Thursday, August 11, 2016

Structured light: Optomechanical tomography

Etienne Brasselet

The understanding that light carries momentum along its direction of propagation dates back to Kepler's astronomical observations of comet tails facing away from the Sun as the mechanical consequence of radiation pressure. Still, the collinearity between the direction that light propagates in and the force it exerts on matter is not a given. An example is the case of a photon impinging on a perfect mirror. The ensuing optical force is always directed perpendicular to the surface of the mirror. In other words, the optical force has in general both longitudinal and transverse contributions with respect to the propagation direction of the incident light. A more intriguing situation occurs when the optical momentum is no longer directed along the propagation direction of light. For instance, this may happen in evanescent waves. These are formed by so-called total internal reflection off an interface between two distinct dielectric transparent media when light propagates from a dense to a rare refractive medium. As their name suggests, evanescent waves have an intensity that vanishes exponentially with the distance from the interface while they propagate along the interface in a direction parallel to the incidence plane.

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