Quantum Effect Changes Direction of Light Waves

In certain materials like topological insulators, light waves can change their direction of polarization. (Source: TU Vienna)

A light wave sent through empty space always oscillates in the same direc­tion. However, certain materials can be used to rotate the direction in which the light is oscillating when placed in a magnetic field. This is known as a magneto-optical effect. After much specu­lation spanning a long period of time, one variant of this type of effect has now been demon­strated at TU Wien for the first time.

Rather than switching the direction of the light wave conti­nually, topo­logical insulators do so in quantum steps in clearly defined portions. The extent of these quantum steps depends solely on fundamental physical parameters, such as the fine-structure constant. It may soon be possible to measure this constant even more accurately using optical techniques than is currently possible via other methods.

“We have been working on materials that can change the direc­tion of oscil­lation of light for some time now,” explains Andrei Pimenov from the Insti­tute of Solid State Physics at TU Wien. As a general rule, the effect depends on how thick the material is: the larger the distance to be travelled by the light in the material, the larger the angle of rotation. However, this is not the case for the materials that Pimenov’s team has now inves­tigated more closely with the assis­tance of a research group from Würzburg. Their focus has been on topo­logical insulators, for which the crucial para­meter is the surface rather than the thickness.

Insulators on the inside, elec­tricity can usually be conducted very effec­tively along the surface of a topo­logical insulator. “Even when sending radiation through a topo­logical insulator, the surface is what makes all the difference,” says Pimenov. When light propa­gates in this 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.

What is most remarkable here is that this rotation takes place in parti­cular portions, in quantum steps, rather than being continuous. The interval between these points is not determined by the geometry or by properties of the material and is instead defined only by funda­mental natural constants. For example, they can be specified on the basis of the fine-structure constant, which is used to describe the strength of the electro­magnetic inter­action. This could open up the possi­bility of measuring natural constants with more precision than has previously been the case and may even lead to new measuring techniques being identified.

The situation is similar for the quantum Hall effect, which is another quantum pheno­menon observed in certain materials, in which case a particular variable can rise only by certain amounts. The quantum Hall effect is currently used for high-precision measure­ments, with the official standard defi­nition of electrical resistance being based on it. Back in 1985, the Nobel Prize in Physics was awarded for the disco­very of the quantum Hall effect. Topo­logical materials have also already been the subject of a Nobel Prize victory – this time in 2016. It is expected that these latest results will also make it possible for materials with special topo­logical charac­teristics to be used for specific technical appli­cations. (Source: TU Vienna)

Reference: V. Dziom et al.: Observation of the universal magnetoelectric effect in a 3D topological insulator, Nat. Commun. 8, 15197 (2017); DOI: 10.1038/ncomms15197

Link: Inst. of Solid State Physics, Vienna University of Technology, Vienna, Austria

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