Nanoplasmonics
Nanoplasmonics is an upcoming field of research on tuned metallic nanostructures used to making tiny optoelectronics devices. Metallic nanoparticles interact strongly with light through localized surface plasmons and act as efficient optical nanoantennas. They can focus light to wavelengths dramatically below the diffraction limit. Nanoplasmonics research has revealed that diamagnetic particles can develop magneto-optical properties and a special type of magneto-optic Kerr effect can be noticed. Localized plasmonic modes have been seen in structures like gold/cobalt/gold nanosandwiches, gold-iron garnet perforated films and gold-coated maghaemite nanoparticles.
Researchers at Chalmers University in Sweden and nanoGUNE in Spain claim to have discovered a fundamentally new property in nanoscale metallic ferromagnets – the ability to control the sign of rotation of polarized scattered light. Researchers have found localized surface plasmons in purely ferromagnetic nanostructures. The researchers studied nickel nanodiscs grown on a glass substrate. Using a longitudinal magneto-optic Kerr effect setup (L-MOKE), the researchers showed "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.
Principle
When light with a certain polarization falls onto a nanosized magnetic ferromagnetic particle, the polarization will slightly rotate because magnetization changes the dielectric properties of the particle. This material is called magneto-optical as it changes the "non-diagonal" elements of a particle's polarizability tensor and such light also excites localized surfaces plasmons in the particle. Localized plasmons also change the particle's polarizability tensor but in its 'diagonal' elements.
Magneto-optics and nanoplasmonics 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 this direction can be changed to the opposite one. This is what happens when Kerr rotation gets reversed. The researchers claim that they have 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
This effect can be used to make biological and chemical sensors, 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 and this effect can be used in label-free sensing. Magnetoplasmonic nanostructures might also be employed as polarization-resolved light modulators in nanophotonics.