Plasmonic Au/Co/Au nanosandwiches with enhanced magneto-optical activity.

Abstract

It is well known that localized surface plasmon resonances (LSPRs) greatly influence the optical properties of metallic nanostructures. The spectral location of the LSPR is sensitive to the shape, size, and composition of the nanostructure, as well as on the optical properties of the surrounding dielectric. The latter effect has been used to develop different types of optical biosensors for which biological reactions near the surface of the nanostructure can be monitored through the changes in the frequency of the LSPR. The induced electromagnetic field associated with the LSPR is greatly enhanced at the metal/dielectric interface, a phenomenon that is the basis for various types of surface-enhanced spectroscopy, such as surface-enhanced Raman scattering. Furthermore, metallic nanoparticles have been shown to have light-guiding capabilities on the nanometer scale. This makes them suitable for the development of nano-optic devices. The overwhelming majority of LSPR studies have focused on Au or Ag nanoparticles because these metals have suitable optical constants for application with visible wavelengths of light. However, once the morphology and composition of a nanostructure have been fixed, it is difficult to change or control the LSPR properties by external means, which would be desirable for the development of active nanoplasmonic devices. One way to overcome this problem could be to embed the metal nanostructure in an active medium, such as a liquid crystal, which can be controlled by an external electrostatic field, or a ferromagnetic garnet, which can be moderated by a magnetic field. An alternative approach could be to let the controlling field act directly on the metallic nanostructure, for instance, using nanoparticles made of ferromagnetic metals. Such metals have strong magneto-optical (MO) activity, that is, their optical properties change markedly even if the applied magnetic field is weak. Unfortunately, this high optical absorption results in a strong damping of any intrinsic LSPR that prevents the development of active plasmonic devices made solely of ferromagnetic metals. A promising route forward could be to combine ferromagnetic materials that would promote strong MO activity with noble metals that could induce plasmonic response. The large enhancement and spatial localization of the electromagnetic field associated with the LSPR suggest that a strong enhancement of the MO properties should be possible. Several attempts to develop these kinds of structures have been carried out using different chemical synthesis methods to fabricate complex onion-like nanoparticles made of noble metals and ferromagnetic materials. These systems do exhibit LSPRs, but so far no MO activity has been reported. On the other hand, continuous thin films made of Au/Co/Au trilayers were found to lead simultaneously to well-defined propagating surface plasmon polaritons and to strong MO activity at low magnetic fields. Moreover, such composite structures also exhibit a magnetic-field-induced nonreciprocal effect in the surface plasmon polariton propagation, that is, forward or backward propagating surface plasmons exhibit different wavevectors. A promising recent application of composite Au/Co thin films is the new type of high-sensitivity “magneto-plasmonic” biosensor reported in Reference [20]. In this Communication, we show that strong magneto-plasmonic effects occur in nanosandwiches composed of stacked Au/Co/Au disks. The Au/Co/Au nanosandwiches, prepared by a self-assembly process, exhibit simultaneous LSPR, magnetic, and MO properties. We show that optical and magneto-optical properties are strongly linked to the LSPR spectrum, which can be tuned by modifying the size of the nanoparticles. Moreover, there is a large enhancement of the MO properties caused by the LSPR effect. To the best of our knowledge, this is the first demonstration of a plasmonic nanostructure that can be controlled by an external magnetic field. The fabrication of the sandwich nanostructures from sputtered Au/Co/Au trilayer films was performed using colloidal lithography (CL). In Figure 1 we show an atomic force microscopy (AFM) image corresponding to the sample obtained using polystyrene spheres of 76-nm diameter. The resulting nanoparticles have a mountainlike shape. The tip radius of the AFM is about 10 nm, therefore, the mountainlike shape is not only due to convolution effects (that would play a role in imaging the edges of the nanostructure), but also to the leftovers of the polystyrene spheres. This residual influence also explains the differences between the nominal heights of the nanosandwiches and those obtained by AFM, which are always higher than 32 nm. The plasmonic and magneto-optical properties of the fabricated nanosandwiches were analyzed by conventional UV/Vis extinction spectroscopy and magneto-optic Kerr spectroscopy, respectively. The magneto-optic Kerr effect consisted of a change in the reflectivity of the magnetic material when a magnetic field was applied. In particular, we employed a polar Kerr configuration in which the magnetic field was perpendicular to the nanosandwiches. In this con[*] J. B. Gonz lez-D az, Dr. A. Garc a-Mart n, Dr. J. M. Garc a-Mart n, Prof. A. Cebollada, Prof. G. Armelles Instituto de Microelectr"nica de Madrid Consejo Superior de Investigaciones Cient ficas Isaac Newton 8 (PTM), Tres Cantos, Madrid, 28760 (Spain) Fax: (+34) 918 060 701 E-mail: gaspar@imm.cnm.csic.es

DOI: 10.1002/smll.200700594

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Cite this paper

@article{GonzlezDaz2008PlasmonicAN, title={Plasmonic Au/Co/Au nanosandwiches with enhanced magneto-optical activity.}, author={J. Gonz{\'a}lez-D{\'i}az and Antonio Garc{\'i}a-Mart{\'i}n and Jos{\'e} Miguel Garc{\'i}a-Mart{\'i}n and Alfonso Cebollada and Gaspar Armelles and Borja Sepulveda and Yury Alaverdyan and Mikael K{\"a}ll}, journal={Small}, year={2008}, volume={4 2}, pages={202-5} }