In the realm of scientific research and technological advancements, ultra-intense ultrashort lasers have emerged as a versatile tool with applications spanning across various disciplines, including basic physics, national security, industrial services, and healthcare. These powerful lasers have revolutionized the field of strong-field laser physics, enabling significant progress in areas such as laser-driven radiation sources, laser particle acceleration, vacuum quantum electrodynamics, and more. Over the years, there has been a remarkable increase in the peak laser power, from the 1-petawatt “Nova” in 1996 to the 10-petawatt “Shanghai Super-intense Ultrafast Laser Facility” (SULF) in 2017 and the 10-petawatt “Extreme Light Infrastructure—Nuclear Physics” (ELI-NP) in 2019.
The surge in peak laser power can primarily be attributed to the shift in gain medium for large-aperture lasers, transitioning from neodymium-doped glass to titanium:sapphire crystal. This transition led to a significant reduction in pulse duration, from approximately 500 femtoseconds (fs) to around 25 fs. However, it seems that titanium:sapphire ultra-intense ultrashort lasers have reached their upper limit at 10-petawatt. As researchers embark on the development of 10-petawatt to 100-petawatt lasers, they are increasingly abandoning the titanium:sapphire chirped pulse amplification technology in favor of optical parametric chirped pulse amplification technology, which relies on deuterated potassium dihydrogen phosphate nonlinear crystals.
Although optical parametric chirped pulse amplification appears promising, it poses significant challenges in terms of low pump-to-signal conversion efficiency and poor spatiotemporal-spectral-energy stability. These limitations present formidable obstacles in the realization and application of future 10–100 petawatt lasers. In contrast, titanium:sapphire chirped pulse amplification technology stands as a mature technology that has successfully implemented two 10-petawatt lasers in China and Europe. Consequently, it still holds immense potential for the next-stage development of ultra-intense ultrashort lasers.
Titanium:sapphire crystal serves as an energy-level-type broadband laser gain medium. The pump pulse is absorbed to establish a population inversion between the upper and lower energy levels, thereby enabling energy storage. Amplification of the laser signal occurs when the signal pulse passes through the titanium:sapphire crystal multiple times, extracting the stored energy. However, transverse parasitic lasing poses a significant challenge, as amplified spontaneous emission noise along the crystal diameter consumes the stored energy and diminishes signal laser amplification.
In response to the transverse parasitic lasing issue, researchers have devised an innovative approach involving the coherent tiling of multiple titanium:sapphire crystals. This breakthrough, as reported in Advanced Photonics Nexus, effectively circumvents the current 10-petawatt limitation of titanium:sapphire ultra-intense ultrashort lasers. The approach involves increasing the aperture diameter of the tiled titanium:sapphire crystal while simultaneously truncating transverse parasitic lasing within each individual tiling crystal.
Yuxin Leng, the corresponding author from the Shanghai Institute of Optics and Fine Mechanics, highlights the successful demonstration of tiled titanium:sapphire laser amplification in their 100-terawatt (0.1-petawatt) laser system. Leng’s team achieved near-ideal laser amplification using this technology, boasting high conversion efficiencies, stable energies, broadband spectra, short pulses, and small focal spots. The coherently tiled titanium:sapphire laser amplification method presents a relatively straightforward and cost-effective means to surpass the current 10-petawatt limit.
The integration of a 2×2 coherently tiled titanium:sapphire high-energy laser amplifier in China’s SULF or EU’s ELI-NP holds immense potential. This addition could increase the current 10-petawatt laser to 40-petawatt, significantly enhancing the focused peak intensity by nearly 10 times or more. Consequently, this innovative approach promises to amplify the experimental capabilities of ultra-intense ultrashort lasers for strong-field laser physics and drive advancements in a multitude of scientific, technological, and industrial domains.
The breakthrough development of coherently tiled titanium:sapphire crystals represents an exciting advancement in the field of ultra-intense ultrashort lasers. By conquering the limitations of transverse parasitic lasing, researchers have paved the way for the realization and application of higher-power lasers. With the potential to unleash unprecedented levels of peak laser power, this innovative approach is poised to propel scientific discoveries and technological progress into uncharted territory.
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