Second-harmonic generation and linear electro-optical coefficients of BN nanotubes

A systematic ab initio study of the second-order nonlinear optical properties of BN nanotubes within density functional theory in the local density approximation has been performed. Highly accurate full-potential projector augmented-wave method was used. Specifically, the second-harmonic generation (chi((2))(abc)) and linear electro-optical (r(abc)) coefficients of a large number of the single-walled zigzag, armchair, and chiral BN nanotubes (BN-NT) as well as the double-walled zigzag (12,0)@(20,0) BN nanotube and the single-walled zigzag (12,0) BN-NT bundle have been calculated. Importantly, unlike carbon nanotubes, both the zigzag and chiral BN-NTs are found to exhibit large second-order nonlinear optical behavior with the chi((2))(abc) and r(abc) coefficients being up to 30 times larger than that of bulk BN in both zinc-blende and wurtzite structures, indicating that BN-NTs are promising materials for nonlinear optical and optoelectric applications. Though the interwall interaction in the double-walled BN-NTs is found to reduce the second-order nonlinear optical coefficients significantly, the interwall interaction in the single-walled BN-NT bundle has essentially no effect on the nonlinear optical properties. The prominant features in the spectra of chi((2))(abc)(-2 omega,omega,omega) of the BN-NTs are successfully correlated with the features in the linear optical dielectric function epsilon(omega) in terms of single-photon and two-photon resonances.