Optical Tweezers & Photonic Force Microscopy

A photonic force microscope (PFM) can measure forces in the range from few femtonewton to several hundred piconewton and it is particularly suited to address forces acting on particles in the bulk. Therefore, it compliments the range of interations that can be probed by atomic force microscopy (AFM) and total internal reflection microscopy (TIRM).

A typical setup comprises an optical trap to hold a probe - a dielectric or metallic particle of micrometer size - and a position sensing system. In the case of biophysical applications the probe is a small dielectric bead tethered to the cell or molecule under study. The probe randomly moves due to Brownian motion in the potential well formed by the optical trap. Near the center of the trap the optical trapping potential is parabolic, i.e. the restoring force is linear in the displacement. The stiffness of such potential can be calibrated from the three-dimensional position fluctuation spectrum. To measure an external force acting on the probe it suffices to measure the probe average position displacement under the action of such force and multiply by the stiffness.

In order to understand the PFM it is necessary to discuss three aspects:

1. the optical forces that act on the probe and produce the optical trap;

2. the statistics of the Brownian motion of the probe in the trap, which are used in the calibration procedure;

3. and the position detection, which permits one to track the probe position with nanometer resolution and at kilohertz sampling rate.