Alexey Yu. Kuksin

Efficient Second-Harmonic Generation in Nanocrystalline Silicon Nanoparticles

Recent trends to employ high-index dielectric particles in nanophotonics are motivated by their reduced dissipative losses and large resonant enhancement of nonlinear effects at the nanoscale. Because silicon is a centrosymmetric material, the studies of nonlinear optical properties of silicon nanoparticles have been targeting primarily the third-harmonic generation effects. Here we demonstrate, both experimentally and theoretically, that resonantly excited nanocrystalline silicon nanoparticles fabricated by an optimized laser printing technique can exhibit strong second-harmonic generation (SHG) effects. We attribute an unexpectedly high yield of the nonlinear conversion to a nanocrystalline structure of nanoparticles supporting the Mie resonances. The demonstrated efficient SHG at green light from a single silicon nanoparticle is 2 orders of magnitude higher than that from unstructured silicon films. This efficiency is significantly higher than that of many plasmonic nanostructures and small silicon nanoparticles in the visible range, and it can be useful for a design of nonlinear nanoantennas and silicon-based integrated light sources.

MICROSCOPIC THEORY AND KINETIC MODEL OF SPALL IN LIQUIDS

Direct molecular dynamics (MD) simulations of the fracture of a simple liquid are performed. The kinetics of void nucleation and growth are obtained from MD simulations with a Lennard‐Jones interatomic potential. A model of fracture based on void nucleation and growth kinetics is proposed. Analysis of fracture kinetics and an estimation of spall strength are presented. The results of the model are in good agreement with direct MD simulations and experimental data (for hexane).

MOLECULAR DYNAMIC MODELING OF PLASTICITY OF Al AND Al‐Cu ALLOYS UNDER DYNAMIC LOADING

The molecular dynamic (MD) simulations presented are devoted to the study of mechanisms and kinetics of shock‐induced plasticity of Al and its alloy with Cu. Dislocation velocities as functions of applied shear stress are calculated in a wide temperature range up to melting point. Dislocation interaction with nanosize obstacles is considered. The critical resolved shear stress necessary for a dislocation to penetrate through an obstacle is determined. The results are used to characterize the effect of temperature on the dynamic yield stress in perfect crystals and crystals with nano‐sized obstacles.

INFLUENCE OF TEMPERATURE ON SPALL STRENGTH: ATOMISTIC SIMULATION

The influence of temperature and strain rate on a spall strength in a single crystal and polycrystal of aluminum is studied by means of molecular dynamics. The different behavior of the dynamic spall strength of the single and polycrystal was observed in shock‐wave experiments at a temperature near the melting temperature. The spall strength of polycrystal tends to be zero at the melting temperature, while for single crystal temperature influences on the spall strength sufficiently less and considerable overheating takes place. This work is devoted to explain such effect.

ATOMISTIC SIMULATIONS OF DISLOCATION NUCLEATION IN ALUMINUM

Homogeneous dislocation nucleation is considered by means of molecular dynamics (MD) simulations. Dislocation nucleation starts from the formation of partial dislocation loop by thermal fluctuations. Stochastic properties of nucleation time are considered. Homogeneous nucleation rates are calculated at different temperatures and shear stresses. The comparison with dislocation theory is presented: the dependence of the loop energy on shear stress and radius, nucleation rate at different temperatures. The activation shear stresses of these mechanism are obtained.

ATOMISTIC STUDY OF NANOPRECIPITATES INFLUENCE ON PLASTICITY AND FRACTURE OF CRYSTALLINE METALS

The molecular dynamics simulation is applied to study the influence of nanoprecipitates on the microscopic mechanisms of the spall fracture initiation and corresponding plastic deformation. The systems under study are single crystals doped with nanoclusters of another metal. The calculation results for the Cu‐Al system is presented. Three models are considered: the triaxial uniform expansion, the shock wave propagation and the reflection of release wave in the impactor‐target model and the model of an edge dislocation interaction with a nanocluster. Different characteristic consequences of the nanoprecipitate presence on material response are analyzed.

Atomistic simulations of fracture in nanocrystalline copper under high strain rates

3D molecular dynamics calculations of plasticity and fracture in nanocrystalline copper under high strain rate are carried out. Grain boundary sliding and dislocation motion are observed. Flow stress dependence on grain size is obtained. Three general properties of spall process are considered: existence of sites of stress concentration, two step process of microcrack formation and stochastic character of microcrack formation. Spall strength dependences on grain size and strain rate are obtained.