Ab initio study of ferroelectric and nonlinear optical performance in BiFeO3 ultrathin films

Density functional theory within the local density approximation plus on-site Coulomb repulsion is applied to investigate the crystal structure, electronic structure, multiferroism, linear, and nonlinear optical response in BiFeO3 ultrathin films, which are fabricated via epitaxial growth on (001)-oriented SrTiO3 substrates and have a strained P4mm phase. Born effective charges are calculated within the Berry-phase method and the spontaneous electric polarization is found to be 79 mu C/cm(2). The second-harmonic generation susceptibility, which can reach 7.5 x 10(-7) esu, undergoes a substantial change between different magnetic orderings. Our calculations demonstrate the existence of robust ferroelectric distortion and nonlinear optical response in these ultrathin films. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3232215]