Toward Athermal Plasmonic Ring Resonators Based on Cu- ${\rm TiO}_{2}$-Si Hybrid Plasmonic Waveguide

Owing to the negative thermo-optic coefficient (TOC) of TiO₂, the real part of effective modal index of Cu-TiO₂-Si hybrid plasmonic waveguide (HPW) will be temperature-independent at a certain thickness ratio between the TiO₂ interlayer and the Si core. Cu-TiO₂-Si HPWs and waveguide-ring resonators (WRRs) are fabricated on SOI wafers using standard CMOS technology and are characterized around 1550-nm telecom wavelengths. The HPWs exhibit a propagation loss of ~0.2 dB/μm and negligible bending loss when the bending radius is larger than ~1 μm. WRRs with radii of 1.5-2.5 μm exhibit an extinction ratio of ~15-30 dB and a Q-value of ~140 . A WRR with a 220-nm Si core and 58-nm TiO₂ is measured to have a thermo-optic effect of ~52 pm/K , which is about half of the SiO₂ counterpart. The TOC of the as-deposited amorphous TiO₂ is then extracted to be ~-1.0-1.5 × 10^-4 K^-1. Athermal plasmonic WRRs are expected by either increasing the thickness of TiO₂ (with -1.0 × 10^-4 K^-1 TOC) to ~120 nm or improving the TOC of TiO₂ (with 58-nm thickness) to the reported value of -1.8 × 10^-4 K^-1.