Near-field polarization analysis for an optical nanofiber

年内，光学纳米纤维的偏振控制得到进一步改善，从而能够研究光学捕获的与纳米光纤相邻的手性粒子的自旋-轨道耦合。电磁波中的自旋角动量不仅是由与椭圆极化相关的场旋转引起的，而且是由场的空间调制引起的。因此，光携带的自旋不必与能量传递平行。横向分量在强受限光中可能很重要，例如瞬逝光场。在这里，横向自旋是通过它在纳米纤维表面的逝去场中直接作用于被光学镊子抓住并自旋的探针粒子来揭示的。这项工作得益于俄罗斯ITMO的同事开发的一个理论模型。这项工作已经在几次会议(包括OPIC 2021)上提出，并正在准备出版。第二项工作也在偏振控制的基础上进行。像Janus粒子这样的复合材料物体可以表现出有趣的动力学，例如响应外部梯度场的推进或由于每个粒子周围产生的局部热梯度而形成的集体组装。在这项研究中，我们展示了金属介电Janus微球的光诱导动力学，这种微球带有一层薄薄的金层，被捕获并推动在纳米纤维的消失场中。我们发现，与未包覆的二氧化硅微球相比，包覆了10 nm或20 nm金的Janus粒子表现出了推进速度的提高。

During the year, the polarization control in optical nanofibres has been further improved enabling studies on spin-orbit coupling for optically trapped chiral particles adjacent to an optical nanofibre. Spin angular momentum in electromagnetic waves is caused not only by field rotations associated with elliptical polarization, but also by spatial modulations of the field. Consequently, the spin carried by light does not have to be parallel to the energy transfer. Transverse components can be significant in strongly confined light, such as evanescent fields. Here, the transverse spin was revealed through its direct action on a probe particle held and spun by optical tweezers in the evanescent field at the surface of a nanofiber. The work has benefitted from a theoretical model developed by colleagues in ITMO, Russia. This work has been presented at several conferences (including OPIC 2021) and is being prepared for publication.A second work has also been conducted building on the polarization control. Composite material objects such as Janus particles can exhibit interesting dynamics, such as propulsion in response to an external gradient field or collective assembly due to a generated localized thermal gradient around each particle. In this study, we demonstrated light-induced dynamics of metallo-dielectric Janus microspheres with a thin layer of gold that are trapped and propelled in the evanescent field of a nanofiber. We found that Janus particles with a 10 nm or 20 nm gold coating exhibit propulsion speed enhancements compared with uncoated silica microspheres.