Modeling of azopolymer surface restructuring under complex irradiation patterns

聚合物可以在光等外界刺激下改变其分子结构，引起了科学界的极大关注。特别著名的是光敏偶氮聚合物，由于其高通用性，在不同领域有许多应用。在现代工程中，一个特殊的兴趣已经引起了一个独特的机会来控制偶氮聚合物薄膜的变形取决于光照射模式。地形重组已被用于生产具有光控润湿性的表面，生物技术中活细胞的模板，光子应用的大规模多路光栅。多种理论解释已经发展到解释表面浮雕（SR）的外观，但其中只有少数考虑到潜在的分子结构特征。我们也一直在研究这个问题，通过追求取向方法，在含偶氮材料的光特性和分子性质之间建立了明确的关系。最近我们提供了一些决定性的证据，证明聚合物骨架沿光偏振方向的重定向是偶氮聚合物薄膜光致变形的主要原因。本项目的最终目标是发展复杂辐射模式的粘塑性建模方法。为了达到这一目的，应将线偏振和圆偏振光束的两种极限情况推广到椭圆偏振的一般情况。该模型将通过比较其预测与空间变化椭圆偏振的干涉图案所刻的SRs的外观来改进。然后重点研究了螺旋形表面起伏和光变形的波前灵敏度。螺旋浮雕是通过用紧密聚焦的螺旋光束照射偶氮聚合物样品的表面来雕刻的。这种光束可以由现代光学元件产生，这大大扩展了基于偶氮聚合物的纳米光刻提供的可能性。螺旋光束的光场相当复杂。我们打算在这样一个场的作用下模拟螺旋起伏的外观，以测试定向方法在极端辐照条件下的有效性。实验中使用的侧链偶氮聚合物的原子模型将提供潜在的分子参数。当我们成功时，不仅将为长期存在的SRs铭文之谜提供最终解决方案，而且还将为现代光学和光子学的实际应用开发一种通用的建模方法。

Polymers that can change their molecular architecture under such an external stimulus as light have attracted great attention in the scientific community. Particularly known are photosensitive azopolymers, which have many applications in different fields due to their high versatility. In modern engineering, a special interest has been evoked by a unique opportunity to control the deformations of thin azopolymer films depending on the light irradiation pattern. The topographical restructuring has been used to produce the surfaces with light-controlled wettability, templates for living cells in biotechnology, large-scale multiplexed gratings for photonic applications. Multiple theoretical interpretations have been developed to explain the appearance of surface reliefs (SR), but only a few of them consider the features of underlying molecular architecture. We have been also working on this problem by pursuing the orientation approach, which establishes a clear relation between the light characteristics and the molecular properties of azo-containing materials. Recently we provided a couple of decisive proofs that reorientation of polymer backbones along the light polarization direction should be the main reason of photoinduced deformations in azopolymer films. The ultimate objective of this project is to develop the viscoplastic modeling approach for complex irradiation patterns. To reach this objective, two limiting cases of linearly and circularly polarized beams should be extended to a general case of elliptical polarization. The model will be refined by comparing its predictions with appearance of SRs inscribed by the interference patterns of spatially varying elliptical polarization. Then we concentrate on the understanding of the spiral surface reliefs and the wavefront sensitivity of photodeformations. The spiral reliefs are inscribed by irradiating a surface of azopolymer sample with tightly focused helical beams. Such beams can be produced by modern optical elements, which significantly extends the possibilities offered by azopolymer-based nanolithography. The optical field of a helical beam is rather complex. We intend to model the appearance of spiral reliefs under action of such a field, to test the validity of orientation approach at extreme irradiation conditions. Underlying molecular parameters will be provided by atomistic modeling of the side-chain azopolymers used in the experiments. When we succeed, not only an ultimate solution of a long-standing mystery of SRs inscription will be provided but also a versatile modeling approach will be developed for practical applications in modern optics and photonics.