Laserlike emission from silica inverse opals infiltrated with Rhodamine 6G
Introduction
Natural opals are silica based materials, where monodispersed submicrometer amorphous silica spheres, with chemically bonded water molecules, are spontaneously arranged in a periodic structure. The mesoscale structure within the mineral causes Bragg diffraction of light producing the beautiful optical effects displayed by these materials [1], [2], [3], [4], [5]. Synthetic opals on the other hand can be prepared from monodisperse latex spheres for example. A simple and efficient method is proposed in [3] where styrene is polymerized in the presence of potassium persulphate without any surfactant. Monodisperse latex spheres suspensions can be obtained with the size of the spheres being controlled by the reaction conditions. The synthetic latex opal can be used as a template to prepare silica based inverse opals. Then, silica nanostructures are obtained with pore size and ordering that can be controlled using three-dimensional (3-D) close packed crystals of submicrometer latex spheres as a template for silica polymerization. The template can be removed in a subsequent step by heat treatment for example [3].
The phenomenon of direction dependent propagation of light in opals is the main characteristic of the so-called photonic crystals. A full photonic band gap is observed in that case depending on the structure and refractive index profile [5]. In opals a pseudo band gap is observed for certain directions. The possibility of spontaneous emission suppression for a given luminescent specie inside the structure is one of the features possible to occur in these photonic structures [5], [6].
On the other hand, laserlike emission from strongly scattering media has been the subject of intense research since its first observation in dye solutions containing TiO2 submicrometer particles [7]. The emission from such system, which was named as laser paint (LP) [7], has characteristics of a multimode laser, although no resonant cavity is used. The phenomenon has been investigated by many authors and several points regarding the characteristics of the laserlike emission have been clarified [8], [9], [10], [11]. Of particular importance to obtain laser emission is the multiple scattering of light which occurs due to the difference between the refractive index of the liquid and the scatterers. The multiple scattering events contribute to increase the effective pathways of the emitted photons leading to interaction with a larger number of molecules. Thus, non-linear processes such as stimulated emission or non-linear absorption may occur and the dominant effect will depend on the specific properties of the active molecules [7], [8], [9], [10], [11], [12].
An analogous process of laser generation is expected to be observed if a periodic arrangement of particles, such as in the case of a synthetic opal, is infiltrated by a laser active medium and excited in a proper manner. In fact, inverse opals are appropriate structures to investigate the effect of laserlike generation because the emission properties of incorporated dyes may change considerably owing to the spatial modulation of the refractive index, as shown in Refs. [13], [14], [15], [16], [17].
In this paper we report a study on the laserlike generation by Rhodamine 6G infiltrated in silica inverse opals. The observation of this effect was previously reported in Ref. [18] and here we present further details of the samples preparation and the emission characteristics.
Section snippets
Experimental
Latex spheres were prepared with a surfactant free methodology [3]. Styrene was added to water at 70 °C under N2 and controlled mechanical stirring (±1 rpm). A solution of potassium persulphate was then added slowly (0.09 mL min−1). After 28 h the latex suspension was isolated. Colloidal crystallization was achieved by centrifugation (5300 rpm at 4 °C during 3 h). The resulting colloidal crystals were dried first in air for 24 h and after at 70 °C for 3 h under vacuum.
Silica based inverse opals were
Results
Monodisperse latex spheres and silica inverse opals (f.c.c. crystalline structure of air holes in a silica host) have been successfully prepared. Samples with holes of 210 nm diameter and 182 m2 g−1 surface area were used in the present experiments. Typical SEM images of the obtained latex opals and silica inverse opals are shown in Fig. 1.
Emission due to the incorporated dye is observed under laser excitation and Fig. 2 shows the fluorescence spectra. By increasing the pump intensity narrowing of
Discussion
The laser beam, with a frequency off the pseudo bandgap in the blue region (that is, light transmission is reduced, but not completely eliminated in the wavelength range inside the band gap), penetrates inside the porous structure exciting molecules along its trajectory. Although silica presents no absorption in the visible range, the laser light is absorbed inside the inverse opal structure due to the presence of the dye molecules. The emitted light propagates inside the opal structure and
Conclusion
Silica inverse opals have been prepared from tetraethoxisilane and latex opals templates obtained by controlled settling of monodisperse latex spheres. The adsorption of Rhodamine 6G at the opal pores walls was performed and spectroscopic characteristics studied. Laserlike emission with frequency off the photonic band gap was observed under 532 nm laser excitation. The emitted light propagates inside the opal structure and amplified spontaneous emission occurs along the random pathway due to
Acknowledgments
Financial support of the Brazilian agencies CNPq, FAPESP, the research program ‘Programa de Nucleos de Excelência (PRONEX)’ and the CAPES (Brazil) – COFECUB (France) cooperation program (project 471/04) is acknowledged.
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