser gain medium mechanism. In this process, pump pulse absorption plays a fundamental role in building up a population inversion between the upper and lower energy levels, thus completing the energy storage. Once the signal pulse passes through the titanium:sapphire crystal multiple times, the stored energy is then extracted for laser signal amplification. However, strong transverse parasitic lasing can lead to a reduction in the signal laser amplification due to an consumed stored energy by amplified spontaneous emission noise along the crystal diameter.

The current generation of titanium:sapphire crystals can barely support lasers that exceed the 10-petawatt threshold. This limitation persists even with larger titanium:sapphire crystals due to the effect of strong transverse parasitic lasing which exponentially increases as the size of the titanium:sapphire crystals is also incrementally increased.

In a spur to overcome this challenging threshold, researchers have adopted an inventive method that involves combining multiple titanium:sapphire crystals coherently. In an article published in the Advanced Photonics Nexus on December 23, 2023, this method was reported as a breakthrough that shatters the existing 10-petawatt limitation of the titanium:sapphire ultra-intense ultrashort lasers. It effectively enlarges the aperture diameter of the entire tiled titanium:sapphire crystal while simultaneously truncating the transverse parasitic lasing within each tiling crystal.

The lead author of this study Yuxin Leng, from the Shanghai Institute of Optics and Fine Mechanics, points out, “Our developed 100-terawatt (i.e. 0.1-petawatt) laser system successfully demonstrated the effectiveness of the tiled titanium:sapphire laser amplification. We were able to achieve near-ideal laser amplification with this groundbreaking technology, including high conversion efficiencies, stable energies, broadband spectra, short pulses, and small focal spots.”

The findings of Leng’s team indicate that the coherently tiled titanium:sapphire laser amplification technique is a relatively convenient and cost-effective means of outpacing the current 10-petawatt constraint. Leng adds, “By incorporating a 2×2 coherently tiled titanium:sapphire high-energy laser amplifier in China’s SULF or EU’s ELI-NP, the current 10-petawatt intensity could potentially be pushed further to a staggering 40-petawatt, and the peak intensity of focused light could be raised by nearly a factor of 10 or even more.”

Such a technique could significantly enhance the experimental capabilities of ultra-intense ultrashort lasers in strong-field laser physics.