Magnetoplasmonic nanoringsnovel architectures with tunable magneto-optical activity in wide wavelength ranges

Resumen

Light-matter interaction is an old but still active and fascinating field, with new phenomena and theories emerging consistently. Thanks to the development of the nanotechnology in the last decades, fabrication of different materials and structures in the nanoscale becomes feasible, therefore the light-matter interaction in nanoscale obtains a vast and intensive research. Among this field, plasmonics, which studies the resonantly oscillating electrons (plasmons) driven by the light field in the metallic nanostructures, draws more and more attention due to its ability to confine the light field efficiently in nanometric scale. This ability, which makes it possible to manipulate light in nanoscale and actually can miniaturize greatly the device volume, has already found potential application in the integrated optical circuits for optical computation. Furthermore, the confined light field in such a small space results in the greatly enhanced localized electromagnetic (EM) field which can be utilized to enhance the other light-matter interaction, e.g. the famous application in surface enhanced Raman scattering (SERS). On the other hand, this enhanced localized field is sensitive to the environmental refractive indices, which makes it useful for the concept of “lab-on-chip” for biological and chemical sensing. For a full light circuit, light manipulators such as light modulator and optical isolator, are necessary, which allow modulating light wavevector and permit the light to pass through only along one direction but the opposite direction is forbidden (non-reciprocal effect), respectively. For this purpose, magneto-optical (MO) effect is a good option to manipulate the light actively (rotate the light polarization or control light reflection intensity) by the active magnetic field since it can realize an ultrafast operation speed (femtosecond level) and relatively large modulation depth. Moreover, the MO effect itself is non-reciprocal. Therefore, by combining the plasmonics and magneto-optics in one nanostructure, the so-called magnetoplasmonics will give us both of their advantages: miniaturization of the devices and active light (or plasmon) manipulation. As a light-matter interaction process, the MO effect can also benefit from the localized field at the plasmonic resonance, which gives rise to the plasmon-enhanced MO effect. On the other hand, instead of measuring the light intensity and peak shift in the plasmonics-based sensing platform, MO signal contains the phase information of the light (e.g. Kerr ellipticity), which actually has been proved to give a much better signal-to-noise ratio as the sensing platform. In this thesis, following these trends, a typical plasmonic nanostructure-nanoring- is studied with the further introduced MO functionality. Both the fabrication protocol and the optical/MO properties are studied. The protocol is implemented based on the hole-mask colloidal lithography (HCL, Fig. 1) technique that allows fabricating multicomponent nanostructures randomly distributed on a large area (over cm2). Fig. 1 Sketch for hole-mask colloidal lithography (HCL) method. The material (Au in the figure) deposits through the hole in the HCL template. With tilted angle deposition and rotation in the substrate, a nanoring structure can be formed. With the HCL method, the specific position of plasmonic and magnetic components (Au and Co in this case, respectively) can be controlled in three dimensions with the precision in nanoscale. Two important parameters in the HCL template, the density of the PS spheres used to define the holes in the template and the thickness of the Au mask, are optimized to adapt to the nanoring structure fabrication. By controlling the overall morphology and the spatial location of the different components (Fig. 2), we are able to finely control the plasmonic resonant characteristics of the nanostructures and as a consequence the MO response, both in terms of spectral position and intensity. Starting from the Au/Co/Au trilayer nanodisk structures which are normally deposited, the unimodal plasmonic resonance evolves to the bimodal resonance of the nanoring as the deposition angle increases to a certain value. The enhanced MO effect of these structures exhibits similar evolution as the structures evolve. This behavior is explained with the plasmonic hybridization model. Furthermore, by increasing the deposition angle during the deposition process, the cross-section of the nanoring structure can be tuned continuously, therefore the spectral positions of the plasmonic resonance and the enhanced MO peaks can be controlled in a fine way. The Co continuous layer in the nanoring can be further varied into opposite 2 Co sectors and 2 Co dots, which results in optical anisotropy, and the MO effect is found to be almost proportional to the Co amount. At last, with the purpose to further improve the MO effect in such structures, a gap is generated in the top Au ring above the Co dot, which can further focus the EM field in the MO-active Co dot region and result in a further enhancement of the MO effect by a factor of 3 compared with the structures without such a gap. Co Au Fig. 2 Structures studied in this thesis. From left to right: Au/Co/Au nanodisk, Au/Co/Au nanoring, Co sectors nanoring, Co dots nanoring, and ring/split ring with a gap on top of Co dot.