Optical Probes inside Photonic Crystals
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Optical Probes inside Photonic Crystals W.L.Vos and A. Polman Introduction The spontaneous emission of an atom is not a property of the atom only; it also depends on the local optical surroundings.1 The simplest demonstration of this effect was provided by the early experiments of Drexhage, who studied the emission rate of luminescent europium ions close to a mirror.2 It was found that while the spectral distribution of the emission remained constant, the emission rate was dependent on the position of the Eu3 ions relative to the mirror. This effect is due to interference of the optical modes incident to and reflected at the mirror. Since then, the modified spontaneous emission of atoms in cavities has been studied extensively.3 More recently, the control of spontaneous emission in solid-state systems has become of great interest because it enables the tailoring of the emission properties of optical materials. It was shown how the spontaneous-emission rate of optical probe ions or dyes inside dielectric films is modified by the presence of a dielectric interface,4,5 in a dielectric multilayer,6,7 or a microcavity.8,9 The dependence of the decay rate on the optical surroundings in these one-dimensional systems can be described in terms of Fermi’s “golden rule,” which states that the decay rate is proportional to the local optical density of states (DOS). The spatial variation in the DOS is due to the interference of optical modes reflected and refracted at the dielectric interface(s). Photonic crystals are predicted to have a radical effect on the spontaneousemission rate.10,11 In crystals with a full bandgap, spontaneous emission at frequencies inside the gap will be fully suppressed because the optical modes do not exist (DOS 0). Photonic crystals with a partial gap can also have a significant effect on the spontaneous-emission rate, and large spatial variations in the local DOS are predicted in some cases.12–14 While great progress has been made in past years in the fabrication of a large variety of photonic-crystal structures, measurements
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of the spontaneous-emission rate are still scarce. In this article, we present recent work in Amsterdam on the modified emission of optical probes incorporated inside photonic crystals and make reference to work by other authors in this area. Applications of controlled spontaneous emission in photonic-crystal lasers are also reviewed.
Optical Probes inside Colloidal Photonic Crystals A large variety of colloidal photonic crystals have been fabricated to date (see the article by Colvin in this issue). If optical probes are incorporated inside the colloids, the effect of band structure on the spontaneous emission can be studied. So far, two classes of light sources have been used: dyes and lanthanide ions. A dye is an organic complex with a luminescent ligand; a large variety of different dyes, with emission spectra varying over the full visible spectral range, are available. Dye processing is compatible with the typical wet-chemical processing routes used in colloid f
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