Sidewall Angle as a Microstructural Degree of Freedom for Intrinsic Electro-Optic Enhancement in X-Cut Thin-Film Lithium Niobate

In X-cut thin-film lithium niobate (TFLN), the electro-optic response is fundamentally governed by the alignment between the applied radio-frequency (RF) electric field and the crystallographic axes. The etch-defined waveguide sidewall angle is established not as a fabrication artifact, but as an active, materials-level microstructural degree of freedom for intrinsic electro-optic enhancement. By precisely engineering sidewall angle to 74.7° using a robust inductively coupled plasma reactive ion etching process while keeping all planar electrode and waveguide dimensions fixed, geometric field engineering is demonstrated, which reorients the RF electric field toward the crystallographic Z-axis. This activates the dominant r33coefficient, yielding an RF propagation loss (αRF) of 1.65 dB/cm at 50 GHz under fixed electrode geometry, an optical loss of 0.28 dB/cm, and a dynamic energy efficiency of 0.82 pJ/bit for 200G PAM4, demonstrating significant improvement over prior art without electrode modification. This work shifts the design paradigm in crystalline photonics from “electrode-centric” to “morphology-aware,” providing a general, fabrication-compatible route to co-optimize optical, RF, and crystallographic responses through deliberate morphological control.