Photonic force microscopy (PFM) is an optical-tweezers-based microscopy technique. A small

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

particle (20 nm to several

micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

s) is held by a strongly focused

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...

beam. The forward scattered light, i.e. the light whose orientation is slightly changed while passing through the particle, and unscattered light are collected by a lens and projected onto a Quadrant Photo-Diode (QPD), i.e. a

Position sensitive device A position sensitive device and/or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure a position of a light spot in one or two-dimensions on a sensor surface. Principles PSDs can be divided into two classes whi ...

(PSD). These two components interfere in the detector and produce signals, which permit the detection of the bead's position in three dimensions. The precision is very good (as low as 0.1 nm) and the recording speed is very high (up to 1 MHz). Brownian motion deflects the bead from the resting position. A time sequence of measured positions allows one to derive the optical potential in which the particle is held. The PFM is sensitive to the environment of the particle and has been used in a variety of different experiments that e.g. monitor space that can be filled by particles inside agarose or the fate of small latex beads captured by macrophages. A similar concept of scanning a bead with an optical trap over a surface was invented in 1993 by Ghislain and W. W. Webb. The name of photonic force microscope was first used in 1997 by Ernst-Ludwig Florin, Arnd Pralle, J. Heinrich Hoerber and Ernst H.K. Stelzer during their stays at EMBL, when they developed 3D position detection and began using the Brownian motion as scanner.

References

Florin, E.-L., Pralle, A., Horber, J.K.H., and Stelzer, E.H.K. (1997) Photonic force microscopy based on optical tweezers and two-photon excitation for biological applications. J. Struct. Biol. 119, 202-211.
*Pralle, A., Florin, E.-L., Stelzer, E.H.K., and Horber, J.K.H. (1998) Local viscosity probed by photonic force microscopy. Appl. Phys. A 66, S71-S73. {{doi, 10.1007/s003390051102
Florin, E.-L., Pralle, A., Horber, J.K.H., and Stelzer, E.H.K. (1998) Photonic force microscopy calibration by thermal noise analysis. Appl. Phys. A 66, S75-S78.

Commercial PFM system

Elliot Scientific Ltd.

JPK Instruments
Microscopes Scientific techniques