FIGURE 2. SEM images of planar Si photonic crystals.

Cross-sectional SEM images are shown as a function of the thickness of the initial opal template for 2 (a), 4 (b), and 16 (c) layers. The Si substrate is snapped, and the fracture propagates up through the photonic crystal. Close examination of the lowest layer reveals that the photonic crystals are completely integrated into the wafer. Apparent defects in c are due to a ripple in the fracture surface that occurs in thicker photonic crystals. d, Sample edge showing the (100) surface, confirming that the crystals are f.c.c. e, A planar (111) surface exposed by RIE. From this image, parameters¹⁹, such as the radius of the Si coating sphere (0.41) and the radius of the air sphere (0.354), both in units of the lattice constant, were estimated. f, A planar (100) surface exposed by RIE. The sphere diameters are 670 nm (c and e), 855 nm (d and f), and 1 m (a and b).