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

TL;DR

This paper investigates how crystal anisotropy influences optical phonon resonances and second harmonic generation in silicon carbide, revealing selective enhancement effects and providing a general method applicable to similar polar dielectrics.

Contribution

It demonstrates the impact of crystal anisotropy on SHG in SiC and offers a general approach for analyzing anisotropic effects in wurtzite-structure polar dielectrics.

Findings

Anisotropy causes selective appearance of phonon resonances in nonlinear susceptibility.

Enhanced SHG observed due to anisotropy factor $$ in transmitted fields.

Method applicable to all wurtzite-structure polar dielectrics.

Abstract

We study the effects of crystal anisotropy on optical phonon resonances in the second harmonic generation (SHG) from silicon carbide (SiC) in its Reststrahl region. By comparing experiments and simulations for isotropic 3C-SiC and anisotropic 4H-SiC in two crystal cuts, we identify several pronounced effects in the nonlinear response which arise solely from the crystal anisotropy. Specifically, we demonstrate that the axial and planar transverse optical phonon resonances selectively and exclusively appear in the corresponding tensor elements of the nonlinear susceptibility, enabling observation of an intense SHG peak originating from a weak phonon mode due to zone-folding along the c-axis of 4H-SiC. Similarly, we identify an anisotropy factor $\zeta \equiv \epsilon_\perp/\epsilon_\parallel$ responsible for a steep enhancement of the transmitted fundamental fields at the axial longitudinal optical phonon frequency, resulting in strongly enhanced SHG. We develop a general recipe to extract all these features that is directly applicable to all wurtzite-structure polar dielectrics, where a very similar behavior is expected. Our model study illustrates the opportunities for utilizing the crystal anisotropy for selectively enhancing nonlinear-optical effects in polar dielectrics, which could potentially be extended to built-in anisotropy in artificially designed hybrid materials.