RUI: Spatial light modulator technology for the on-demand fabrication of optical microstructures in polarization-sensitive materials

光学微结构是一种材料表面的变形，其尺度太小，人眼无法看到，但它可以有效地重新定向光线，从而使表面呈现出彩色甚至自发光的外观。这种结构是自然发生的，例如，蝴蝶翅膀的绚丽色彩就是由这种结构形成的。光学微结构的合成制造在信息存储和显示等领域具有深远的技术影响，并且通常基于将光敏薄膜暴露在灯或激光的规定照明模式下。虽然这种技术已经很成熟，但它并不适合快速原型。一个单独的限制是，一旦薄膜加工完成，制造的结构在薄膜表面是不移动的。然而，人们发现一种新型材料——高度着色的有机分子与柔性聚合物偶联——对光照方向敏感，而对亮度不敏感。这种对光偏振的敏感性可以用于制造动态表面微结构，不仅可以实现永久的光学微结构，而且可以根据光的响应重新配置结构。另外，最近出现的可编程空间光调制器使得产生快速可重构的偏振光场成为可能。该研究将新型偏振敏感聚合物与空间光调制器技术相结合，为静态和动态光学微结构的按需制造创造了一个台式系统。这样的系统将使光衍射表面的快速原型设计为广泛的光学制造商所接受，同时也使依赖于使用微尺度表面来研究细胞反应的生物工程新技术成为可能。该项目还将有利于以本科院校为主的天津大学的教育和培训。该项目为理科生提供了一个获得综合经验的绝佳机会。本科研究人员将成为一个多学科和国际研究项目的合作伙伴，该项目利用了光子学和材料科学领域的新兴研究。光学微结构是通过将光敏薄膜暴露于光强模式来制造的。传统的光敏膜响应于光强，需要曝光后的化学处理，而新的超分子偶氮聚合物薄膜响应于光偏振，其表面微观结构在光照下立即生长，不需要后续处理。为了最好地利用这些新的偏振敏感材料，需要一个可编程的源，可以投射线性偏振光的空间定义模式，如空间光调制器。因此，提出的研究将致力于开发一种基于超分子偶氮聚合物材料和数字偏振光学的新型微结构制造系统。实现这一发现的第一步是确定空间光调制器可获得的光学表面结构的振幅、分辨率和表面形貌。在加入多元件校正光学元件后，预计分辨率为500 nm，表面振幅为2 μ m。第二个目标是利用空间光调制器的光电扫描能力，利用适当确定的曝光时间和相移的多次曝光叠加来傅立叶合成非正弦表面图案。此外，空间光调制器数字光学系统的动态可编程性将被用于探索动态表面微结构。这种移动的表面结构只能在偶氮聚合物等材料中产生，这些材料表现出可逆的光力学响应。通过对特征尺寸和结构的探索，我们将使用纳米压印光刻技术研究偶氮聚合物薄膜上光学表面微结构的复制。原子力和扫描电子显微镜将补充这一努力，并将用于评估复制保真度。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.