Second harmonic generation from Si m Ge n superlattices

We report the observation of optical second harmonic generation (SHG) from SimGen short-period superlattices grown on different substrates. For a 775 nm fundamental beam, the SHG signals from odd-odd, even-even, and even-odd specimens are comparable, apart from substrate-dependent strain enhancement. It is concluded that the lack of a large bulk dipole-allowed signal in odd-odd superlattices predicted by Ghahramani, Moss, and Sipe [Phys. Rev. Lett. 64, 2815 (1990)] is due to the inevitable presence of monatomic steps on the substrate which lead to antiphase domains; a possible circumvention of this limitation is outlined based on the use of a single domain vicinal Si(001) substrate.