Synergistic diffraction and multibounce reflection interference within single iridescent microstructures

Multibounce reflection interference produces structural coloration due to interference between light rays traveling by different paths of total internal reflection in a microscale cavity. However, as microscale structures are reduced in dimension, it is possible that a ray-based framework may become inaccurate as contributions from wave-based phenomena, like diffractive effects, are expected to be increasingly important to the observed interference. Here, we employ Fourier plane imaging microscopy to analyze angle-resolved interference spectra from individual microstructures at the 10 μm scale to reveal how edge diffraction events, synergistically coupled with multibounce total internal reflection, contribute to the observed far field interference. We show that hemicylindrical microstructures on these length scales, particularly those with lower contact angles, exhibit interference signatures that cannot be fully explained by the conventional multibounce ray-based framework but align with full-wave simulations. By selectively illuminating and collecting angularly resolved interference spectra from specific regions within isolated hemicylinders, we identify and separate the contributions from light entering and exiting along different paths to the far field interference. To bridge the gap between ray-based and wave-based interpretations, we develop an extended ray model incorporating edge diffraction and use experimental data to guide our estimation of the relative intensity contributions of diffracted ray trajectories. This work not only demonstrates the unique utility of Fourier plane microscopy and spectroscopy for optical characterization of iridescent materials but also contributes fundamental insight into the complex optical effects occurring within small radius of curvature microscale surfaces.