Appl. Phys. B 81, 205–208 (2005) Applied Physics B DOI: 10.1007/s00340-005-1821-6 Lasers and Optics E. PALACIOS-LID ´ ON 1 H.M. YATES 2 M.E. PEMBLE 2 C. L ´ OPEZ 1, ✉ Photonic band gap properties of GaP opals with a new topology 1 Instituto de Ciencia de Materiales de Madrid (CSIC) C/Sor Juana In´ es de la Cruz 3, 28049 Madrid, Spain 2 Institute of Materials, Cockcroft Building, University of Salford, Salford, M5 4WT, UK Received: 18 January 2005 Published online: 15 July 2005 • © Springer-Verlag 2005 ABSTRACT In this paper, we propose that the “anomalous” optical response exhibited by GaP and InP infiltrated opals is due to the peculiar morphology shown by these materials when grown within the pores. In order to account for their optical response, we propose a new structural model con- sisting of a network of high dielectric spheres located in the pores of the bare opal, interconnected by cylinders of the same material. A fair agreement between the theoretical predictions using this model and the experimental measurements has been found. We also show that the inverse structure presents very interesting optical properties. PACS 42.70.Qs; 42.25.Fx; 42.25.Gy 1 Introduction The study of synthetic opals as photonic crystals (PC) has been a topic of increasing interest in the last few years [1]. These materials are prepared from colloidal systems in which silica or polymer spherical particles settle down into a closed-packed face centre cubic (fcc) lattice. PCs exhibit a periodic modulation in the refractive index and, consequently, peculiar properties for photon propagation are expected [2, 3]. By filling the interparticle voids with different materials, it is possible to obtain structures apt to be used as sensors [4], photonic inks [5], etc. When the infiltrated material has a high enough refractive index (n > 2.8), and after the removal of the spheres’ matrix, the resulting inverse opal shows stop bands for all directions of photon propagation [6, 7] known as full photonic band gaps (PBG). Up to now, inverse opals of semiconductors, such as Silicon [8] and Germanium [9], presenting a PBG have been obtained. However, the presence of very high absorption in the infrared range becomes an issue in their use as devices working in the visible range. A fine solution to this problem is the infiltration of opals with certain III–V compounds. These materials have high refractive index and are transparent in the visible range (GaP) or near infrared (InP, GaAs) [10]. Additionally, these materials are very important in many technological fields because they ✉ Fax: +34 91 334 9019, E-mail: cefe@icmm.csic.es Present address: NMRC, Lee Maltings, Prospect Row, Cork, Ireland present interesting properties, such as luminescence [11] non- linear optics effects [12], etc., that make them suitable in many device applications. A periodic macroporous network formed with one of these materials would offer the possibility of coupling the photonic and electronic properties. However, infilling with these type of compounds is a difficult task and (in fact, only a few inverse opals have been demonstrated) more so with III-nitrides [13]. Lee et al. [14] reported the preparation of a Gallium Arsenide (GaAs) inverse opal infilled by electrochemical deposition, although they concluded that there was little chance of obtaining other III–V materials, such as GaP or InP using this method. Recently, we showed for the first time the possibility of Indium Phosphide (InP) or Gallium Phosphide (GaP) opals infiltration by using a Metal-organic Chemical Vapour De- position (MOCVD) method [15]. Although X-ray diffraction and Raman studies revealed they are crystalline III–V materi- als of very good quality, their optical response (Fig. 1) could not be explained by the usual infiltration model termed sur- face templating [16]. In such a model the infiltrated material is assumed to grow uniformly around the opal spheres making shells of increasing thickness. As the infiltration progresses the shell radius gets larger and larger until, eventually, the inter-sphere voids become filled. This behaviour has been re- ported in many studies using materials, such as Si [17], Ge [18], and SiO 2 [19] for instance. 2 III–V infiltration According to this model, as the infilling degree in- creases the Bragg peak, corresponding to the first pseudogap in the (1 1 1) direction, shifts toward higher wavelengths. If the infilled material has a refractive index n > 3, this shift may amount to a few hundred nanometres [8]. At the same time the width of this Bragg peak dramatically increases. GaP infiltrated opals present, however, a very different behaviour, as can be seen in Fig. 1. Surprisingly, the reflectance spectra show a Bragg peak with no appreciable shifting with reference to the bare opal. According to the above model, this would mean that all studied samples would have a negligible filling fraction. The appearance of intense high-energy peaks in the reflectance spectra is in clear contradiction with the assump- tion of a low degree of infiltration. Another important fact to be highlighted is that (apart from small differences caused