Particle image velocimetry for visualizing laser-induced motion of nanoparticles

Abstract

The pressure of electromagnetic waves, experimentally discovered in the early twentieth century,1 produces forces that affect objects scattering or absorbing light. With ordinary light sources these forces are feeble indeed and were considered unimportant before the invention of lasers in the 1960s. In the early 1970s, Arthur Ashkin demonstrated that a focused laser beam provides light pressure sufficient to displace and even levitate microscopic objects.2 This work eventually led to the development of ‘optical tweezers’3 employed for motion control of microscopic objects in physics, chemistry, and biology.4 Applying the concept of light pressure force to nanoparticle and laser technologies represents a further instrumental advance. In the past decade, noble metal nanoparticles have found broad application in biotechnology and nanomaterials.5 But the displacement of nanoparticles by light pressure requires much less intensity than is required to trap it in ’optical tweezers’. Laser-driven nanoparticles have a variety of potiential uses in nanofluidics, nanobiotechnology, and biomedicine. Although the possibility of optically trapping gold nanoparticles was demonstrated in 1994,6 developing tweezers for nanoparticles is not straightforward. The gradient forces that are responsible for trapping, fall off with particle size. Moreover, the non-negligible absorption by particles leads to a dramatic temperature rise at high power, destablizing single beam optical traps.7 Recently we visualized nanoparticles moving along a laser beam under light pressure by micron-resolution particle image velocimetry (PIV).8 PIV records images of two successive moving particles, separated by a known time delay. Each image Figure 1. The microscope-based experimental setup. Inset shows the illumination geometry: A – microscope objective; B – laser beam; C – light scattered by nanoparticles; D – micro cell with nanoparticles colloidal solution; E – illuminating light; Fdark field condenser.