Periodic Reporting for period 1 - BoX-BOOM (Biomimetic eXtremely-Birefringent Organic Optical Materials and devices)
Okres sprawozdawczy: 2023-09-01 do 2026-02-28
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.
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.
The activities within the scope of the project encompassed several subdomains. The first involves development of methods for growing large, uniform and oriented single extremely birefringent crystals on a substrate. For this end we worked on both liquid-phase and gas-phase deposition methods, with the former leading to much more significant success. We have also elucidate the growth mechanism of these crystals and as such understood the origins of their uniform thickness as well as the limitations to their dimensions due to epitaxial mismatches. The second involved both developing methods and implementing them on single microscopic crystals to perform spectroscopy of the refractive properties of extremely birefringent materials. We have implemented micro-ellipsometry in the lab using an imaging ellipsometer procured as part of the project (Accurion EP4) and developed our own version of a home built quantitative phase microscope which, when combined with AFM data, can provide similar results. The goals of this effort are twofold - to see whether extremely birefringent crystals can outperform commonly used inorganics in terms of the relationship between absorption and dispersion, and to provide a mechanistic understanding of the role of crystal structure and of hydrogen bonding in terms of determining refractive indices and birefringence. Finally, we have continued our work on biological optical systems, aiming to resolve new mechanisms used to expand the parameter space of organic crystal-based devices.
Our main achievements within this period are:
1. Development of a method for growth of highly uniform and well-oriented thin single crystal domains with areas of the order of 1mm2, as well as the means to lithographically pattern these layers to form polarization-sensitive holograms.
2. Revealing how extremely birefringent materials can saturate limits set by the Kramers-Kronig relations on the relationship between dispersion and refractive index, opening the pathway for high index transparent materials with minimal absorption.
3. Elucidating structural dispersion as the mechanism through which biological opals maintain a high degree of color saturation despite not having sufficient control over the individual component uniformity. The combination of a tunable degree of crystallinity with extreme birefringence makes this a potent tool for refractive index modulation.
Our main achievements within this period are:
1. Development of a method for growth of highly uniform and well-oriented thin single crystal domains with areas of the order of 1mm2, as well as the means to lithographically pattern these layers to form polarization-sensitive holograms.
2. Revealing how extremely birefringent materials can saturate limits set by the Kramers-Kronig relations on the relationship between dispersion and refractive index, opening the pathway for high index transparent materials with minimal absorption.
3. Elucidating structural dispersion as the mechanism through which biological opals maintain a high degree of color saturation despite not having sufficient control over the individual component uniformity. The combination of a tunable degree of crystallinity with extreme birefringence makes this a potent tool for refractive index modulation.
The ability to grow near-perfect crystals via solution epitaxy is not unprecedented but is an extreme example of how controlling growth mechanisms enables exquisite control over the end product. The preliminary findings that such crystals can readily self-heal under certain types of damage incurred is perhaps not entirely new but is very surprising. The main hurdles we are facing here currently relate to the difficulty of transferring these crystals from one substrate to another and on growing a second layer on top of the first crystalline layer. These would enable us to build stacked devices.
As for the notion of structural dispersion – this is a methodology that we stumbled upon by trying to reverse engineer the inner workings of biological opals. This finding was very surprising to us (as often occurs when observing biological optical systems) and follows the same line of discovery of the significant role of extreme birefringence in biological optics which is at the heart of the BoX-BOOM research grant.
As for the notion of structural dispersion – this is a methodology that we stumbled upon by trying to reverse engineer the inner workings of biological opals. This finding was very surprising to us (as often occurs when observing biological optical systems) and follows the same line of discovery of the significant role of extreme birefringence in biological optics which is at the heart of the BoX-BOOM research grant.