Prof. Andrei Rode (Laser Physics Centre, Australian National University)

Ultrashort laser pulses focussed down to a micron-size spot subsurface of transparent materials can achieve energy densities on the order of MJ/cm3, resulting in solid-density plasma formation, followed by a microexplosion and shockwave compression of the surrounding substance. Subsequent rapid quenching leads to the formation and preservation of novel non-equilibrium material states.  Reaching extreme pressures is of fundamental interest for the formation of new material phases, and  also for the study of the Warm Dense Matter, reproducing the state of the cores of planets in table-top laboratory experiments.

Ultrafast laser induced microexplosion in confined geometry has already demonstrated the potential to create and preserve new thermodynamically non-equilibrium state of matter such as bcc-Al [1] and two tetragonal phases of Si [2].  These new phases have been predicted to exist theoretically, but have never before observed in nature or in laboratory experiments.  In this talk I’ll present a new way for increasing the shock wave affected volume by using a micro-Bessel beam with a 100:1 aspect ratio [3,4]. The experimental results show an effective formation of voids when such a beam focused inside sapphire crystal, which is a clear indication of significantly increased efficiency of new phase formation when compared with the previous experiments with a Gaussian laser beam.  The results open up a new way for increasing the quantity of high-density/pressure phases and help to increase sensitivity in search of new phases using X-ray and electron diffraction analysis.

[1] A. Vailionis, et al., “Evidence of superdense aluminium synthesized by ultrafast microexplosion”, Nat. Comm. 2, 445 (2011).
[2] L. Rapp, et al., “Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast laser-induced confined microexplosion”, Nat. Comm. 6, 7555 (2015).
[3] L. Rapp, et al., “High aspect ratio micro-explosions in the bulk of sapphire generated by femtosecond Bessel beams” Sci. Rep. 6, 34286 (2016).
[4] E. G. Gamaly, A. V. Rode, “Ultrafast re-structuring of the electronic landscape of transparent dielectrics: new material states (Die-Met)”, Appl. Phys. A 124, 278 (2018).