Colloidal suspensions in modulated light fields

Periodically-modulated potentials in the form of light fields have previously been applied to induce reversible phase transitions in dilute colloidal systems with long-range interactions. Here we investigate whether similar transitions can be induced in very dense systems, where inter-particle contacts are important. Using microscopy we show that particles in such systems are indeed strongly affected by modulated potentials. We discuss technical aspects relevant to generating the light-induced potentials and to imaging simultaneously the particles. We also consider what happens when the particle size is comparable with the modulation wavelength. The effects of selected modulation wavelengths as well as pure radiation pressure are illustrated.