Monolithic tantalum pentoxide athermal microrings with intrinsic Q factors exceeding 5 × 10⁶ without SiO₂ cladding

Tantalum pentoxide (Ta₂O₅), as a silicon-photonic-compatible material platform, has garnered significant attention for high-performance wafer-scale integrated photonics due to its exceptional properties: a broad transparency window spanning from 0.28 µm to 8 µm, a moderate refractive index of 2.05 at 1550 nm, an impressive nonlinear refractive index of 1.0 × 10^-14 cm²/W, and thick-film deposition with low residual stress at room-temperature. Despite these advantages, achieving low-loss fabrication of monolithic microrings on the Ta₂O₅ platform remains challenging due to its inherent hardness and brittleness, which often result in rough sidewalls and significant scattering losses. In this work, we successfully demonstrated monolithic Ta₂O₅ microring resonators with exceptionally high intrinsic and loaded quality (Q) factors, without covering with SiO₂ cladding to preserve its intrinsic material properties. This was accomplished through the innovative application of photolithography-assisted chemo-mechanical etching (PLACE) technique for producing the device with an ultra-smooth surface. By optimizing the coupling region between the microring and the bus waveguide, near-critical coupling was achieved. The device exhibited loaded Q factors of 3.89 × 10⁶ in the telecom band without employing expensive electron-beam lithography, showing an intrinsic Q factor as high as 5.49 × 10⁶ and a low propagation loss of only 0.0696 dB/cm-representing the highest results reported for strongly confined Ta₂O₅-based microrings to date. Moreover, the suppressed temperature-induced resonant wavelength shift rate of only 9 pm/°C was also demonstrated, showing impressive thermal stability. This work paves the way for the development of advanced photonic devices on the Ta₂O₅ platform with low manufacturing cost, including low-threshold microlasers, highly sensitive sensors, broad-bandwidth supercontinuum sources, and optical frequency combs.