Department of Physics, University of Colorado, Boulder, CO 80309, USA.
Proc Natl Acad Sci U S A. 2011 Dec 27;108(52):20891-6. doi: 10.1073/pnas.1112849108. Epub 2011 Dec 12.
Noncontact optical trapping and manipulation of micrometer- and nanometer-sized particles are typically achieved by use of forces and torques exerted by tightly focused high-intensity laser beams. Although they were instrumental for many scientific breakthroughs, these approaches find few technological applications mainly because of the small-area manipulation capabilities, the need for using high laser powers, limited application to anisotropic fluids and low-refractive-index particles, as well as complexity of implementation. To overcome these limitations, recent research efforts have been directed toward extending the scope of noncontact optical control through the use of optically-guided electrokinetic forces, vortex laser beams, plasmonics, and optofluidics. Here we demonstrate manipulation of colloidal particles and self-assembled structures in nematic liquid crystals by means of single-molecule-thick, light-controlled surface monolayers. Using polarized light of intensity from 1,000 to 100,000 times smaller than that in conventional optical tweezers, we rotate, translate, localize, and assemble spherical and complex-shaped particles of various sizes and compositions. By controlling boundary conditions through the monolayer, we manipulate the liquid crystal director field and the landscape of ensuing elastic forces exerted on colloids by the host medium. This permits the centimeter-scale, massively parallel manipulation of particles and complex colloidal structures that can be dynamically controlled by changing illumination or assembled into stationary stable configurations dictated by the "memorized" optoelastic potential landscape due to the last illumination pattern. We characterize the strength of optically guided elastic forces and discuss the potential uses of this noncontact manipulation in fabrication of novel optically- and electrically-tunable composites from liquid crystals and colloids.
非接触式的光学捕获和操控微米级和纳米级的粒子通常是通过使用高强度的聚焦激光束来实现的，这些激光束可以产生力和扭矩。虽然这些方法在许多科学突破中起到了重要作用，但由于其操纵面积小、需要使用高激光功率、对各向异性流体和低折射率粒子的应用有限以及实施复杂等原因，它们在技术上的应用很少。为了克服这些限制，最近的研究努力已经转向通过使用光引导的电动作用力、涡旋激光束、等离子体和光流控学来扩展非接触式光学控制的范围。在这里，我们通过使用单分子厚的、光控表面单层来演示胶体粒子和自组装结构在向列液晶中的操控。使用强度为传统光镊的 1000 到 100000 倍的偏振光，我们可以旋转、平移、定位和组装各种大小和组成的球形和复杂形状的粒子。通过控制单层的边界条件，我们可以操纵液晶指向矢场和由此产生的各向异性弹性力场对胶体的作用。这使得可以在厘米尺度上进行大规模的并行粒子和复杂胶体结构的操控，通过改变照明或根据最后一个照明模式的“记忆”光弹势能景观组装成静态稳定的配置，可以动态控制这些粒子和复杂胶体结构。我们对光引导弹性力的强度进行了表征，并讨论了这种非接触式操控在由液晶和胶体制造新型光和电可调复合材料方面的潜在用途。
