Observation of Multiple Stop Bands in Photonic Bandgap Structures Doped with Organic Dyes** By Petr Nozar,* Davide DiDomenico, Chiara Dionigi, Maria Losurdo, Michele Muccini, and Carlo Taliani The dramatic growth of the number of papers dealing with photonic bandgap (PBG) structures indicates the importance of the subject. [1±15] In this paper we report on the optical properties of self-as- sembled periodic structures of monodisperse doped polysty- rene beads (LATEX). [16±20] Our aim is to study the enhance- ment of the refractive index contrast using an organic dye (Oil Red EGN) infiltrated into the periodic structure and the influence of the dye absorption on the optical properties of the assembly. We test the validity of the simple analysis of op- tical spectra in the framework of the Bragg and Snell law. Some considerations regarding dimensions of LATEX suit- able for experiments are also drawn. Similar experiments have been performed by Park et al. on LATEX doped with the organic dye Oil Blue N aggregated as a thin layer (»12 lm). [20] Measurements of optical spectra re- ported in the literature [17,20] were performed in transmission configuration; they suffer from some basic disadvantages: Firstly, the wavelengths of the stop band of doped and un- doped samples are generally different; secondly, due to the multiple scattering in photonic crystals (PCs) the effective op- tical thickness of doped and undoped samples are different even if the physical thickness is the same; and thirdly, the ab- sorption in the doped PC is strongly amplified due to multiple scattering, in contrast to the disordered doped sample. There- fore, it seems to be inadequate to use both ordered undoped PCs and disordered doped samples as references for the ab- sorption in the ordered doped PC. Indeed, from our transmis- sion measurements (not reported here) it follows that even at the lowest concentration of dye (1.3  10 ±3 M) used, the ab- sorption of the dye reaches saturation, which manifests itself as an apparent bandgap in the transmission spectrum, inde- pendent of the incident angle of the light beam. Therefore, we measured reflectivity spectra of thick sam- ples (»1.5 mm). As there is zero transmitted intensity, the absorption equals the difference between the intensity of the incoming and reflected beam. Within the gap, the attenuation ratio appears smaller due to absorption, which lowers the re- flected intensity of the light (absorption has an opposite effect with respect to the transmission experiments). However, the measured width of the bandgap and its shift with the change of the incident beam angle should not be significantly influ- enced by absorption. In the article we use the following terminology: i) The stop band is the energy interval associated with the reflection of light from one ªcrystallographicº system of planes. ii) If the stop bands of all ªcrystallographicº systems of the planes have non-zero overlap, the energy interval of this overlap is called a full photonic bandgap. iii) The strong nonlinear behavior of the refractive index as a function of wavelength at the absorp- tion is called the resonance, for convenience. The synthesis of charged LATEX, their aggregation, doping by an organic dye (Oil Red EGN), and the characterization by atomic force microscopy (AFM) and transmission electron microscopy (TEM) methods is reported elsewhere. [21] Figures 1a and 1b show typical results of absolute reflec- tance measurements on undoped LATEX and doped LA- TEX, respectively. The spectrum of the sample of undoped beads exhibits one main reflectivity maximum, which corre- sponds to the reflection from the system of (111) planes of the sample. With increasing incident angle c of the light beam, this maximum is shifted towards lower wavelengths (blue shift), and the intensity of the maximum gradually decreases. Adv. Mater. 2002, 14, No. 15, August 5 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1508-1023 $ 17.50+.50/0 1023 COMMUNICATIONS ± [*] Dr. P. Nozar, Dr. D. DiDomenico, Dr. C. Dionigi, Dr. M. Muccini, Prof. C. Taliani Istituto per lo Studio dei Materiali Nanostrutturati, CNR Via P. Gobetti, 101, I-40129 Bologna (Italy) E-mail: nozarpetr@libero.it Dr. M. Losurdo Plasma Chemistry Research Centre, CNR Via Orabona, 4, I-70126 Bari (Italy) [**] Work was supported in part by the European Communities' IST Program under contract IST-2001-33057, ILO and by the European Communities' Human Potential Program under contract HPRN-CT-2000-00135, LAMI- NATE. a) b) Fig. 1. a) Reflectivity spectra of bare LATEX (D = 295 nm) for different c (15 £ c £ 45; Dc = 5 between two adjacent curves). The numbers in paren- theses indicate systems of planes in the position of reflection. Intensities of the spectra in the interval of 360±450 nm are multiplied by 5 for clarity. Reflectivity spectra of the same batch of LATEX doped by dye Oil Red EGN are reported in the inset. b) Reflectivity spectra of a sample of dyed LATEX (D = 245 nm) for different c.