Almost a hundred years on from its first proposal, a number of questions remain around what spin is. While understood to be a basic property characterizing and distinguishing fundamental particles just as charge or mass do, it is still not clear what exactly spin describes. Photon spin has been understood as a feature of circularly polarized light, and there remains a generally accepted association between spin and polarization – no polarization, no spin so the general understanding goes. Now researchers at the LCN in collaboration with scientists in Germany, Japan, France and the US, have demonstrated that even this basic assumption is no longer valid.
Notions of spin have already had a reset over the past ten years along with a broadening in the possible descriptions of the polarization of light. For laser beams or light at any significant distance from its source, light rays can be considered to travel parallel to each other (paraxial rays), in which case the polarization is always confined to the plane orthogonal to the direction of propagation. With light circularly polarized in this plane, the spin will consequently always be aligned either along the direction of propagation or directly opposite to it. However, subject the beam to total internal reflection, as for example in waveguides or just a prism, and a non-propagating evanescent field results, where it turns out the polarization has components in all three dimensions. Tightly focused beams can also result in 3-dimensional polarization. For 3D polarization states, spin transverse to the direction of propagation is possible, as researchers at King’s College London shown several years ago.
“We saw some interesting phenomena in electromagnetism that were hard to explain,” says Francisco Rodríguez Fortuño, a researcher at King’s College London, recalling the first time they realised 3D polarization could give rise to transverse spin. “Then everything made sense.” Soon observations of transverse spin following transformations akin to reflection or focussing were flooding in for light in various experiments. However, according to theory even totally unpolarized light can lead to transverse spin. The reasoning follows from the expansion from 2D the polarization states to describe a paraxial beam and the 3D polarization states used once it has been transformed into an evanescent field by total internal reflection. Even a beam totally unpolarized in 2D would have some order of polarization when the polarization is described in 3D due to the lack of longitudinal components of the spin. The next challenge was to observe transverse spin experimentally from a totally unpolarized beam.
Diane Roth and Luke Nicholls were among the researchers at King’s College London demonstrating this in the experiments with evanescent fields, led by Anatoly Zayats. As Roth points out, one of the obstacles to experimental validation of this theory was the difficulty in sourcing completely unpolarized light bright enough for the effect to be measured, since lasers and the usual lab light sources are all polarized in one or another degree. In the end they used a humble light bulb and sure enough they observed signatures of transverse spin. Their collaborators in Germany working with tight focussing and a different source of unpolarized light were also able to corroborate the theory. The results suggest that transverse spin may be present in instances as common as totally internally reflected sunlight. Meanwhile a valid definition of what is meant by that spin remains ever more elusive.
Image: A beam transformed: Optical transforms like tight focussing and total internal reflection, which gives rise to an evanescent field, can lead to out-of-plane polarization components and transverse spin, even for totally unpolarized input light. (Courtesy: Konstantin Bliokh/Diane Roth)