Angular shaping of fluorescence from synthetic opal-based photonic crystal
© Boiko et al.; licensee Springer. 2015
Received: 6 November 2014
Accepted: 23 January 2015
Published: 28 February 2015
Spectral, angular, and temporal distributions of fluorescence as well as specular reflection were investigated for silica-based artificial opals. Periodic arrangement of nanosized silica globules in the opal causes a specific dip in the defect-related fluorescence spectra and a peak in the reflectance spectrum. The spectral position of the dip coincides with the photonic stop band. The latter is dependent on the size of silica globules and the angle of observation. The spectral shape and intensity of defect-related fluorescence can be controlled by variation of detection angle. Fluorescence intensity increases up to two times at the edges of the spectral dip. Partial photobleaching of fluorescence was observed. Photonic origin of the observed effects is discussed.
Modification and enhancement of the fluorescence in photonic structures is important for development of optical sensors  and improvement of the fluorescence efficiency  and light harvesting ability in solar cells . Most papers (see, for example, reviews [1,4]) deal with experimental and theoretical aspects of the fluorophores embedded in the multilayered films forming one-dimensional (1D) photonic crystals. The behavior of such structures is determined by the interference of light that leads to decreased reflectance and enhanced fluorescence at certain directions of observation. Fluorescence has increased intensity and higher degree of polarization at these angles, in comparison to the background emission. The increase of light intensity at certain directions may be explained by resonant coupling of fluorescence with the waveguiding leaky modes in the 1D structure that can result in shorter fluorescent lifetimes and higher radiative rates [5,6]. In particular, resonantly enhanced directional fluorescence with decreased lifetime was detected experimentally for a dye doped in 1D photonic crystals  and explained in terms of increased density of states near the photonic bandgap. Directional emission of light has also been reported for the multilayered films doped with quantum dots  and rare-earth ions .
Considerably less attention has been paid to the modification of light emission properties of fluorophores incorporated within three-dimensional (3D) photonic crystals, such as artificial opals composed of closely packed dielectric globules. Photonic stop zones in 3D structures have been already proved useful for blocking undesirable light emission . In particular, this effect was used to avoid leakage of light from the dye-sensitized solar cells , to suppress the radiative channels , and in such a way to improve the Förster resonance energy transfer between the dye molecules situated inside the photonic structure .
In this work, we investigate the influence of photonic stop zones on the intrinsic fluorescence of 3D photonic crystal made of closely packed silica globules. Special attention is paid to the changes in the spectral shape of fluorescence as a function of the detection angle and the angular shift of photonic stop zone. It is demonstrated that the self-fluorescence of silica material can be enhanced at certain detection angles, near the spectral edges of the photonic stop zone.
Preparation of photonic crystals based on synthetic opals was carried out in several steps. First, the silica globules were prepared by the hydrolysis of alkyl orthosilicate. Second, sedimentation and close-packing of these globules from the solution was achieved by centrifugation. Finally, the precipitated samples were annealed to obtain solid samples with size of several cm. Two samples (marked as 1 and 2) of compacted silica globules of slightly different sizes were selected for the study.
The samples were investigated by using optical and spectroscopic methods. SEM images of opals were obtained with EPMA SEI JXA-8200 microscope (JEOL Ltd., Akishima-shi, Tokyo, Japan). Reflectance spectra at the normal incidence were measured on JASCO V-570 spectrophotometer (JASCO International Co. Ltd., Tokyo, Japan), whereas the angular dependence of reflectance was acquired on a custom goniometric setup . Fluorescence was excited with Nd:YAG laser emitting at 266 nm. Fluorescence spectra were detected by means of Andor SR303i spectrograph equipped with a CCD camera.
Results and discussion
In principle, one could employ the Brewster law for the estimation of effective refractive index of the samples, based on the angular dependences of reflectance, measured spectrally far from the photonic stop band (Figures 4b and 5). It was revealed, however, that the diffusely reflected light restricts this possibility.
For the sample 1, we have λ 1 = 532 nm, θ 1 = 46° (Figure 3a) and λ 2 = 593 nm, θ 2 = 25° (Figure 3b). Substituting these values into the Equation 1 yields a system of two equations with two unknown parameters D and n eff. The solution of this system gives D = 276 nm and n eff = 1.38.
The spectra may belong to electronic transitions in the defect centers of -Si-O-Si-O- lattice which have the energies in the visible range . Particularly, the maximum at 450 nm in spectrum 2 can be associated with twofold Si-oxygen deficiency center O2 = Si: [22,23]. Fluorescence bands with maxima near 500 nm in spectra 1 and 3 can be related to the hydrogenated ≡ Si-H defects, which are formed by attaching H and OH groups to the disrupted ≡ Si• and ≡ Si-O• bonds . Red luminescence in the range of 600 to 700 nm (spectra 2 and 3) can be associated with non-bridging oxygen hole centers  or OH groups on the surface of the silica .
Similar photobleaching of blue fluorescence has been reported earlier for the laser-treated silica waveguides . This process may be caused by photoconversion of the silica twofold deficient centers into the E′-centers initiated by the two-photon absorption . Further measurements were conducted after a preliminary UV irradiation of samples in the saturation region between 800 and 1,200 s (Figure 8), where the changes of fluorescence intensity caused by photobleaching are already minimal.
Fluorescence photobleaching may be affected by light confinement inside the photonic crystal and slow non-radiative migration of excitation between the defects in SiO2 material. As a consequence, the excitation energy may decrease before reaching the defect-related fluorescence center. The longer the migration time, the smaller the energy reaching the luminescent center. Therefore, less energetic red fluorescence could become predominant after a prolonged irradiation. Similar effect and its origin have been discussed before .
An interesting feature in the recorded fluorescence is a dip in the spectrum 2 (Figure 7), with a minimum at 507 nm that is absent in spectra 1 and 3. This spectral feature overlaps with the maximum of reflectance, caused by photonic stop zone (Figure 7, spectrum 2′). Since the photonic stop zone of the sample 1 is almost outside the spectral range of fluorescence (Figure 7, spectrum 1′), the dip in fluorescence spectrum 1 is absent. The presence of the photonic dip in spectrum 2 demonstrates fundamental opportunity to control the spectral shape of fluorescence spectrum by 3D photonic structure of opal.
In addition to the angular shift of the dip in fluorescence spectra, which is clearly of photonic origin, an enhancement of the fluorescence intensity was observed near the edges of spectral dip. For example, the fluorescence intensity at the detection angle of 70° is almost two times higher than that at the angle of 0° at the wavelength of 490 nm (marked by dashed line in Figure 9). We suppose that such angular enhancement in fluorescence also has photonic nature and is caused by blue shift of the photonic stop band at large angles. Similar effect was described for the light transmitted through a thin photonic crystal film . The overlap of diffractional resonances associated with different systems of crystallographic planes would also lead to a redistribution of light intensity on the edges of the stop bands, which is visible for the transmitted light in Figures 2 and 3 in Ref. .
We demonstrated that intrinsic fluorescence of opal-based photonic crystals can be influenced by photonic stop zone. A decrease of the fluorescence at the wavelengths within the photonic stop band and its increase near the edges of stop band were observed. This effect could be proved by comparison of the fluorescent spectra detected at different angles, despite the undesirable photobleaching of the samples.
This work was supported by the Marie Curie ILSES project no. 612620, Russian-Ukrainian project 27-02-14, NATO SPS project NUKR.SFPP984702, Nanotwinning FP7 project (ID 294952), and European Regional Development Fund project TK114 and RFBR project 14-02-90406.
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