Optical traps are often described as springs made of light, and their linearity makes them excellent force measurement tools. However, a more detailed look reveals that a conservative force field is not the whole story, and there are subtle but measurable forces which are not conservative. In real-life optical tweezers systems, these forces are usually attributed to "imperfections" in the system, such as scattering or absorption of the trapping light from a particle, or to asymmetries in the object being trapped. Saberi and Gittes describe a simple system, composed of four light rays and a sphere, which still exhibits non-conservative effects. What makes their model particularly interesting is that it does not require inclusion of forces due to reflection or absorption of light by the particle (often termed the "scattering" force).

Four rays of light are used to trap a refractive sphere, arranged such that the rays are all in the X-Y plane and they enter the sphere in the +X, -X, +Y and -Y directions (as illustrated in Fig. 2b in the paper). If the rays all pass through a single point, the system is conservative and we observe only a linear spring (for small displacements of the trapped sphere). However, if we displace the rays such that they don't cross, we make the system chiral; this is reminiscent of a Laguerre-Gaussian beam, for example. Particles trapped in Laguerre-Gaussian beams have been theoretically predicted and experimentally observed to circulate around the beam axis. However, this is primarily due to reflection or absorption of light by the particles. Modelling only refraction, the authors show similar behaviour in their much simpler system. It is worth noting, however, that the trapped object circulates in the opposite direction; the scattering force drives a particle in the direction of the Poynting vector, whereas the refractive forces described here push it around the other way.

Paring down the physical model to a very simple system is not just an interesting theoretical exercise; it is an important part of our understanding of these phenomena. An appreciation of the non-equilibrium aspects of optical forces is increasingly important as techniques emerge that work with asymmetric objects such as micro-tools, for example the need to take account of non-equilibrium thermodynamics when making force measurements. The particular model described here also bears a resemblance to counterpropagating optical traps, where two opposing, low Numerical Aperture light beams trap a particle. Misaligning such a system, such that the two counterpropagating beams are not collinear, could provide a way of realising this simple system experimentally.

---

---

 Add Comment

You must log in to add comments.