Modification of opal photonic crystals using Al2O3 atomic layer deposition

Aluminum oxide (Al2O3) atomic layer deposition (ALD) on synthetic opal was explored as a model system to understand the growth of ALD films on photonic crystals. Al2O3 ALD was used to coat the silica spheres in synthetic opal with conformal Al2O3 films. Using Al2O3 ALD to modify the interstices of the opal allowed for the tuning of the position and intensity of the Bragg reflection from the opal structure. Numerical transfer matrix method (TMM) simulations were used to predict the optical effects from the modified photonic crystal. The TMM simulations assumed an ideal face-centered cubic (fcc) crystal structure with conformal Al2O3 ALD on all surfaces. The experimental Bragg wavelengths from the Al2O3-coated photonic crystals were observed to shift to longer wavelength versus Al2O3 ALD thickness before asymptotically reaching a limiting wavelength. This red shift was attributed to the higher effective refractive index produced when Al2O3 ALD coats the SiO2 spheres in the opal structure. The experimental and predicted Bragg wavelengths showed excellent agreement for thin Al2O3 ALD films. This agreement indicated that the Al2O3 ALD inside of the opal structure occurs conformally as assumed by the model. Differences between experiment and simulation were observed for high filling fractions of the opal structure. These discrepancies were attributed to a higher free volume in the opal structure than the free volume predicted by the fcc geometry. The reflectance of the Bragg peak also decreased versus Al2O3 ALD thickness. This reduction in intensity was attributed to the decrease in the refractive index difference between the SiO2 spheres and their surroundings.