Study of crystalline defect induced optical scattering loss inside photonic waveguides in UV-visible spectral wavelengths using volume current method.

In this work, we study the crystalline defect induced optical scattering loss inside photonic waveguide. Volume current method is implemented with a close form of dyadic Green's function derived. More specifically, threading dislocation induced scattering loss inside AlN waveguides in UV-visible spectrum wavelengths are studied since this material is intrinsically accompanied with high densities of dislocations (typically on order of 108-1010cm-2). The results from this study reveal that threading dislocations contribute significant amount of scattering loss when material is not MOCVD grown. Additionally, the scattering loss is strongly dependent on polarization and waveguide geometries: TM modes exhibit higher scattering loss compared with TE modes, and the multimode large core waveguides are more susceptible to threading dislocations compared with single mode waveguides and high-aspect-ratio waveguides. Conclusions from this work can be supported by several recently published investigations on III-N based photonic devices. The model derived from this work can also be easily altered to fit other material systems with other types of crystalline defects.

Characterizations of the nonlinear optical properties for (010) and (2¯01) beta-phase gallium oxide.

We report, for the first time, the characterizations on optical nonlinearities of beta-phase gallium oxide (ß-Ga2O3), where both (010) ß-Ga2O3 and (2¯01) ß-Ga2O3 were examined for two-photon absorption coefficient, Kerr nonlinear refractive index, and their polarization dependence. The wavelength dependence of two-photo absorption coefficient and Kerr nonlinear refractive index were also estimated by a widely used analytical model. ß-Ga2O3 exhibits a two photon absorption (TPA) coefficient of 1.2 cm/GW for (010) ß-Ga2O3 and 0.6 cm/GW for (2¯01) ß-Ga2O3. The Kerr nonlinear refractive index is -2.1 × 10-15 cm2/W for (010) ß-Ga2O3 and -2.9 × 10-15 cm2/W for (2¯01) ß-Ga2O3. In addition, ß-Ga2O3 shows stronger in-plane nonlinear optical anisotropy on (2¯01) plane than on (010) plane. Compared with GaN, TPA coefficient of ß-Ga2O3 is 20 times smaller, and the Kerr nonlinear refractive index of ß-Ga2O3 is also found to be 4-5 times smaller. These results indicate that ß-Ga2O3 have the potential for ultra-low loss waveguides and ultra-stable resonators and integrated photonics, especially in UV and visible wavelength spectral range.

Low loss GaN waveguides at the visible spectral wavelengths for integrated photonics applications.

We perform comprehensive studies on the fundamental loss mechanisms in III-nitride waveguides in the visible spectral region. Theoretical analysis shows that free carrier loss dominates for GaN under low photon power injection. When optical power increases, the two photon absorption loss becomes important and eventually dominates when photon energy above half-bandgap of GaN. When the dimensions of the waveguides reduce, the sidewall scattering loss will start to dominate. To verify the theoretical results, a high performance GaN-on-sapphire waveguide was fabricated and characterized. Experimental results are consistent with the theoretical findings, showing that under high power injection the optical loss changed significantly for GaN waveguides. A low optical loss ~2 dB/cm was achieved on the GaN waveguide, which is the lowest value ever reported for the visible spectral range. The results and fabrication processes developed in this work pave the way for the development of III-nitride integrated photonics in the visible and potentially ultraviolet spectral range for nonlinear optics and quantum photonics applications.