A photonic-force microscope is a device consisting of three essential elements: a tightly-focused laser, a spherical particle, and a quadrant photodiode. The device is operated as follows: first, the spherical particle must be trapped by the laser. Optical interactions create weak forces such that the particle will be attracted towards a point close to the focal point of the laser, but interactions with the medium in which the particle is found will tend to dislodge it. In particular, Brownian motion is an especially interesting cause of its whereabouts. The induced movements of the particle then modify the characteristics of the transmitted laser beam, and these modifications are recorded by the quadrant photodiode. All of this finally results in the knowledge of the evolution of the position of the particle through time near the focal point of the laser. Because they are produced by the Brownian motion, the fluctuations are random; but since they depend on the local properties of the medium, by analyzing the trajectories it is hoped to recover the properties themselves.

Additionally, the position of the sample been investigated can be controlled by high-precision actuators. This offers the opportunity to perform a spatial scanning of the sample to establish a map of the properties such as, for example, anisotropic viscosity.