Fig. 4.

Observation of grating diffraction from the periodic surface structures of photonic balls. (A and B) SEM and optical micrographs of photonic balls prepared from 400-nm colloids. Diffraction-grating colors, arising from the periodic arrangements of the colloids at the ball’s surface, are visible toward the rims of the ball. (Scale bars, 10 µm.) (C) Model of the photonic ball’s surface as a one-dimensional, curved diffraction grating and angular distribution of colors at different parts of the curved surface. Only light that is reflected back with a small range of angles (defined by the N.A. of the objective, indicated as a dotted cone in the image) close to normal to the surface from the grating is picked up by the objective lens and appears as color on the ball’s surface. Because the incident angle increases from point 1 to point 4, light with increasing wavelengths is back-reflected into the lens. (D) Spatial distribution of the color predicted by the model, plotted on a circular surface mimicking the top-view image of the ball. The positions of color in the model and micrograph coincide.