The Goos–Hänchen effect is an optical phenomenon where a beam of light appears to shift slightly along a surface when it's reflected. Instead of the light bouncing off exactly where you'd expect it to, it seems to travel a tiny distance along the surface before being reflected. This effect is very small but is important in various applications, particularly in fibre optics and some types of sensors.
This spatial shift of a reflection was first suggested by Isaac Newton in the 17th century. Later, in 1947, Goos and Hänchen used light to provide experimental evidence of Newton's suspicions and so was born the "Goos-Hänchen shift".
The first observation of Goos–Hänchen for particles was of neutrons using the OffSpec instrument at ISIS, back in 2010. The theory predicts a spatial shift on reflection, and a corresponding change in polarisation, but neither had been observed experimentally. A team led by Victor de Haan from Delft University of Technology (Netherlands) in collaboration with Robert Dalgliesh and Sean Langridge from ISIS reflected a beam of polarised neutrons onto a film of magnetic material and observed this minuscule change in polarisation. Their paper was the first to be published using results from ISIS' target station 2.
However, for some in the community, these changes were considered too miniscule, and not conclusive enough to prove the Goos-Hänchen shift. Now, a team led by Roger Pynn from the University of Indiana (USA), including Victor and Robert from the original research team, have developed a specially designed multilayered magnetic structure and repeated the experiment on the newer Lamor instrument at ISIS to prove the existence of the shift for neutrons.
In their study, published in Physical Review Letters, the sample, prepared by Peter Boni from Swiss Neutronics, enabled them to measure a much larger change in polarisation after reflection.
“As well as the polarisation change, in this experiment we were also able to observe the spatial shift directly," says Roger Pynn. “These two results verify both aspects and provide unambiguous evidence of the Goos–Hänchen shift."
The work at ISIS not only supports the ideas and work of Newton, Goos and Hänchen, but also proves that when particles are reflected, they behave in exactly the same way as light, as predicted from quantum theory. The results provide insights for the design of neutron waveguides - important components in neutron optics that are used to channel neutrons along a specific path.
“These results show that particles, in this case neutrons, behave in exactly the same way as light when they are reflected," says Professor Sean Langridge, ISIS Associate Director. “The results provide us with further experimental evidence of the accuracy of quantum mechanics and how the quantum nature of materials will be increasingly relevant to our lives through the development of new quantum technologies."
The full paper can be found at DOI: 10.1103/PhysRevLett.134.093803
