The last decade has seen light sources becoming more efficient and more easily controllable following the advances in LED technology. A research team from the United States has proposed a practical approach to utilize just one layer of photonic crystal in order to enhance the selective emission of light from active media.

Photonic crystals are structures with periodic modulation of the refractive index. They appear in nature, e.g., in the pigment of butterfly wings or in opals, giving them their famous spectacular appearance. Photonic crystals attracted attention of scientists several decades ago, with tremendous research efforts directed towards creating and utilizing their properties for controlling light for various applications, from enhancing spontaneous radiation of quantum emitters to optical communications and signal processing. One of the main obstacles slowing applications of photonic crystals is the complexity and high cost of their fabrication along with difficulty to scale their fabrication to larger volumes. Planar structures with modulated index of refraction, which are also closely related to diffraction gratings, can be produced with several simpler techniques, e.g., nanoimprint technology, allowing fabrication of larger volumes of photonic structures.

In this work the authors propose a new scalable method for fabrication of planar photonic structures and demonstrate their use for enhancing the radiation efficiency of quantum dots. The method resembles the nanoimprint technology, due to the fact that it utilizes an inverted re-usable template. During the fabrication process, the template is coated with dielectric structure, and with a polymer containing quantum dots. Then, the layer is exfoliated from the template, and the template can be used again. The proposed structures are just arrays of rectangular holes, with the period of these arrays different in two orthogonal directions. The quantum dots in the obtained flexible films can be excited with an ultraviolet source, for example an ultraviolet LED. The authors demonstrate nearly a 6-times enhancement of the emission of quantum dots compared to the case when there is no periodic structure involved. The proposed structure can be also used for controlling the directionality of the light emission, so that the light goes only in a narrow range of angles. The fabricated structure is flexible and this can potentially make it useful for applications in flexible displays.

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