(a) Scanning electron micrograph of the hetero-opal, (b) schematic of transmission measurements in the triple-film hetero-opal, and (c) the first Brillouin zone of the fcc lattice. The thick dashed line shows the scanning direction .
Transmission spectra of the triple-film opal in comparison with the spectra of its constituent (dotted lines) and (dashed lines) films at (a) and (b). At the and (111) bands overlap in the energy range from 1.65 to 1.75 eV.
Transmission surfaces obtained along and directions for the (a) superposition and (b) triple-film hetero-opal. The lines marked by , , and indicate branches of the transmission minima that appear in the transmission surface of the superposition but erased in the transmission surface of the triple-film opal.
Transmission surface of the interface transmission function. The inset shows an enlarged view of a fraction of the interface transmission function marked by the black rectangle.
Detailed view of transmission surfaces of Fig. 3 in the frequency and angle range of 2.25–2.55 eV and , respectively. The traces and indicate the branches of the transmission minima, of which the branch is not observed in the hetero-opal. The numbers show the appearance of additional features in the transmission surface of the hetero-opal.
Dispersion of transmission minima in the triple-film opal (crosses), in the opal film (squares) and in the opal film (circles). The Bragg fits are plotted for the (111), , (002), and (220) diffraction resonances (lines) as indicated. Numbers 1 and 2 denote the corresponding new features shown in Fig. 5(b).
The schematic of light refraction at the interface in the hetero-opal. (a) Isofrequency contours of light at a frequency well below the lowest order PBG in and opals superimposed on the Brillouin zone sections of (solid line) and (dashed line) opals. The arrow stands for the vector of the incident light. There is no refraction at the interface, because is the same for both films. (b) Isofrequency contours of (thick solid lines) and (thick dashed lines) opals at a frequency in the lowest order PBG range. The solid (dashed) arrows show vectors of light propagating in the opal (a). Case 1 represents the light refraction at the interface. Cases 2 and 3 demonstrate blocking of the light propagation by the directional PBG in the and opals, respectively.
(a) Transmission at the (111) diffraction minima of the film : in the triple-film opal (filled squares) and in the superposition spectra (open circles); (b) a close-up plot of Fig. 6 for the spectral range of 1.60–1.85 eV; (c) and (d) show the calculated corresponding mode dispersions and group velocities for the and films, respectively.
Parameters obtained from fitting experimental dispersion of transmission minima to the corresponding diffraction at the (111), , (002), and (200) planes according to the Bragg law, where is the diameter of PMMA particle, is the, effective RI, and is the angle that , (002), and (200) planes make with the (111) plane. The parameters highlighted in bold correspond to the Bragg fit plotted in Fig. 6.
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