&ball; Phys. Rev. Focus 26, 13
Two research teams used a ring-shaped probe to directly characterize the magnetic field of infrared light in a small cavity.
Light is a wave of electric and magnetic fields, but when these waves hit matter, the weaker effect of the magnetic component is almost impossible to detect directly. Now two groups have independently demonstrated that a tiny metal probe interacts strongly with the magnetic field of light waves trapped in some kind of semiconductor “box.” As described in a pair of articles from September 17 Physical examination lettersa similar setup could be used either to measure the high-frequency magnetic properties of individual nanoscale objects or to map the magnetic field inside so-called metamaterials that can control light in new ways.
When light interacts with matter, the dominant action is often a “jerk” of electrons up and down in response to the electric field. This interaction is generally 10,000 times greater than the “swirling” action of the magnetic field of a light wave. The case is different in metamaterials, which contain small components like metal rings that are often designed to have an enhanced response to magnetic fields. Thanks to this sensitivity, light passing through a metamaterial can bend in unusual ways, making devices such as super-lenses and invisibility cloaks feasible.
Previously, researchers could only measure the magnetic interaction between light and some form of matter by subtracting the dominant electrical interaction from the total effect of light. Now two experimental groups have succeeded in directly isolating the effect of the magnetic field. They worked with a type of two-dimensional device called a photonic crystal microcavity. The crystal is made by punching a thin layer of semiconductor with a pattern of small holes, like a micron-sized punched card. The cavity is made by leaving a small “unperforated” region and letting the surrounding array of holes act as mirrored walls that keep infrared light bouncing around the cavity as standing waves.
For several years, researchers have been characterizing the light trapped in photonic cavities by bringing the tip of a needle-shaped optical fiber a few nanometers from the surface. This probe disrupts the electric field and shifts the trapped light to longer wavelengths. The new experiments used a fiber tip coated with a thin layer of aluminum that covers all but the bottom of the tip. This metal “tube” acts like a ring a few hundred nanometers in diameter.
The two research teams were initially surprised to find that these metal rings caused a blue shift in the trapped light. But later they realized that according to classical electrodynamics, the oscillating magnetic field of light induces a current in the metal ring of the tip, which creates a secondary magnetic field that points in the opposite direction to the original. . This field cancels part of the magnetic field in the cavity and thus reduces the volume of trapped light. Less volume means shorter, bluer wavelengths. “It’s like playing the guitar,” says Tobias Kampfrath of the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam. “If you shorten the guitar strings, the resonant wavelengths will also shorten.”
A team including Kampfrath, AMOLF’s Kobus Kuipers and others were able to measure a blue shift of about 0.03% with their cavity and probe. They combined this with an estimate of the cavity’s maximum magnetic field to obtain the magnetic properties of their nanometer-sized ring. With the results matching theoretical expectations, the authors suggest that this method could be used to measure the magnetic response of other small objects, such as carbon nanotubes or even single atoms. They also recently performed a variation of this experiment, in which they used a tip with an open ring to probe the magnetic field of traveling (untrapped) light. .
The other group’s experience was similar, except that they gleaned different information from the magnetic interaction. Instead of measuring the properties of the ring, Silvia Vignolini, now at the University of Cambridge, Diederik Wiersma of the European Laboratory for Nonlinear Spectroscopy (LENS) in Florence, Italy, and their collaborators, scanned their tip coated with metal on the photonic crystal. surface to construct an image showing the spatial patterns of the magnetic field.
“The imaging itself is nice,” says Claus Ropers of the University of Göttingen in Germany, “but the real breakthrough of this work lies in the quantitative extraction and potential control of local magnetic interactions and forces of coupling.” Harald Giessen of the University of Stuttgart compares the experiments to the late 19th century work of Heinrich Hertz, who used a ring-shaped antenna to map the magnetic fields of radio waves. Giessen thinks these new probing techniques will prove useful in the fabrication of new optical devices.
Michael Schirber is corresponding editor for Physics based in Lyon, France.
- M. Burresi, D. van Oosten, T. Kampfrath, H. Schoenmaker, R. Heideman, A. Leinse and L. Kuipers, “Probing the magnetic field of light at optical frequencies”, Science 326, 550 (2009)