NSF Award
1920840
The range of colors and hues in the natural world are astonishing - from colorful flowers, birds and butterflies to underwater creatures like fish and cephalopods. The mechanisms underlying this color are as varied as the species, but can be broken into two main categories: pigmentation and structural color. In pigment producing cells, molecules absorb part of the visible spectrum while leaving the rest to be scattered back to the viewer. Structural color, on the other hand, is produced through combinations of reflection, scattering and interference within groups of cells or external nanostructures. Of these two classes, structural color serves as the primary color generating mechanism in several extremely vivid species. The key aspect of these color displays in nature is that they are consistently and uniformly formed from self-assembled nanostructures on thin, flexible and curvilinear surfaces. This in contrast to state-of-the-art manmade displays, which remain vastly rigid, brittle and based on top down processing.<br/> Here, we propose a large area, highly reproducible self-assembling technique where aluminum particles are formed on the surface through a temperature and pressure dependent thin film growth mechanism in an ultra-high vacuum electron beam evaporator. The system supports localized surface plasmons confined to the gaps between particles and the mirror which demonstrate a high degree of independence on the angle of incident light. The spectral location of the resonance can be tuned based on the size distribution of the aluminum particles and the index of the surrounding media. Light which is not absorbed by the surface is reflected back, resulting in a vivid perceived color. By integrating the self-assembled surface with liquid crystal cells, actively tunable plasmonic displays can be obtained. The process is also amiable to large-scale flexible and diffusive substrates which can result in novel plasmonic surfaces/displays with engineerable material and scattering properties. <br/> The proposed work is important for the development of low cost reflective displays on flexible substrates. The newly developed self-assembling techniques will enable large area patterning of nanostructured surfaces for low cost manufacturing. The program provides a good platform for interdisciplinary research (including integrated optics, nanofabrication and materials science and engineering) and graduate education. The research will generate exciting scientific content for enriching the PI's graduate and undergraduate teaching. The program will integrate outreach activities that span Autistic students, K-12 students and other underrepresented minorities. The PI collaborates with Orlando Science Center to informally educate the broader community and increase public awareness on displays and color generation in general.<br/><br/>Technical: There is much to understand and develop in the field of actively tunable plasmonics. A fast response, angle independent and diffusive Liquid Crystal (LC) based tunable displays which can actively shift the color of its pixels is now possible only after years of interdisciplinary research. We plan on furthering this field through an in depth analysis of the plasmonic response of self-assembled nanostructured surfaces, LC orientation and how they influence each other. A key objective of the proposed system is to design angle independent and diffuse color surfaces based on a self-assembled resonance which doesn't depend on angle of illumination or viewing angle. While apt at producing color, these plasmonic surfaces cannot produce deep black states needed for displays due to intrinsic narrow band absorption. A vital aspect of these display devices is the ability to control the amount of light reflected from them. One possible way of achieving black and intermediate gray states is the use of liquid crystal shutter. The proposed device can readily be fabricated on flexible substrates as the low temperature fabrication process is compatible with low glassing temperature polymers such as PET.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
