Characterizing the thermo-optic coefficient of gallium phosphide-on-insulator platform using high-quality ring resonators

Characterizing a material's thermo-optic coefficient lays the foundation for optimizing thermal tuning of photonic integrated devices, a key feature for applications in optical communication, sensing, and signal processing. Unlike traditional bulk measurements, determining the thermo-optic coefficient (TOC) in microscale photonic devices offers significant advantages in data processing and provides more direct relevance to real-world device performance. In this work, we characterize the TOC of gallium phosphide (GaP) films using an air-cladded ring resonator, built on a GaP-on-insulator (GaP-OI) architecture. The resonator is fabricated via an optimized “etch-n-transfer” process, which incorporates silicon dioxide hard masks to enhance the precision of pattern transfer and improve the waveguide surface cleanliness, reducing defects and ensuring better device performance. The fabricated resonator exhibits a loaded quality factor of (2.18 ± 0.1)×104 at 1550 nm by using contact lithography, with a waveguide propagation loss of 23.8 ± 0.3 dB/cm. At 780 nm, the propagation loss decreases to 16.7 dB/cm. The resonator also shows a temperature-dependent wavelength shift of 65.8 pm/K, allowing us to extract a TOC of 1.19 × 10−4/K for GaP. This high temperature sensitivity empowers the GaP-OI platform particularly well-suited for rapid thermal turning, which is beneficial for a range of applications including optical sensing, optical signal processing, and highly efficient nonlinear conversion.