Broadband Chromatic Dispersion of Thermo‐Refractive Coefficients and Its Impact in Silicon Nitride Nonlinear Photonics

ABSTRACT

The thermo‐refractive effect is a cornerstone of frequency and phase tuning in photonic integrated circuits. In particular, it enables control of phase‐matching for integrated nonlinear processes. Chromatic dispersion of the group and effective refractive indices and modal confinement are standard considerations in design, but material thermo‐refractive coefficients (TRCs) are typically taken to be fixed for the guiding and cladding materials. Here, we demonstrate that the assumption of non‐dispersive TRCs across an octave of bandwidth between the telecom and visible results in a significant discrepancy between measured and simulated resonance frequencies of an integrated / microring resonator. We uncover a 7% variation in and material TRCs across this range, finding that the variation of from material TRCs is 1.3 times that from modal confinement. This matches a temperature‐dependent Lorentz oscillator model describing their chromatic dispersion. By integrating these dispersive TRCs into a multi‐physics finite‐element model, we achieve precise correspondence with experimentally measured temperature‐dependent resonance frequency shifts across the octave, including in the context of second harmonic generation devices. Our results provide a physical framework and a universal predictive workflow for the design of high‐efficiency, multi‐wavelength nonlinear optical processes, fundamentally improving the thermal control of integrated photonic devices.