  We propose an original and efficient method to engineer a defect between two
well-ordered silica opals by sputtering silica on the top of the first one. As
the amount of sputtered silica can be well controlled, it is also the case for
the thickness of the layer and consequently for the spectral position of the
defect mode. The optical response of these sandwich structures is studied in
terms of specular reflection and transmission spectroscopy. Tunable highly
transmitted and reflected optical modes are evidenced. The very good agreement
between the experimental results and the simulations, run without fitting
parameters, demonstrates the almost perfect order of the synthesized
structures.
