Optical Activity Modulation in Helicoidal Crystal Films

NON-TECHNICAL SUMMARY Optically active materials that differentially transmit or refract circularly polarized light enable modern technologies, from 3D displays to biological sensors. Researchers at New York University, with support from the Solid State and Materials Chemistry Program in NSF’s Materials Research Section A, develop a new class of materials whose optical activity can be tuned with light or heat. Optical activity can be generated in a wide variety of simple molecular materials that spontaneously grow from melts as bundles of twisted crystalline fibers. When constituent molecules change color upon irradiation or undergo temperature-sensitive phase transformations that do not disrupt the medium's large-scale structural architecture, the distinct sensitivities to handed photons can be turned on and off. These materials can lead to advanced technologies in fields ranging from optical data encryption to support U.S. national security to medical imaging for improved healthcare. Students trained in light-matter interactions, thin film processing and characterization, and computational modeling of molecules and materials contribute to the U.S. STEM workforce. Hands-on demonstrations to reveal the quantum nature of light are designed to engage the public by leveraging the intrinsic appeal of crystals and the interactions of crystals and light central to this research. TECHNICAL SUMMARY Materials with tunable optical activity typically require complex synthetic or patterning strategies of chiral components or mechanical reconfigurability to generate and modulate chiroptical responses. With support from the Solid State and Materials Chemistry Program in NSF’s Materials Research Section A, researchers develop simple, non-mechanical polycrystalline films with tunable circular birefringence (CB) and circular dichroism (CD). Their strategy exploits the growth of many simple compounds as helicoidal crystal lamellae that are endowed as grown with extrinsic optical activities arising from the dissymmetric arrangement of linearly anisotropic crystallites. Linear anisotropies of individual helicoidal crystals within bundles generate chiroptical properties through sense-specific inhomogeneities along the light path that couple the misoriented complex constitutive permittivity tensors to produce differential responses to circularly polarized light. CB and CD can then be modulated as a consequence of their responses to prescribed changes in linear birefringence and linear dichroism. Optical properties are characterized in situ with home-built, rapid imaging Mueller matrix microscopes. This research articulates mechanisms governing helicoidal crystal growth and how the dissymmetric organization of crystal fibers transforms the polarization state and diattenuation of light. 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.