Quantum Effect Changes Direction of Light Waves

In a variation of the magneto-optical effect, special materials called “topological insulators” (TI) demonstrated the ability to switch the direction of a light wave in clearly defined quantum leaps rather than continually. This “quantized magneto-electric effect” could open up new and highly accurate methods of measurement.

The extent of these quantum steps was found to depend solely on fundamental physical parameters, such as the fine-structure constant. Researchers believe that it may soon be possible to measure this constant even more accurately than is currently possible using optical techniques.

Researchers from Vienna University of Technology (TU Wien) used monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semitransparent gate to selectively address surface states. In high external magnetic fields, they observed a universal Faraday rotation angle equal to the fine structure constant α = e²/2hc (in SI units) when a linearly polarized THz radiation of a certain frequency passed through the two surfaces of a strained HgTe 3D TI.

In certain materials, light waves can change their direction of polarization. Courtesy of TU Wien.

The magneto-optical effect, which involves use of certain materials to rotate the direction in which light oscillates, typically depends on how thick the material is. The greater the distance the light must travel through the material, the larger the angle of rotation.

However, the TU Wien team, with the assistance of a research group from Würzburg, found that this was not the case for the TI materials. Electricity can be conducted effectively along the surface of a topological insulator. For TIs, the surface rather than the thickness of the material was the crucial parameter.

“Even when sending radiation through a topological insulator, the surface is what makes all the difference,” said professor Andrei Pimenov.

When light propagates in TI material, the oscillation direction of the beam is turned by the surface of the material twice — once when it enters and again when it exits.

This rotation takes place in quantum steps, rather than being continuous. The interval between the steps is not determined by the geometry or properties of the material. It is instead defined by fundamental natural constants — for example, based on the fine-structure constant, which is used to describe the strength of the electromagnetic interaction.

The team believes that its discovery could open up the possibility of measuring natural constants with more precision than has previously been the case, and may even lead to the identification of novel measuring techniques. The findings offer insight into the axion electrodynamics of TIs and could potentially be used for a metrological definition of basic physical constants.

“We have been working on materials that can change the direction of oscillation of light for some time now,” said Pimenov.

The research was published in Nature Communications (doi:10.1038/ncomms15197).