infillInP

InP infill

InP is an
example of a high dielectric constant (e ~ 12.5 in the visible) semiconductor. As such, it is a good
choice for boosting the photonic properties of opaline materials as it can be grown
therein by conventional techniques.

FIG. 1. SEM
images of internal {111} (top) and {100} (bottom) facets of an artificial opal partially
filled with InP. InP granules on the sphere surfaces can be seen and their size estimated.

In Fig. 1, two images of
different cleaved edges of an InP infilled opal are shown. The loading, in this case fills
a 4% of the pore volume. Semiconductor crystallites with an average size of 60 nm can be
clearly seen on the spheres surfaces. SEM images demonstrate that the InP is very
homogeneously distributed inside the matrix and, as a consequence, the guest material
inherits the 3D periodicity of the host.

FIG. 2. a)
Theoretically calculated photonic pseudogap at the vicinity of L Brillouin zone point and
b) reflectance spectra of both bare and 4 vol.-% loaded InP opals. Sphere diameter is 380
nm.

In the right panel of Fig. 2
we show normalized reflectance spectra of both a f=380 nm sphere bare sample and the same
sample infilled with InP. A few interesting features can be extracted from these spectra.
On the one hand, the peak wavelength suffers a red shift under infilling, as expected and,
on the other, its linewidth is slightly reduced. A deeper theoretical analysis based on
order-N formalism shows that both effects are in good agreement with photonic band
structure modification induced by the presence of the infill (see left panel of Fig. 2).
However, the main change observed in InP infilled opals when compared with the bare ones
is the enhancement of the PX properties.

FIGURE 3. Diffuse reflectance
of the bare opal compared to the same opal after InP loading (4 vol.-% of the pore). Three
cases are shown for opals of increasing diameter: a) 311 nm, b) 359 nm, and c) 380 nm. As
can be seen the Bragg peak can be tuned across the bulk InP band gap (at 900nm).

In Fig. 3 reflectance spectra
of opals with different particle size, all having a 4% of the pore loaded with InP, is
shown. In spite of the small concentration of semiconductor, the intensity of the (111)
diffraction from the composite is about twice as large as that coming from the bare
structure. This result, caused by the increase of the dielectric contrast in the
structure, is in good agreement with the predictions. The thin InP coating of the silica
particles allows PBG effects to show, since light can cross several sphere crystalline
planes. Another interesting feature concerns the tunability of the PBG. The Bragg peak
arising from the opal-semiconductor composite can be tuned across the InP fundamental
absorption edge simply by varying the diameter of the silica spheres.