Optical circuits at the nano scale
Shining light on the mysteries of the quantum world is already leading to novel materials and devices. Synchronised oscillations of electron charge density at interfaces in some metallic nanostructures are called localised surface plasmon resonances (LSPRs). Active plasmonic devices that use a chemical or physical input to control a plasmonic system have great potential for use in optical circuits at the nano scale, including advanced biosensors. If that input is a magnetic field, the result is a magnetoplasmonic system. The EU-funded project 'Imaging the plasmonic activity of magnetic nanostructures' (IPMAGNA) was designed to investigate LSPRs via local probe microscopy techniques in nanostructures exhibiting magneto-optic (MO) properties. In order to do so, scientists sought to illuminate the nano-structured materials to induce LSPRs and then use magnetic force microscopy (MFM) measurements during illumination to detect the magnetic components of their field distribution (MFM detection upon illumination). Researchers modified a commercial atomic force microscopy (AFM) probe tip with a metal coating to be used in MFM measurements. They developed nano-fabrication techniques to create nanodisks and nanoholes in thin films. Further, they were able to tune the wavelength of the LSPRs (their frequency of oscillation) to the wavelength of the laser used for MFM by controlling the composition and dimensions of the nanostructures. Optimisation of the MFM detection technique followed by minimising laser interference between the excitation laser and the detection laser, as well as increasing the signal-to-noise ratio. Finally, scientists used a new detection mode enabling a 20 % increase in lateral resolution and a tip-sample distance of only 2 nanometres. Although investigators did not detect a reliable magnetic signature associated with LSPRs, evidence suggests that stronger laser power should overcome the inherent challenges. In the meantime, the novel magnetic detection techniques developed by the team should find immediate use in both academic research and industry, where MFM is a common technique employed in evaluating magnetic storage media.
