Current Opinion in Solid State and Materials Science 7 (2003) 151–157 Photonic band gap structures by sol–gel processing * Rui M. Almeida , Sabine Portal ´ Departamento Engenharia de Materiais, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Received 17 February 2003; accepted 5 June 2003 Abstract Since the launching of the photonic bandgap concept in 1987, the development of corresponding structures has expanded very rapidly, in particular two-dimensional semiconductor-based structures. In the case of sol–gel derived materials, the main emphasis for the past year has been on one-dimensional multilayer stacks and, in particular, on three-dimensional structures of the opal and inverse opal type.  2003 Elsevier Ltd. All rights reserved. Keywords: Photonic band gap; Photonic crystal; Sol–gel; Bragg reflector; Fabry–Perot microcavity; Artificial opal; Inverse opal 1. Introduction sional (3-D) structures, characterized by possessing a full PBG in a certain frequency range. In addition, by intro- ducing defects into a PBGs, light can be localized, i.e. Photonic band gap structures (PBGs), also known as trapped in the defect. The properties of the defect mode photonic crystals (PC), evolved from an original idea of (frequency, symmetry, etc.) can then be easily tuned. Eli Yablonovitch [1] who was inspired by semiconductor In view of the above, many interesting applications are materials where electrons cannot have certain energy possible. Sol–gel processing [4] can be used to prepare a values due to the existence of a periodic electrostatic large variety of high optical quality materials and this is in potential. PBG materials are optical analogues of semi- fact one of the simplest and cheapest available methods to conductors, in the sense that the propagation of photons fabricate PBGs. This paper is a review of the origin and within a particular energy range is also forbidden. These development of PBG materials and structures made by structures have a periodic refractive index (or dielectric sol–gel processing since 1987, with an emphasis on the constant), on a length scale of the order of optical most recent developments during 2002. wavelengths [2]. This periodicity prevents light from propagating through the material due to Bragg reflection, in a wavelength range of the order of the spatial period of the PBGs. The modified form of Bragg’s law for the 2. Types of PBG structures optical region, which takes into account Snell’s law of refraction, is [3]: PBGs may be broadly classified as one-dimensional ]]]] (1-D), two-dimensional (2-D) or 3-D. 1-D PBGs usually 2 2 l 5 2d n 2 sin u (1) œ eff consist of a stack of layers (typically dielectric) with alternating high and low refractive indices. 2-D PBGs can where l is the free-space wavelength of the light, d the either be planar structures, with a periodic pattern in two interplanar spacing, n the average refractive index of the eff directions, or PBG fibers (often called PC fibers [5] ), such structure and u the angle between the incident radiation as the hollow core, or ‘holey’ fibers, or the depressed index and the normal to the set of planes. solid core PBG fibers, where there is a PBG in the Some PBGs are characterized by a certain frequency cladding. 3-D PBGs have a refractive index which is range where light is not allowed to propagate, regardless of periodic in three dimensions and are generally of the opal, its direction or polarization state. These are three-dimen- or inverse opal type, as described later. There is also the possibility of 1-D PBGs with 3-D *Corresponding author. Tel.: 1351-218-418-110; fax: 1351-218-418- sub-structures [6]: each layer of the 1-D PBGs consists of 132. E-mail address: rui.almeida@ist.utl.pt (R.M. Almeida). a 3-D ordered array of close-packed colloids and the 1359-0286 / 03 / $ – see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S1359-0286(03)00045-7