Structural instability of transition metals upon ultrafast laser irradiation

Abstract

Precision in laser material structuring is critically defined by the energy flow during the irradiation process, particularly in ultrafast regimes. Alternative to thermal evolutions, nonequilibrium electronic excitation can exercise a direct influence on the atomic bonding delivering a potentially rapid destructuring process. In this context, the atomic disordering of transition metals (Cr, Ni and Ti) induced by non-equilibrium electronic excitation typical for ultrafast laser processing is studied with an emphasis on the role of the d-band filling and crystalline structure. The density functional theory is used to obtain, from first principles, structural stability criteria and non-thermal disordering pathways on timescales shorter than picosecond electron-phonon dynamics. We show that the hot electrons distort charge distribution in the cold crystalline arrangement and increase the entropy of the system thus driving the mechanical expansion. The calculated nonequilibrium freeenergy potentials indicate that a solid destabilization is possible when electron temperature reaches a universal value of around 2 eV for all considered metals. Under a uniaxial lattice relaxation expected in the laser ablation of surface layers the charge redistribution and loss of lattice stability is shown to be oriented along specific crystal directions. Moreover, we show that the interatomic potential destabilization is energetically more favorable for Ni than for Cr and Ti. Lattice dynamics is affected by space charge redistribution induced by Fermi smearing associated with anisotropic expansion due to electron pressure gradients. This ultrafast destructuring mechanism is shown to be a general feature of partially-filled-d-band transition metals.

Energy Relaxation and Electron–Phonon Coupling in Laser-Excited Metals

“…If the initial state is a solid, the lattice is heated due to electron–phonon energy transfer to a new thermal equilibrium over several (tens to hundreds of) picoseconds. During this evolution, many interesting effects such as the ultrafast electron and non-equilibrium phonon dynamics [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ], changes in lattice stability [ 20 , 21 , 22 , 23 , 24 ], phase transitions [ 25 , 26 , 27 , 28 , 29 , 30 ], and non-equilibrium electron–phonon interactions [ 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ] take place. It should be pointed out that these various physical processes are all driven by electronic excitations.…”

Super-efficient drilling of metals with ultrafast non diffractive laser beams

“…The metal target complex refractive index 55 is where optical properties at relatively high electronic temperatures (30,000 K) were used to emulate laser-material interaction and the excitation of the metal. This electronic temperature marks the beginning of material destructuring 67 and is regarded as a reasonable hypothesis for interaction with hot but not ablated walls.…”

Effect of Nonequilibrium Transient Electronic Structures on Lattice Stability in Metals: Density Functional Theory Calculations

“…Lattice dynamics are of fundamental importance in a variety of fields, including phase transition [1], superconductivity [2], and thermal conductivity [3] It is reported that under intense laser radiation, the melting of the semiconductor will be athermal due to softening of interatomic bonds, which takes place long before the conventional thermal melting due to the process of energy transfer from electrons to ionic lattice [4]. For metals, however, the phenomenon of structural instability, phonon hardening, and phonon softening has been reported [4][5][6][7][8][9][10][11]. Recently, the lattice dynamics of Au-Cu alloys at warm dense state (WDM) have also been investigated [12].…”