  The physical origin of resonant Wood's anomalies in the reflection spectra of
three-dimensional (3D) opaline photonic crystals covered with
Ge$_2$Sb$_2$Te$_5$ (GST225) is discussed. For this purpose, the optical
reflection spectra are studied for different incident angles of light both
experimentally and theoretically. The performed eigenmode analysis reveals that
the Wood's anomalies originate from the quasiguided modes which appear in the
GST225 capping layer. This conclusion is supported by the simulated
electromagnetic near-field distributions of incident light at resonant
frequencies. The experimental reflection spectra are in a good agreement with
theoretical calculations performed by the Fourier modal method in the
scattering matrix form.
