Developments in magnetoplasmonics and nanoplasmonics

Citation

(2012), "Developments in magnetoplasmonics and nanoplasmonics", Sensor Review, Vol. 32 No. 3. https://doi.org/10.1108/sr.2012.08732caa.002

Developments in magnetoplasmonics and nanoplasmonics

Article Type: Nanosensor update From: Sensor Review, Volume 32, Issue 3

Researchers at Chalmers University in Sweden and CIC nanoGUNE Consolider, a recently established Spanish research centre, claim to have discovered a fundamentally new property in metallic, nanoscale ferromagnets: the ability to control the direction of rotation of polarised, scattered light. This is the “Kerr rotation reversal” effect and could be exploited in novel biosensors and chemical sensors.

Nanoplasmonics is a relatively new field involving metallic nanostructures that can be used to fabricate miniature optoelectronics devices. Metallic nanoparticles interact strongly with light via localised surface plasmons and so act as efficient optical nanoantennas which can focus light to wavelengths significantly below the diffraction limit. Since many metals are magnetic, nanoplasmonics often spills over into nanomagnetism research and several intriguing effects have already been observed. For example, diamagnetic particles can also develop magneto-optical properties and a special type of the magneto-optic Kerr effect that arises from propagating or localised plasmonic modes has also been observed in structures such as gold/cobalt/gold nano-sandwiches, gold-iron garnet perforated films and gold-coated maghaemite nanoparticles. In contrast to such studies of hybrid plasmonic and ferromagnetic materials, the researchers at Chalmers and nanoGUNE have now investigated localised surface plasmons in purely ferromagnetic nanostructures. They 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, they observed a magnetoplasmonic Kerr effect, whereby the polarisation of light reflected by the disks depends on both the magneto-optical coupling and the simultaneous excitation of localised plasmons in the material. When light with a certain polarisation is shone onto a nanosized ferromagnetic particle, the polarisation will rotate slightly because magnetisation changes the dielectric properties of the particle. Strictly speaking, it changes the non-diagonal elements of a particle’s polarisability tensor. Magneto-optics and nanoplasmonics thus work hand in hand, making the particle magnetoplasmonic. Without plasmons, the intrinsic magneto-optical effect rotates polarised light in one direction but when the particle is magnetoplasmonic, this direction is reversed. This is Kerr rotation reversal and has potential applications in sensing, as localised plasmons are very sensitive to their immediate dielectric environment. If a solution surrounding the plasmons is changed, the plasmon resonance in the material changes too – something that can be exploited in label-free biosensing. 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, biological and chemical sensors utilising this effect would be very sensitive because they track this rotation instead of just simply looking at the plasmon resonance itself.

Vortex nanogear transmissions

Despite offering innovations in biosensing and THz metamaterials design, plasmonics faces fundamental physical limitation in the visible region due to the high absorptive losses of metals. A significant part of the energy is converted into an inherently lossy kinetic motion of free electrons and is dissipated rapidly as heat. Now, work in the Department of Chemistry at Boston University has shown that it is possible to achieve significant performance improvement to nanodevices fabricated with standard plasmonic materials such as silver, gold and aluminium. The Boston team has demonstrated a new way to trap, enhance and manipulate light in nanoscale structures and nanopatterned thin films. This novel approach can significantly improve the performance of photonic and electronic devices such as nanosensors and thin-film solar cells. It is fundamentally different from the traditional way of designing plasmonic/nanophotonic structures, where plasmonic components are treated as nanoscale analogues of RF antennas and waveguides. Drawing its inspiration from hydrodynamics, it offers a new way to route and re-circulate optical energy outside of the metal volume within plasmonic nanostructures. In particular, optical energy flow is “moulded” into optical vortices – tornado-like areas of circular motion of energy flux which are “pinned” to plasmonic nanostructures. In this way, nanostructures can be engineered to couple counter-rotating optical vortices into transmission-like sequences termed vortex nanogear transmissions (VNTs, Figure 6).

Figure 6 Schematic of total light absorption by a graphene film

This new approach allows the reduction of dissipative losses, increasing the amount of energy that can be accumulated within a sub-diffraction volume and tailoring the optical spectra of plasmonic nanostructures. This provides a basis for developing robust schemes for long-range, on-chip energy transfer/routing as well as active nanoscale field modulation and spatial control, which are still largely missing in conventional nanoplasmonic circuitry. The group anticipates that their findings will dramatically change the approach to engineering plasmonic nanocircuits. In the biosensing context, the high Q-factors of surface plasmon modes in VNTs translate into high spectral resolution. In particular, the researchers predict that novel biochemical sensors based on the proposed concept will exhibit almost an order of magnitude improvement in figure of merit values compared to current designs. Further, the high field intensity generated in VNTs at several wavelengths, e.g. pump and Raman-shifted wavelengths or pump and emission wavelengths, offers opportunities for improvements to Raman and fluorescence spectroscopy.