|
Manipulation and trapping of particles have taken a huge relevance in recent years thanks to many applications with revolutionary contributions to diverse fields. Several experiments have demonstrated that thermal effects can improve the current micromanipulation techniques such as DNA manipulation or assembly of colloidal crystals. In this work, we present the effect of laser-induced thermal effects, such as convection currents and thermophoresis, on the trap stiffness (spring constant) constant of an optical trap of 3-micrometer particles suspended in water. These effects are a consequence of light absorption in a thin layer of hydrogenated amorphous silicon (a-Si:H) deposited at the bottom of the chamber which generates a thermal gradient. Since these effects (and its correspondent forces) are symmetric around the beam focus, trapped particles, experience an increment in the trapping force. Around the beam focus, the drag force associated with convective currents is directed upwards and are compensated by optical scattering force. Depending on the laser power, the trap stiffness increases significantly, so a trapped particle can be dragged along the cell (by displacing the sample and leaving the beam fixed) at velocities around 90 μm/s without escaping the trap, whereas in the absence of the a-Si:H film, the escape velocity of the particle in the trap drops to velocities around 30 μm/s. This presents a simple, yet effective, option for optical manipulation at low powers (<5 mW) and its possible applications in the manipulation of a variety of biological micro samples.
|
