Controlling emission using one dimensional integrated photonic fluorescent collectors
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Controlling emission using one dimensional integrated photonic fluorescent collectors Thomas S. Parel and Tomas Markvart Solar Energy Laboratory Faculty of Engineering and the Environment University of Southampton Southampton SO17 1BJ United Kingdom ABSTRACT It is known that photonic crystals can be used to suppress spontaneous emission. This property of photonic crystals has been investigated for suppressing and decreasing the propagation of photons within loss cones in fluorescent collectors. Fluorescent collectors can concentrate light onto solar cells by trapping fluorescence through total internal reflection. In an ideal fluorescent collector the major obstacle to efficient photon transport is the loss of photons through the top and bottom escape cones. One possible method to decrease this loss and improve the efficiency of these devices is to fabricate one-dimensional photonic crystals doped with fluorescent molecules. If these photonic crystals are tuned to exhibit a photonic band gap in the escape cone directions and at the emission frequencies of the fluorescent molecules, a suppression of the escape cone emission and an enhancement of the edge emission is expected. In this paper, we detail the fabrication of a one dimensional integrated photonic collector and show the suppression of the escape cone emission. This suppression of the escape cone will be shown to correspond to the photonic band gap and the modifications to the edge emission will be shown to correspond well with so called Fabry Perot modes. The control of emission inside fluorescent collectors opens up a number of additional possibilities for efficiency enhancements that will also be discussed. INTRODUCTION Optical features that we now call photonics have been used for visual reception and display by biological organisms for many millions of years [1]. The underlying science, now understood in terms of the density of photon states, includes an early theoretical observation by Lord Rayleigh [2] and, more recently, the suppression of spontaneous emission in periodic structures (photonic crystals) [3,4]. A prominent feature of photonic crystals is the presence of a photonic bandgap, characterized by the absence of quantum states available to receive the emitted photons. One dimensional photonic crystals (multilayered films or Bragg stacks) do not display a photonic gap in all directions but can, however, be engineered to reflect light incident externally under any angle onto the face of the structure, in other words, they act as omnidirectional reflectors [5,6]. In comparison with a homogeneous dielectric layer such structures display the absence of an escape cone for light emitted from within the structure, a feature which forms the principal point of study in this paper. From a practical point of view, the control of light emitted inside a dielectric layer and the suppression of emission in the escape cone is key to the operation of fluorescent collectors [710]. These devices operate over a dual frequency range, transforming the normally incide
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