Photonic force microscope

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Photonic force microscopy (PFM) is an optical tweezer based microscopy technique. A small dielectric particle (20 nm to several micrometres) is held by a strongly focused laser beam.

The forward scattered light, i.e. the light that passes through the bead, can be collected by a lens and projected onto a Quadrant Photo-Diode (QPD) or a Position Sensing Detector (PSD), which permits it to detect the position of the bead in 3D with very high accuracy (down to 10 nm) and at a very high rate (up to 1 MHz). From the measured positions it is possible to derive the optical potential in which the particle is held.

If then a force is applied on the particle, this can be estimated from the displacement of the particle itself.