Abstract View
Methods for Optical Trapping and Active Orientation Control of Airborne Microparticles
JESSICA ARNOLD, Aimable Kalume, Gorden Videen, Chuji Wang, Yong-Le Pan, U.S. Army Research Laboratory
Abstract Number: 183
Working Group: Instrumentation and Methods
Abstract
Manipulating microparticles through rotation has a wide range of applications including creating micro-motors, controlling the orientation of biological and chemical samples, and nanofabrication. Various methods have been invoked to trap and rotate particles suspended in liquid. However, when developing techniques for the detection and identification of aerosols, it is advantageous to directly study particles trapped in air. A limited number of studies on manipulating airborne particles have focused on periodic circular motion of trapped particles. In these cases, the observed rotational motion was a consequence of combined angular momentum and forces applied on the trapped particle, and the rotational periodicity is dependent on fixed properties including the particle shape, absorptivity, and laser beam configuration/power. Once the particle and trapping laser configuration are determined, the rotation frequency is fixed and cannot be controlled.
We present innovative methods for controlled rotation of optically trapped particles in air. We show examples of inducing circular or rotational motion by shaping the trapping beam with a liquid crystal on silicon spatial light modulator (SLM). Absorbing particles can be manipulated using the SLM to place a hollow ring onto the center of a Gaussian beam. The hollow beam is distorted into an elliptical shape and passed through a focusing lens. Once a particle is trapped at the focal point, the pattern is rotated which induces a circular, rotational motion in the particle. Another method is to create the hollow trapping beam before it reaches the SLM using two axicons. The SLM is then used to dim the intensity at two sets of diametric points (four total) which are then rotated. With both of these techniques the rotation rate of the trapped particle is externally controlled, and a given orientation of the trapped particle can held for data collection over the required length of time.