Thin film optical technology is very commonplace in our everyday life, from the anti-reflection coatings on our glasses to the mirrors inside lasers commonly used in household appliances. Thin film optical elements are traditionally prepared by vapor phase growth of a handful of inorganic solids, most notably magnesium fluoride, lanthanum fluoride, zinc oxide, silica, alumina, tantalum oxide and titania. These are deposited as layers of amorphous solids with extremely well controlled thicknesses in industrial processes. As these materials are amorphous, they are optically isotropic, meaning that they are characterized by a single refractive index. While thin film technology now enables the production of exquisitely controlled structures, enabling, for example, ultrasharp spectral filters, there are numerous limitations to such structures due to their isotropy, such as the inability to fabricate polarization elements at normal incidence.
In contrast with man-made technology, biological optical reflectors, such as the ones found in fish scales, various types of insects and in the eyes of many marine animals, are often made from organic crystals, mostly of pterin and pteridine molecules. The most notable feature of these materials is their extreme birefringence, which endows them with much broader design flexibility. This enables, for example, to fabricate reflectors with a much better control of the angle-dependent spectral reflectivity, to for organic crystals with extremely high indices along certain polarization directions, and to form metamaterials with a reduced or an extreme polarization dependence. Such structures are currently inaccessible by man-made technology. The BoX-BOOM project aims to bridge this gap, developing new ways to grow single-crystalline domains with controlled orientation and thickness using epitaxial growth, either from a liquid or from a solid phase, to grow heterostructures of such layers, and to provide a better understanding of the fundamental relation between structure and optical properties in small molecule organic crystals. In particular, a better understanding of the role of hydrogen bonds in controlling the refractive index will enable the design of new, environmentally benign and simple to deposit organic materials with extremely high refractive indices and exhibiting extreme bireferingence.
When these methods become available, they will open the door to a plethora of new thin-film optical devices such as thin film reflective polarizers at normal incidence, thin film spectral filters exhibiting an achromatic angular response and a new capability of biodegradable and environmentally benign thin film optical devices.
