Optical Tweezers When a colloidal particle is illuminated by an electromagnetic wave, it polarizes itself. It then emits an electromagnetic field, but its dipole also interacts with the incident field. If it is highly focused, then a radial force is exerted onto the particle, perpendicular to the axis of propagation of light. This force is attractive: the particle is trapped at the focal point. It is thus possible to manipulate the particle with the incident electromagnetic field. In a typical Optical Tweezers experiment, a single molecule is attached between two micron-sized beads. One of the bead is trapped by laser light, the other can be held by suction on a micro-pipette (one hand of the molecule can also be directly attached to a glass slide). Because the pipette (glass slide) is placed on a piezo-electric element, it can me moved with nm accuracy and the mechanical properties of the molecule (e.g. DNA) can be studied This type of set-up is best suited for the study of DNA/proteins interactions. The spatial resolution of Optical Tweezers can be greatly improved if one decouples the instrument from the environment. This can be achieved by separating the incident light into two components (s and p polarizations) and allows to reach sub-nm resolutions (bandwidth: few Hz).