inverse opal

https://doi.org/10.1351/goldbook.13832
Colloidal crystal of spherical voids with a periodic structure within a solid polymer continuum.
Note: An inverse opal is usually produced by assembling spheres, filling the interstitial spaces with solid, and removing the spheres.
Source:
PAC, 2012, 85, 463. (Terminology for aggregation and self-assembly in polymer science (IUPAC Recommendations 2013)) on page 475 [Terms] [Paper]