Material phase transformation by laser-induced confined microexplosion

The quest for recreating in laboratory experiments conditions of extreme pressure and temperature that exists inside planets and stars has been a driving force for researches for more than a century. At pressures above 100 GPa (1 Mbar) and temperatures above 104 K, common materials form exotic dense phases with unusual physical properties due to compacted atomic arrangement.

High power laser-matter interaction conditions open the synthesis avenue to novel functional material structures which are formed above the pressure limitation of a diamond anvil cell and cannot be produced by other means. Femtosecond laser pulses can deposit a volume energy density up to several MJ/cm3 in a sub-micron volume. This creates highly non-equilibrium, hot, dense and short-lived plasmas with conditions favourable for arrangement of atoms into unusual material phases. The TPa-range pressure, which is higher than the strength of any material, combined with temperature in excess of 100,000 K and ultra-high cooling rates ~1014 K/s, give access to novel, nonequilibrium material states.

The ultra-short laser-induced confined microexplosion method has extended the range of possible new phases by initiating the highly non-equilibrium plasma state deep inside a bulk material. The excessive pressure inside the focal volume transforms a material and leads to the formation of a cavity surrounded by a shell of compressed material. Ultra-high quenching rates can help to overcome kinetic barriers to the formation of new metastable phases, while the surrounding pristine crystal confines the affected material and preserves it for further study.

Electron microscope images of laser produced voids beneath the SiO2 surface of the oxidized wafer. (a) SEM image of FIB-opened section showing an array of voids produced by 170 fs, 800 nm, 300 nJ single laser pulses focused 2 μm apart with ×150 objective; the inset shows a magnified transverse crosssection of voids. (b)–(d) TEM side views of voids at the Si/SiO2 interface produced by single 400 nJ pulses focused with ×20 objective to 2 J/cm2 (c) and with ×150 objective focused at 42 J/cm2 (c) and 95 J/cm2 (d). The TEM images (b)–(d) demonstrate the elongation of voids toward the laser pulse with an increase of laser fluence, starting from the Si–SiO2 interface at 2 J/cm2 just above the threshold of optical breakdown.

The discovery of a new stable super-dense phase of body-centered-cubic aluminium (bcc-Al) from sapphire crystal

Formation of new phase of aluminium within the compressed sapphire provides evidence of unusual and unexpected phenomenon, namely, the spatial separation of Al- and O-ions at the plasma stage and then freezing the bcc-Al nanocrystals during the transition to ambient conditions. The analysis reveals that spatial separation of ions with the different masses is possible in highly non-equilibrium, hot, dense and short-lived plasmas, where the temperatures of electrons and ion species are significantly different. The essential distinctive feature of laser-driven microexplosion is that the modified material remains compressed and confined in a strongly localized region inside a bulk, and can be investigated later by Raman spectroscopy, electron beam, x-ray, and other structural diagnostics techniques.

Application of the microexplosion technique to the domain of non-transparent material, and discovery of two novel silicon phases, the tetragonal bt8-Si and st12-Si, which have also been theoretically predicted but never observed before

Electron diffraction patterns from tetragonal Si phases. Predominantly (a) bt8-Si signatures are produced by a laser fluence of 95 J/cm2 and (b) st12-Si signatures by a laser fluence of 48 J/cm2. In addition, in both cases evidence of further tetragonal phases such as Si-VIII and t32-Si is observed. In both SADPs the dc-Si diffraction rings used for calibration are indicated by white dash-dotted circles. The bt8-Si and st12-Si diffraction rings are depicted by yellow and light-green dotted circles, respectively. The d-spacings used for these rings are calculated from the experimental lattice parameters of bt8-Si and st12-Si. Corresponding pairs of reflections are connected by radial dashed lines to guide the eye.

New original approach

An original approach now consists in using high-aspect ratio micro-Bessel beams to enhance the ultrafast laser microexplosion conditions generating inside material an increase in quantity of the new so-created phases together with increasing the fabrication speed. The invariance of the shape of the beam during the nonlinear propagation is a key issue and by employing a Bessel beam with higher order of topological charge we can enhance the laser radiation absorption and increase the energy density in microexplosion and microimplosion experiments.

Scanning electron microscopy images of nanochannels in sapphire generated by a Bessel beam of 2.5 µJ at 275 fs with associated magnifications (A and B).

The phase created by ultrashort laser pulse gives further insights into the high-pressure behaviour of materials. The results demonstrate and confirm that high-power ultrafast lasers represent new perspectives for studies of high-pressure materials at the laboratory tabletop, and unfolds new routes for formation of exotic material phases.

Selected publications

L.A. Smilie, M. Niihori, L. Rapp, B. Haberl, J.S. Williams, J.E. Bradby, C.J. Pickard, A.V. Rode, Micro-Raman spectroscopy of ultrashort laser induced microexplosion sites in silicon, Phys. Rev. Materials 4 (2020) 093803
R. Buividas, I. Aharonovich, G. Seniutinas, X. W. Wang, L. Rapp, A.V. Rode, T. Taniguchi, S. Juodkasis, Photoluminescence from voids created by femtosecond laser pulses inside cubic-BN, Optics Letters (2015) 40 24
L. Rapp, B. Haberl, C.J. Pickard, J.E. Bradby, E.G. Gamaly, J.S. Williams, A.V. Rode, Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast-laser-induced confined microexplosion, Nature Communications 6, 7555 (2015)
L. Rapp, B. Haberl, J.E. Bradby, E.G. Gamaly, J.S. Williams, A.V. Rode, Confined microexplosion induced by ultra-short laser pulse at SiO2/Si interface, Applied Physics A 114 1 (2014) 33-43
E. G. Gamaly, L. Rapp, V. Roppo, S. Juodkazis and A. V. Rode, Generation of high energy density by fs-laser-induced confined microexplosion, New J. Phys. 15, 025018 (2013)
E. G. Gamaly, A. Vailionis, V. Mizeikis, W. Yang, A. V. Rode, S. Juodkazis, “Warm Dense Matter at the bench-top: Fs-laser-induced confined micro-explosion” High Energy Density Physics, 8, 13-17 (2012)
R. Buividas, G. Gervinskas, A. Tadich, B. C. C. Cowie, V. Mizeikis, A. Vailionis, D. de Ligny, E. G. Gamaly, A. V. Rode, S. Juodkazis “Phase Separation in Laser-InducedMicro-Explosion in Olivine (Fe,Mg)2SiO4” in press, Optical Materials Express (2013)
A. Vailionis, E. G. Gamaly, V. Mizeikis, W. Yang, A. V. Rode, S. Juodkazis, “Evidence of superdense aluminium synthesized by ultrafast microexplosion” Nature Communications 2, 445 (2011)
L. Bressel, D. de Ligny, E. G. Gamaly, A. V. Rode, S. Juodkazis “Observation of O2 inside voids formed in GeO2 glass by tightly-focused fs-laser pulses”, Optics Materials Express 1, 1150-1157 (2011)

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