A new method proposed by Shanghai Institute of Optics and Mechanics to describe the structure of amorphous silicon oxide and its dynamic process of laser damage induced shock

Recently, the Metal Organic Framework (MOF) force field model parameters proposed by the research team of the Thin Film Optics Laboratory of the Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences, can not only accurately describe the structure of amorphous silicon oxide, but also efficiently simulate the hydrodynamic process of amorphous silicon oxide in laser damage impact, Relevant achievements were published in Optical Materials Express under the title of "Structure and shock properties of ambient silica predicted by a metal organic framework force field".

Amorphous silicon oxide is the most widely used optical material in high power laser system components. It controls laser energy transmission in the form of windows, substrates, films, etc. Under the irradiation of high-power laser, the nano absorption center in amorphous silicon oxide materials deposits laser energy with band gap collapse, forming complex hydrodynamic processes such as local high temperature and high pressure driven shock wave transmission, inducing dense phase transition and topological defects, which has an essential impact on the catastrophic damage and fatigue effect of laser components. Although there are many force fields that can accurately describe the structure of amorphous silicon oxide, the traditional long range Coulomb interaction method (Ewald summation) for calculating the ionic bond contribution of silicon oxide depends on the periodic boundary conditions, which requires a huge amount of computation when simulating the open boundary of hydrodynamic processes. In addition, the traditional Buckingham method for calculating the covalent bond contribution of silicon oxide has a short-range atomic condensation with no physical significance. Under the impact load driven by high temperature and high pressure, silicon oxygen atoms have a greater probability to cross the short-range energy barrier to form the above atomic condensation phenomenon.

In order to solve the above problems, researchers used the MOF model to describe the covalent bonding of silicon oxide, introduced Grimme correction on the basis of Buckingham method, eliminated the short range atomic condensation with no physical significance, shielded the long range action of ion charge with spherical Gaussian charge distribution, avoided the requirement of periodic boundary conditions, and optimized a set of force field parameters that can accurately describe the topological order of amorphous silicon oxide structure and impact the Hugoniostat equation of state. Taking the amorphous silicon oxide structure composed of 24000 atoms as an example, the efficiency of static structure optimization and shock Hugoniostat equation of state can be improved from 5.2 ns/day to 33.5 ns/day. The removal of periodic boundary conditions means that the force field model can effectively describe the laser induced hydrodynamic behavior of free surfaces such as optical element interface/surface/crack. In view of the close relationship between the hydrodynamic process and the high pressure phase and topological defects, this work has important scientific significance for the life and repair of laser components.

Relevant research has been supported by the Science and Technology Innovation Action of Shanghai Municipal Science and Technology Commission, the National Key R&D Plan, the Shanghai Youth Science and Technology Talent Sailing Plan, the Youth Promotion Association of the Chinese Academy of Sciences, and the National Natural Science Foundation of China.

Fig. 1 Silicon oxide interaction potential (the solid line is the method in this paper, and the dotted line is the traditional method)

Fig. 2 Effect of impact pressure on (a) compression ratio (b) compression ratio below HEL (c) coordination number (d) ring size

Source: Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences