R6G_infill

Rhodamine 6G infill

In this study
we have introduced rhodamine 6G by simply soaking the crystal in a methanolic solution of
the said dye and leaving it to dry at ambient. By this method the dye enters the whole
crystal and is homogeneously distributed.

The optical
characterization of the bare opal is performed by studying the Bragg reflections produced
by the stacking of, mainly, (111) planes as a function of incidence angle. Other planes
can also be used, for which it is more convenient to perform transmission measurements
since the orientation of the sample is then less complicated. To measure the PL spectra
excitation with 514.5 nm Ar laser line was used keeping the exciting power as low as
possible in order to prevent degradation of the dye for as long as possible.

Fig. 1. Reflectance
measurements showing Bragg diffraction spectra from an artificial opal made up of 288 nm
spheres. Inset shows the fit to Bragg law. The scale is in arbitrary units as the
measurements are not absolute. Absolute reflectance measurements cast Bragg reflectance
values of about 50%.

In Fig. 1 a
typical Bragg reflectance experiment is shown. In the inset the wavelength for which
maximum reflectance is attained is plotted vs. incidence angle and fitted to Bragg law
which can be expressed in the form:

where d is the sphere diameter,n is the
average refractive index of the structure, and q is the incidence angle measured with respect to the normal to the sample,
that is, to its (111) planes. The constant 0.816=2/3½
accounts for the spacing between the close packed (111) planes in sphere diameter units.
As can be seen, the data can be very fairly accounted for with this simple model. In this
case we are studying the pseudogap between the first and second bands and the width to
centre ratio can be expressed as:

where e0 is the average dielectric
constant of the composite (zeroth Fourier component), UG is the Fourier
component of the dielectric constant relative to the reciprocal lattice vector G.
In our case G corresponds to the L point of the Brillouin zone which lies in the
(111) direction and the ratio results to be 0.054 whereas the experiment yields 0.08 due
to the presence of domains.

Fig. 2. Luminescence from
rhodamine 6G (full line) and rhodamine 6G-filled opal for different collection apertures
at right angles to the sample (111) surface (scaled to fit in the same plot). Transmission
of the bare opal is also shown as a dotted line where more than a decade of attenuation
can be appreciated. This is on top of a background produced by the diffuse scattering
arising from the polycrystalline character of the opal.

In Fig. 2
photoluminescence (PL) from Rhodamine 6G is shown in two circumstances: when alone (full
line), and when introduced in an opal for which Bragg reflection occurs at wavelengths
contained in the emission spectrum. For the latter case three spectra corresponding to
three numeric apertures of the collection optics are shown. Along with the luminescence,
the figure shows the optical transmission spectra of the bare opal for right angles
incidence (dashed line). As can be seen, the PL band coming from the dye presents a dip at
the wavelength that is Bragg reflected (lower transmission of the opal) for that specific
configuration. One must keep in mind that the attenuation produced by the opal is a
lossless process in the sense that no absorption is taking place since we are in the
transparency range for silica. This phenomenon, then, can be interpreted in the following
manner. Luminescence is only collected by the detector if it has been allowed to travel
the photonic crystal (not reflected by Bragg planes) and it emerges in the solid angle
captured by the aperture of the collecting lens. For a given geometrical configuration a
different wavelength in the PL produced inside the system is reflected by the (111) set of
planes (external face of the opal) thus being unable to escape the crystal. So, for low
numerical apertures of the collection optics, a narrow dip in the spectrum is obtained
inasmuch as only a well defined wavelength (narrow range) is Bragg reflected and missing
from the PL spectrum. This is the case for f/4 in Fig.2. When the collection angle is
increased, every wavelength has a wider range of angles for which Bragg law does not hold,
and consequently, may leave the system and be collected in the spectrum. The dips in the
spectra smear out for lower f numbers as seen for the cases of f/1.8 and f/1.1 (increasing
apertures) due to wider range in angles available for wavelengths to travel the photonic
crystal. The overall decrease in intensity results from the decrease in field depth of the
wide aperture lenses that overpowers collection angle.

Fig. 3. Luminescence
from free rhodamine (continuous line) compared with that in the opal taken at 0º
(squares), at 20º (circles) and at 30º (diamonds) with respect to the surface normal
that corresponds to a (111) plane.

When the light collection axis orientation
is varied, the position of the dip is shifted across the PL spectrum as can be seen in
Fig. 3. Here the collection optics has been kept untouched while the angle formed by the
(111) planes of the sample and the optic axis was varied. In this case the numeric
aperture has been kept as low as possible by collecting PL with the monochromator slit at
one focal length from the collecting lens while the sample was far away on the other side.
In this circumstances the collection aperture can be considered virtually zero as for 250
mm slits and a collecting lens of 50 mm focal length the solid angle subtended is 1.9´10-5 sr. For normal (0º spectrum)
collection the inhibition band occurs at around 620 nm in agreement with reflectance
measurements in Fig. 1. For higher angles the wavelengths involved are lower and the
inhibited range occurs eventually on the low wavelength tail of the PL. See the 30º
spectrum in Fig. 3 where inhibition can be appreciated at 560 nm. This figure demonstrates
that the inhibition of the PL is produced by the photonic structure of the host material.
In this case the range of PL wavelengths that verify Bragg law and, according thereto are
inhibited, can be tuned through the angle.