Abstract
Chalcogenides are a promising platform for infrared nonlinear optics but are susceptible to structural changes during fabrication that affect their linear and nonlinear optical properties. We analyze the structure and optical properties of thermally evaporated and annealed chalcogenide films. Thermally evaporated Ge28Sb12Se60 has an increased selenium content, bandgap, and concentration of heteropolar bonds. The concentration of heteropolar bonds can be reduced by annealing above the glass transition temperature, resulting in improved optical nonlinearity. We demonstrate a 4-fold enhancement of third-order nonlinearity in Ge28Sb12Se60 chalcogenide waveguides by thermal annealing and a decrease in propagation loss from 2.5 dB/cm to 1 dB/cm as an added benefit.
Highlights
The nonlinearity of optical materials allows for ultrafast modulation of light as well many light-matter interaction phenomena
Though chalcogenides are a promising platform for nonlinear photonics, the changes their optical properties undergo during fabrication has yet to be fully characterized
We have demonstrated the enhancement of nonlinearity in thermally evaporated chalcogenide through thermal annealing
Summary
The nonlinearity of optical materials allows for ultrafast modulation of light as well many light-matter interaction phenomena. Most thin film deposition processes require the evaporation or dissociation of the chalcogenide source material into a vapor or liquid phase This causes deviations in the optical properties of thin films from the bulk glass. Z-scan measurements have demonstrated an n2 of 11.3×10−18 m2/W in melt-quench prepared bulk Ge28Sb12Se60 [16], while the n2 of thermally evaporated Ge28Sb12Se60 waveguides has been measured to be 0.5×10−18 m2/W [1,30] This decrease in nonlinearity shows that the deposition process has affected the nonlinear optical properties of the chalcogenide. Our annealing study covers temperatures over this whole range and characterizes what changes have occurred by measuring the linear index, bandgap, and bond structure of the films By performing this systematic analysis, we have determined that the decreases in the crystallinity of the films can be used to predict increases in nonlinearity. The roughness of the chalcogenide waveguide is found to decrease with annealing, resulting in a lower propagation loss
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