Field enhancement (FE) plays a pivotal role in boosting the four‐wave mixing conversion efficiency (FWM CE)in photonic resonant cavities. While previous studies typically attributed this enhancement to intracavity power buildup, fewer have linked it to the effective nonlinear interaction length. In this work, we propose a different perspective that FE can be interpreted as an elongation of effective length. Moreover, we establish a unified framework that bridges nonresonant waveguides and resonant cavities through this generalized effective length concept. We validate the model experimentally using direct‐bonded gallium phosphide‐on‐insulator waveguides and microresonators. We achieve an intrinsic quality (Q) factor of 2.8 × 104 for the TM00 resonance at 1546.15 nm wavelength. The TM00 mode FWM CE is −37.12 dB in a 25 µm‐radius microresonator, 18 dB higher than that in a 4.8 mm‐long waveguide of identical cross section and polarization under the same on‐chip power of 10 dBm. The FWM CEs of both the waveguides and the microresonators exhibit a well‐correlated relation versus their effective lengths, confirming the unified theoretical framework. These experimentally validated findings offer a fresh perspective on Kerr nonlinearity and pave the way toward advancing the development of highly efficient nonlinear photonic devices and products.
Cheng et al. (Sun,) studied this question.