Femtosecond laser ablation of carbon: From spallation to formation of hot critical plasma

Strong ablative shock waves were generated on a highly oriented pyrolytic graphite surface by intense IR femtosecond laser pulses in a broad fluence range and their basic parameters (pressure, velocity) were acquired by means of a non-contact broadband ultrasonic technique. At moderate laser fluences (F ≥ 0.3 J/cm2), ultrafast formation of a dense, strongly-heated (supercritical) carbon phase is expected, which expands on a fluence-dependent picosecond timescale in the form of a point-like three-dimensional explosion, driving a multi-GPa shock wave both in ambient air and the graphite target. At higher laser fluences (F > 5 J/cm2), critical electron-ion plasma is formed instantaneously during the exciting femtosecond laser pulse, with its strong plasma absorption initiating a TPa-level shock wave (the maximum shock wave pressure ≈ 3 TPa is more than twice the previous maximum for carbon). Because of the ablative removal of the laser-heated surface carbon layer, the following time-integrated structural studies indicate the formation of a mixture of diamond and graphite nanocrystallites or, in other words, glassy carbon only in the redeposition products.