远距离激光牵引及空气中微粒的光学囚禁与光谱检测的研究

Optical pulling with lasers is the attraction of objects back to the light source by the use of optically induced “negative forces”, which is counterintuitive and has long been attractive. It is commonly expected that when illuminated by a light beam, an object will be accelerated by radiation pressure along the light propagation direction. However, the light-absorption of the airborne particles results in the rise in its temperature, and could induce a negative photophoretic force due to the momentum transfer between the hot particle and the surrounding gas molecules, which can be much greater than the radiation pressure and gravitation force of the particle. Recent experiments have demonstrated a small-scale of tractor beams from a few micrometers to centimeters. In this project, we propose the use of a collimated Gaussian beam for long-distance optical tractor of airborne absorption particles over 1-10 meters. We will study the mechanisms of optical pulling and trapping of airborne particles using Gaussian beams; how the particles move in the tractor beam; how the density or pressure of gas molecules, particle’s size, shape, materials and absorption properties affect the pulling of the airborne particles; and how to combine optical trapping and Raman or fluorescence techniques with optical pulling in order to collect and chemically characterize the dangerous particles and microorganisms in atmosphere. The study of long-distance laser pulling technology in free space opens up potential applications for the collection and analysis of atmospheric particles and star dusts in deep space.

激光牵引是利用光场诱导产生的负作用力吸引物体，使其朝着光源方向运动的操控技术。空气环境中悬浮着大量的未知微粒，当空气微粒受到激光束的照射时，光场辐射压力通常将微粒沿着光束方向推离。然而，颗粒的光学吸收使其温度升高,并与周围气体分子进行动量交换，可产生远大于辐射压力及重力的负光咏动力，使颗粒被囚禁在光束中反方向运动。本项目拟采用平行高斯光束将微粒远距离反向牵引到光源位置(牵引距离在1-10米以上)，然后用聚焦光束将微粒囚禁，并测量其物理和化学特性。我们将研究高斯光束牵引操控及囚禁空气微粒的物理机制；研究微粒在牵引光束中的运动轨迹；研究空气压力和微粒性质(尺寸、形态等)对光学牵引效应的影响；研究光学囚禁技术及拉曼/荧光光谱技术与激光牵引技术的联用，以实现对大气环境中有害微粒和微生物颗粒的收集和直接检测分析。本项目的研究将改变远距离传送物体的方式，研究成果可应用于高空或太空环境中尘埃的分析。

激光牵引是利用光场诱导产生的负作用力吸引物体，使其朝着光源方向运动的操控技术。空气环境中悬浮着大量的未知微粒，当空气微粒受到激光束的照射时，光场辐射压力通常将微粒沿着光束方向推离。然而，微粒的光学吸收使其温度升高，并与周围气体分子进行动量交换，可产生远大于辐射压力及重力的负光泳力，使微粒被囚禁在光束中反方向运动。按照原定项目计划，通过改进光束质量和提高牵引系统的稳定性，本项目实现了采用平行高斯光束将微粒远距离反向牵引到光源位置（牵引距离在10米以上），并研究了高斯光束中被牵引微粒的动力学行为；将激光牵引与光学囚禁和拉曼光谱分析联用，实现微粒的物性分析；为了明确激光牵引和光学囚禁空气微粒的物理机制，我们还研究了光强调制下被囚禁微粒的动力学行为，探明了光泳力及微粒温度的时间响应（时间常数在秒的量级），并从实验上明确了重力在其中的准直作用。此外，我们通过实验研究了作用在弯曲叶片上的光辐射力（一种跟光泳力同源的力）。我们发现，在温度均匀的叶片上通过改变叶片的几何形貌同样可以产生光辐射力。通过模拟计算，我们阐明了弯曲叶片产生光辐射力的微观机制。