150 Years After Maxwell, Scientists Discover Fundamental Property of Light

by Monica Joshi

We've written quotes about it. The Bible makes grand claims about God creating it. We think we know everything that there is to know about it – until now. Scientists have just uncovered a new fundamental property of light that sheds light on the 150-year-old classical theory of electromagnetism. This could lead to some interesting new applications for manipulating light at the nanoscale.

As it is super unusual for a pure-theory physics paper to make it into the journal Science, it is definitely worth a second-glance. In the new study, researchers explore the connections to the following theories: James Clerk’s Maxwell’s famous theory of light, the quantum spin Hall effect and topological insulators.

Seems like a whole lot of hard stuff to swallow, without a chaser. To understand how all of this works, let's begin by considering the behaviour of electrons in the quantum spin Hall effect. Electrons possess an intrinsic spin, constantly rotating about their axis. This spin, is a quantum-mechanical property. There are, however, special rules that apply: the electron only has two options open to it. Either it can spin clockwise or anti-clockwise, spin-up or spin-down respectively. Although, the magnitude of the spin is always fixed.

The spin of the electron, in certain materials, can have a big effect on the way that electrons move. This effect if called the "spin-orbit coupling". This is kind of like in soccer, when you freekick the ball with a spin, the soccer player can deviate the ball either to the left or the right as it travels through the air. The direction of movement of the ball, thereby, would depend on the way in which the ball would spin.

While a normal electrical current consists of an equal mixture of moving spin-up and spin-down electrons, due to the spin-orbit effect, spin-up electrons will be deflected one way, while spin-down electrons will be deflected the other. The deflected electrons will reach the edges of the material and be able to travel no further. The spin-orbit coupling thus leads to an accumulation of electrons with different spins on opposite sides of the sample.

This effect is known as the classical spin Hall effect, and quantum mechanics transforms it in a completely unique way. The quantum-mechanical wave nature of the travelling electrons organises them into neat channels along the edges of the sample. In the bulk of the material, there is no net spin. But at each edge, there form exactly two electron-carrying channels, one for spin-up electrons and one for spin-down. These edge channels possess a further remarkable property: the electrons that move in them are impervious to the disorder and imperfections that usually cause resistance and energy loss.

This precise ordering of the electrons into spin-separated, perfectly conducting channels is known as the quantum spin Hall effect. This is a classic example of a “topological insulator”. A topological insulator is a material that is an electrical insulator on the inside but that can conduct electricity on its surface. Such materials represent a fundamentally distinct organisation of matter and promise much in the way of spintronic applications.

Explanation time over. Time to focus on the new study. The new study suggests that the seeds of this spin Hall effect is actually all around us. So, it focused on light.

In Maxwell’s theory, light is an electromagnetic wave. This means that light travels as a synchronised oscillation of electric and magnetic fields. The new research studies the way in which these fields rotate as the wave propagates. The researchers were able to define a property of the wave, the “transverse spin”, that plays the role of the electron spin in the quantum spin Hall effect.

This spin is exactly zero in any homogeneous atmosphere – such as air. But, at the interface between two media (air and gold, for example), the character of the waves change dramatically. A transverse spin, therefore, develops. The direction of this spin is precisely locked to the direction of travel of the light wave at the interface. Thus, when viewed in the correct way, we see that the basic topological ingredients of the quantum spin Hall effect that we know for electrons are shared by light waves.

Understanding the spin-orbit effect could create new possibilities for controlling light at the nanoscale. Optical connections, for example, are seen as a way of increasing computer performance, and in this context. The spin-orbit effect could be used to rapidly reroute optical signals based on their spin. With applications proposed in optical communications, metrology, and quantum information processing, the future of this theory could be quite fascinating to say the least.