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's surface) and so act as efficient optical nanoantennas. They can focus light to wavelengths dramatically below the diffraction limit.
Since many metals are magnetic, nanoplasmonics research often spills over into research on nanomagnetism and a host of intriguing effects have already been observed. For example, diamagnetic particles can also develop magneto-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 and Paolo Vavassori of nanoGUNE in Spain and colleagues have now looked at localized surface plasmons in purely ferromagnetic nanostructures. The researchers studied nickel nanodiscs that were 60, 95 and 170 nm wide and 30 nm thick, grown on a glass substrate. Using a 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?
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, explains 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," said Dmitriev. "This is what happens when Kerr rotation gets reversed."
The team 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.
Applications in sensing and nanophotonics
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 the plasmons is changed, the plasmon resonance in the material changes too – something that can be put to good use in label-free sensing. He explained: "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."
Such magnetoplasmonic nanostructures might also be employed as polarization-resolved light modulators in nanophotonics.
The work was detailed in Nano Letters.