How to measure the magnetism of light?
Researchers have found a way to directly measure the rather weak magnetic component of an electromagnetic light wave. The proposed technique can be used in the future to measure the magnetic properties of individual nanoscale objects, as well as so-called metamaterials, Sci-lib.com reports.
Light is an electromagnetic wave, which, as you know, has two components: electric and magnetic. But when such a wave travels through matter, a rather weak magnetic component is almost impossible to detect directly. The “main” reaction of a substance to electromagnetic waves is the oscillatory motion of electrons under the influence of a change in the strength of the electric field. This interaction is usually 10,000 times greater than the “circular” component of the force arising from the magnetic field. The only objects for which this statement is not true are the so-called metamaterials, often containing metal rings that react extremely strongly to a magnetic field. Due to this, light passing through the metamaterial can change its trajectory in the most unexpected way, which makes it possible to create super-lenses and even the so-called invisibility cloaks.
Previously, scientists could measure the magnitude of the interaction of an electromagnetic wave with matter only by subtracting the dominant electrical component from the full recorded effect. However, almost simultaneously, two independent groups of researchers reported on the successful observation of the effect of only one magnetic component. To do this, they used a two-dimensional device called a “photonic crystal microcrack.” This device was made from a semiconductor crystal, in which (as in a punched card) small holes with dimensions of the order of several microns were created. “Microcrack” was formed by the absence of perforation on a certain area of the semiconductor. In this case, the surrounding structure acted as a “mirror” for infrared light, creating a standing wave over the “smooth” area.
For several years now, scientists have been investigating the “trapped” light in this way. For their measurements, they used a needle-shaped fiber, which was placed at a distance of several nanometers from the surface. With such an instrument it was possible to work with the electric field of a standing wave, which causes its frequency to shift further into the red region of the spectrum.
Now scientists have used optical fiber, covered everywhere, except for the tip itself, with a thin layer of aluminum. This additional coating behaves in relation to the magnetic field like a metal ring with a diameter of several nanometers. The “tool” interacts with a magnetic field, causing a flow of charges in the metal ring, which, in turn, induces a secondary magnetic field that shifts the frequency of the standing wave towards the violet region of the spectrum.
The researchers believe that such a technique could in the future not only help to better understand the nature of light, but also be used to measure the magnetic field of any small objects, including nanotubes and even individual atoms.