Effects of Crystal Anisotropy on Optical Phonon Resonances in the Mid-Infrared Second Harmonic Response of SiC

摘要

We study the effects of crystal anisotropy on optical phonon resonances inthe second harmonic generation (SHG) from silicon carbide (SiC) in itsReststrahl region. By comparing experiments and simulations for isotropic3C-SiC and anisotropic 4H-SiC in two crystal cuts, we identify severalpronounced effects in the nonlinear response which arise solely from thecrystal anisotropy. Specifically, we demonstrate that the axial and planartransverse optical phonon resonances selectively and exclusively appear in thecorresponding tensor elements of the nonlinear susceptibility, enablingobservation of an intense SHG peak originating from a weak phonon mode due tozone-folding along the c-axis of 4H-SiC. Similarly, we identify an anisotropyfactor $\zeta \equiv \epsilon_\perp/\epsilon_\parallel$ responsible for a steepenhancement of the transmitted fundamental fields at the axial longitudinaloptical phonon frequency, resulting in strongly enhanced SHG. We develop ageneral recipe to extract all these features that is directly applicable to allwurtzite-structure polar dielectrics, where a very similar behavior isexpected. Our model study illustrates the opportunities for utilizing thecrystal anisotropy for selectively enhancing nonlinear-optical effects in polardielectrics, which could potentially be extended to built-in anisotropy inartificially designed hybrid materials.