Optical Binding

In a new experiment at UCL we have realised evanescent wave optical binding of microscopic spheres.  A drop of liquid containing a suspension of microspheres is placed on top of a prism and a laser beam focused onto the prism surface from below.  When the beam is incident on the glass-water interface at just greater than the critical angle for total internal reflection an evanescent field penetrates a short distance into the water.  Under these conditions we see the microparticles drawn together into short optically bound chains.

Part (a) of the diagram represents out set-up.  The beam is retro-reflected and re-focused by the concave mirror in order to balance the radiation pressure in each direction.  Part (b) shows a photograph of optically bound chains of 2 micron diameter microspheres.