Photograph taken under white-light illumination in air (left) and acetone (right), showing clear changes in
structural colour. All images were captured in the direction perpendicular to the SIO film. b, Photonic band structure of the SIO in air (blue) and acetone
(red) calculated using the plane-wave expansion method. The refractive indices used in the simulation were 1.54 for silk, 1.00 for air and 1.36 for acetone.
The unshifted and shifted pPBG are highlighted in the figures. Frequency-domain reflectance spectra for the SIO with lattice constant L ¼ 300 nm are
plotted on the right to compare with the band structure. c, Measured reflectance spectra obtained from L ¼ 300 nm (left) and L ¼ 240 nm (right) opal.
Reflectance peaks are redshifted when acetone is deposited on the SIO. Inset: images of individual silk opals undergoing colorimetric shift