Unveiling Structured Optical Coherence in Nonlinear Optics
arXiv:2607.13296
The paper studies how incoherent light sources affect second‑harmonic generation, deriving and experimentally confirming a far‑field coherence function that depends on both the source distribution and the nonlinear crystal structure, and showing that coherence can increase as light propagates inside the crystal.
Abstract
Nonlinear optical interactions are typically driven by coherent sources. Recently, however, incoherent sources have also been shown to offer new opportunities in nonlinear optics. Here we study the effects of the source incoherence in second harmonic generation. We derive and experimentally validate an analytic expression for the far-field coherence function at the second harmonic frequency, which is a generalization of the well-known van Cittert-Zernike theorem. Unlike the linear optics case, here the results depend not only on the source spatial distribution, but also on the nonlinear structure of the crystal. We further show, by performing Young's double-slit measurement, that the coherence at the second harmonic increases as the light propagates inside the nonlinear crystal. Our results establish nonlinearity as a degree of freedom for optical coherence, opening a new research area at the interface of statistical optics and nonlinear optics.
8 pages, 5 figures