Anomalous light reflection

The Goos-Hänchen effect and the Spin-Hall effect of light

The Law of Reflection of a light ray incident upon a mirror was first formulated by Euclid around 300 BC in his book Catoptrics; it has been a tenet of geometrical optics ever since. However, small deviations of the Law of Reflection have been observed for a physical light beam, when this is regarded as the implementation of a ray. Plane waves (or light rays) are infinitely extended and for this reason they cannot exist in nature. A light beam is the closest approximation of a light ray that it is possible to implement in a laboratory. The reflected beam can be spatially translated with respect to the prediction of geometrical optics. The Goos-Hänchen effect and the Spin-Hall effect of light are the name of these beam shifts according to the fact that the shift takes place in the plane of incidence or orthogonal to it. Apart from spatial effects, angular deviations of the Law of Reflection have been observed for a light beam. All the phenomena just introduced are diffractive corrections to geometrical optics due to the finite extension of physical light beams. These effects are polarization dependent. For the Goos-Hänchen effect, the relevant polarizations are s and p. For the Spin-Hall effect of light the relevant polarization are the circular plus and the circular minus, i.e. it depends on the spin of the incident light. The fact that a light beam with different polarization suffers different shifts is an experimental key for observations. It has been shown that not only the intrinsic angular momentum of the incident photon but also the orbital angular momentum (OAM) influences beam shifts in optical reflection. The dependence on the OAM is uncoupled with respect to the polarization state of the incident beam. All these properties give to the Goos-Hänchen effect and the Spin-Hall effect of light a great versatility. In recent years the technology related to the measurement of these small shifts of a light beam has considerably progressed. These phenomena were observed with a precision down to the nanometer.
We use the Goos-Hänchen effect and the Spin-Hall effect of light as new nano-optical tools for studying a broad range of interfaces. Possible applications of these techniques include the analysis of structured surfaces and optical sensing.

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