Grand Séminaire d'Institut

Vendredi 9 Septembre 2016 à 11h00.

Magneto-optical methods for nanomagnetism and magnetoplasmonics

''Magneto-optics in its many different forms exhibits several striking features: its high sensitivity allows probing magnetic materials down to the monolayer; its time resolution can shed light onto magnetic phenomena at time scales inaccessible to most other techniques; not less importantly, magneto-optics combines magnetism with spectroscopy, thus enabling to probe a rich series of complex effects. A research field in which magneto-optical spectroscopies are critically important is magnetoplasmonics: this discipline is part of the broader field of active plasmonics and studies the effect of a magnetic field on plasmon resonance. A peculiarity of magnetoplasmonic modulation is that the effect is strictly dependent on the polarization of light. As a consequence, the magnetic field can be held at a fixed value and high-frequency light modulation (attainable in the 104 Hz range with photoelastic modulators) can be used as the analog of a 100% inversion of the magnetic field. Fast modulation of the plasmonic response can be used to dramatically increase the figure of merit in refractometric sensing:[1, 2] this is particularly appealing in view of localized plasmon resonance-based sensing.[3] The effect of the magnetic field on non-magnetic plasmonic materials –such as gold or silver– is rather small: in order to try and boost this effect, we have been working on hybrid nano-architectures encompassing a magnetic moiety to flank the plasmonic one. We found that the magnetic-plasmonic interaction is strongly dependent on the nature of the magnetic part. [4] With this knowledge, we were able to obtain a set of precious guidelines for the design and preparation of efficient magnetoplasmonic nanostructures that give rise to increased magnetic modulation of their plasmonic properties. The term molecular plasmonics gathers several disciplines studying the interactions of molecules with plasmonic systems. [5] The best known and most spectacular instance of molecular plasmonics is surface-enhanced Raman scattering. There is, however, a score of other spectroscopies that take advantage of the interplay between plasmon resonances and molecules. We recently carried out experiments on thin layers of a molecular nanomagnet, TbPc2, deposited via thermal evaporation on glass-supported gold nanodisks. We found two different regimes of behaviour in the magneto-optical response: for very small deposits of TbPc2 (~2 nm) and strong peak overlap we found a relevant increase of the molecule’s magneto-optical signal. In the second regime, where there is no significant overlap of molecular and plasmonic resonances, the magneto-optical signal originating from the gold nanodisk shows a non-linear magnetic field dependence. We speculate that the observed non-linearity originates from the interplay with the magnetic molecular layer. This is the first report in which a magnetic molecule is coupled to plasmonic structures and the magneto-optical response of both components is taken into account. Therefore, this kind of approach has a high potential and could quickly develop into the new field of molecular magnetoplasmonics. [1] F. Pineider et al., Nano Lett. 13, 4785 (2013) [2] V. Bonanni et al., Nano Lett. 11, 5333 (2011) [3] J. N. Anker et al., Nature Mater. 7, 442 (2008) [4] Pineider, F. et al. ACS Nano 7, 857 (2013) [5] F. Della Sala, and S. D'Agostino, Handbook of Molecular Plasmonics (CRC Press, 2013) Curriculum Vitae : Francesco Pineider received his PhD degree in Chemical Sciences from the University of Florence in 2009. During this period, he studied the formation and behavior of single-molecule magnets on metallic surfaces, under the scientific guidance of Prof. Roberta Sessoli. During his post-doctoral experience, he specialized in the synthesis of magnetic-plasmonic colloidal nano-heterostructures and studied the effects of magnetic fields on localized surface plasmon resonance, a field of research known as magnetoplasmonics. After spending three years at ISTM-CNR in Padua, where he worked at the preparation of plasmonic and magnetoplasmonic materials using sol-gel techniques, he returned to the University of Florence to develop magneto-optical instrumentation and study the interaction of plasmonic nanostructures with single-molecule magnets. Starting in 2016, he took the position of researcher at the Department of Chemistry of the University of Pisa, where he continues pursuing his interests in magneto-optics and magnetoplasmonics.''