Bond model of second-harmonic generation in wurtzite ZnO(0002) structures with twin boundaries

We describe second-harmonic generation (SHG) in ZnO(0002) wurtzite structures with [11¯00]/(1102) twin boundaries using the simplified bond hyperpolarizability model (SBHM). We show explicitly how the reflective second-harmonic generation (RSHG) intensity profile for the s-incoming fundamental and s-outgoing SHG polarized light arise via superposition of the SHG dipole fields. The nonlinear fields originate from anharmonic motions of electric charges along each bond. The total dipole fields from within the ZnO bulk sum up to zero but produce a constructive SHG radiation from the unreduced surface and twin boundary. In addition, we compare the third-order susceptibility tensor obtained from group theory and the SBHM and calculate the values for the nonzero components. We found that the off-resonance RSHG intensity data in diatomic wurtzite structures even with the presence of twin boundaries can be modeled using only three independent fitting parameters, namely, the effective bulk, reduced surface, and twin boundary SHG hyperpolarizability. The results show that the SBHM can be used to investigate impurities and surface defects in ZnO as well as their contribution to nonlinear radiation with potential application to frequency conversion in nanoscale optical circuitry.