The transmission curves of samples, which were prepared at four different temperatures, are shown in Fig. . Sample 1, 2, 3, and 4 were prepared at 18, 14, 10, and 6 °C, respectively. It can be seen that when the preparation temperature decreases a blue shift of transmission peak occurs, while the transmissivity of incident light, corresponding to the first Bragg condition peak, increases simultaneously. Especially for sample 4, the stop band around 350 nm could hardly be observed and the transmission curve is similar to that of ordinary AAM. This phenomenon may be explained as follows: while the temperature decreases, either the oxide formation or chemical dissolution is decelerated, which directly induces the reducing of thickness of both main and branched channel layer (d1
), furthermore the Bragg wavelength, λm
, (m is the order of the Bragg condition), will become diminished according to the equation shown as below [14
The transmission spectra of AAM DBRs prepared at 18 °C (curve a), 14 °C (curve b), 10 °C (curve c) and 6 °C (curve d), respectively
where n1 and n2 stand for the refractive index of main and branched channel layer, respectively. To prove this, the samples prepared under different temperatures were characterized by FE-SEM as shown in Fig. . Figure a shows an FE-SEM image of the sample 1 prepared at 18 °C, and Fig. b corresponds to sample 2 prepared at 14 °C. It can be clearly seen that the lower the temperature was, the thinner the main and branched channel became. With the temperature decreasing to 10 °C, the main and branched channel layers turned further thinner and some branched channels began to vanish, shown in Fig. c. In Fig. d corresponding to the sample prepared at 6 °C, we can observe that due to slow growth rate at low temperature most branches disappear. As a result the morphology of the channels seems the same as that of straight ones in the ordinary AAM. The distance of remaining adjacent branched channel layers bears less than 100 nm in thickness.
The SEM image of samples prepared ata18 °C,b14 °C,c10 °C, andd6 °C, respectively
Figure shows the optical photograph of AAM DBRs prepared at different anodization temperature from 7 to 14 °C. It is clearly demonstrated that these AAM DBRs almost contain every color of the whole visible light region.
The optical photograph of AAM DBRs prepared for 24 h at different anodization temperatures from 7 to 14 °C
Another issue to be considered is the increasing of transmissivity as the temperature drops, which is brought by the inadequate growth of branches. It is well known that sufficiently high dielectric contrast of two different dielectric materials comprised in the photonic related structures could bring high reflectance to a certain incident light. The reflectivity of a periodical multilayer structure composed of alternating layers with different refractive indices nL
on the substrate with the index of nS
is given as below [15
where the nL and nH is the refractive index of the low and high index dielectric, respectively. Here, the branched channel layer stands for low index dielectric material and the main stem layer corresponds to the high one. The M is the number of the layer pairs. Every two neighboring layers, i.e., main channel layer and branched channel layer, constitute one layer pair. In our experiment, we could consider the air play the role of substrate with nS as 1. The refractivity R is inversely proportional to the value of nL/nH, which means the transmissivity T is directly proportional to this ratio. When the anodization temperature decreases, the branched channel layers will not grow sufficiently, so the contrast of effective refractivity of two layers turns to be less. Since the ratio of nL/nH is augmented, the transmissivity of the whole structure therewith raises. Hence, the effect of inhibition to the incident light becomes weakened.