Optical microstructures are deformations of a material surface on a scale too small to be seen with the human eye, but which can efficiently redirect light so as to give the surface a colored or even self-illuminated appearance. Such structures occur naturally, and for example lead to the brilliant colors of a butterfly wing. Synthetic fabrication of optical microstructures has had far-reaching technological impact in fields such as information storage and display, and is typically based on exposing a photosensitive film to a prescribed illumination pattern from a lamp or laser. While this technique is well-established, it is not suited for rapid prototyping. A separate limitation is that the fabricated structures are immobile on the film surface once the film processing is complete. However, an emerging material class of highly colored organic molecule coupled with a flexible polymer have been found to be sensitive to the direction of illumination and not its brightness. This sensitivity to light polarization can be exploited to make dynamic surface microstructures, enabling not only permanent optical microstructures, but also structures which can be reconfigured in response to light. Separately, the recent emergence of programmable spatial light modulators makes possible the generation of rapidly reconfigurable polarized light fields. The proposed research combines new polarization-sensitive polymers with spatial light modulator technology to create a benchtop system for the on-demand fabrication of both static and dynamic optical microstructures. Such a system will make rapid prototyping of light-diffracting surfaces accessible to a wide range of optical manufacturers, while also enabling new techniques in bioengineering that rely on the use of microscale surfaces to study cellular response. The project will also benefit education and training at TCNJ that is a primarily undergraduate institution. The project offers an outstanding opportunity for undergraduate science students to gain integrative experience. Undergraduate researchers will become partners in a multidisciplinary and international research program that leverages emerging research in photonics and material sciences.
Optical microstructures are fabricated by exposing photosensitive film to an optical intensity pattern. While conventional photosensitive films respond to optical intensity and require post-exposure chemical processing, new supramolecular azopolymer films respond to optical polarization, with the surface microstructure growing immediately in response to illumination, with no subsequent processing required. To best leverage these new polarization-sensitive materials requires a programmable source that can project spatially-defined patterns of linearly polarized light, such as a spatial light modulator. The proposed research will therefore pursue the development of a new microstructural fabrication system based on supramolecular azopolymer materials and digital polarization optics. The first step towards this discovery is to establish the amplitude, resolution, and surface topographies of optical surface structures obtainable with the spatial light modulator. Resolutions of order 500 nm and surface amplitudes of 2 µm are expected following the incorporation of multielement corrected optics. A second goal is to exploit the optoelectronic scanning capability of the spatial light modulator to Fourier synthesize nonsinusoidal surface patterns using the superposition of multiple exposures with appropriately determined exposure times and phase shifts. In addition, the dynamic programmability of the spatial light modulator-enabled digital optics system will be exploited to explore dynamic surface microstructures. Such moving surface structures can only be induced in materials such as azopolymers which exhibit a reversible photomechanical response. Throughout this exploration of feature sizes and structures, the replication of optical surface microstructures on azopolymer films will be studied using nanoimprint lithography. Atomic force and scanning electron microscopy will complement this effort and will be used to assess replication fidelity.
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.
