Nanotechweb: new property of nanoscale metallic ferromagnets
As reported in nanotechweb, researchers at nanoGUNE and Chalmers University have discovered a fundamentally new property in nanoscale ferromagnetic nanoantennas – their ability to control the sign of rotation of polarized scattered light. This “Kerr rotation reversal” effect arises from the interplay between magneto-optical and nanoplasmonic properties of such nanoantennas and could be exploited to make novel biochemosensors and in a variety of nanophotonics applications. The work was published in Nano Letters.
Nanoplasmonics is a relatively new and upcoming field of research on tailored metallic nanostructures that can be used to making tiny optoelectronics devices. Metallic nanoparticles interact strongly with light via localized surface plasmons (collective oscillations of electrons on a metal surface) and so act as efficient optical nanoantennas. They can focus light to wavelengths dramatically below the diffraction limit.
Nanoplasmonics research often spills over to research in nanomagnetism and a host of intriguing effects have already been observed. For example, diamagnetic particles can also developmagneto-optical properties. And, a special type of magneto-optic Kerr effect that comes about thanks to propagating or localized plasmonic modes has also been seen in structures like gold/cobalt/gold nanosandwiches, gold-iron garnet perforated films and gold-coated maghaemite nanoparticles, to name but three examples.
In contrast to such studies of hybrid plasmonic and ferromagnetic materials, Alexandre Dmitriev and Valentina Bonanni of Chalmers University of Technology in Sweden, Paolo Vavassori of nanoGUNE and colleagues have now looked at localized surface plasmons in purely ferromagnetic nanostructures. The researchers studied nickel nanodisks that were 60, 95, and 170 nm wide and 30 nm thick, grown on a glass substrate. Using longitudinal magneto-optic Kerr effect setup (L-MOKE), they unearthed a “magnetoplasmonic” Kerr effect – whereby the polarization of light reflected by the disks depends on both magneto-optical coupling and simultaneous excitation of localized plasmons in the material.
What is happening?
In the lab When light with a certain polarization is shone onto a nanosized magnetic ferromagnetic particle, the polarization will slightly rotate because magnetization changes the dielectric properties of the particle, explained Dmitriev. More strictly speaking, it changes the non-diagonal elements of a particle’s polarizability tensor – this is why these materials are called magneto-optical. “Now, we have found that such light also excites localized surfaces plasmons in the particle,” he told nanotechweb.org. “Localized plasmons also change the particle’s polarizability tensor but this time its diagonal elements instead.”
Magneto-optics and nanoplasmonics thus work hand in hand, making the particle magnetoplasmonic. “Without plasmons, the intrinsic magneto-optical effect rotates polarized light in one direction but when the particle is magnetoplasmonic, we can flip this direction to the opposite one,” says Dmitriev. “This is what happens when Kerr rotation gets reversed.”
The team, which includes researchers from Sweden, Iran and Spain, says that it has in fact discovered an extension of the “ordinary” magneto-optic Kerr effect, which describes how the polarization of light reflected by a ferromagnetic surface changes when an external magnetic field is applied.
The effect might be exploited to make biological and chemical sensors, suggests Dmitriev, because localized plasmons are very sensitive to their immediate dielectric environment. If the solution surrounding them is changed, the plasmon resonance in the material changes too – something that can be put to good use in label-free sensing. He explains: “In magnetoplasmonic nanostructures, the environment-induced variation of the optical resonance produces a change in the position of the Kerr rotation reversal. Since Kerr rotation sign changes can be detected very precisely, biochemosensors taking advantage of this effect would be much more sensitive because they track this rotation instead of just simply looking at the plasmon resonance itself.” Further, the ease of spatial manipulation of ferromagnetic nanoparticles due to magnetism and mentioned functionality for remote optical sensing and thermal management due to nanoplasmons open possibilities for simultaneous diagnostics and therapy applications. In nanophotonics, such magnetoplasmonic nanostructures can be employed, for example, as polarization-resolved light modulators.