Designer magneto-optics with plasmonic magnetic nanostructures, PhD Thesis by Nicolò Maccaferri


Nicolò Maccaferri, Pre-doctoral Researcher at the Nanomagnetism Group at CIC nanoGUNE, received his PhD at the University of the Basque Country (UPV/EHU) after the defense of his thesis project on 9 December 2016. His research work, entitled “Designer magneto-optics with plasmonic magnetic nanostructures", has been developed under the supervision of Dr. Paolo Vavassori, Nanomagnetism group coleader.

An international committee including leading researchers in the field was selected to assess the research project:

The defense consisted of a presentation by the candidate on the main aspects of the Ph.D. thesis project followed by an extended discussion based upon the questions that each one of the members of the committee raised in relation to the research work that has been carried out by Nicolò Maccaferri during his PhD studies. After its final deliberation, the committee decided to award the candidate the Doctor Degree with the highest mention existing at UPV/EHU (cum laude).

Nicolo Maccaferri at the magnetoopctics laboratory, at CIC nanoGUNE.

After the defense, we asked Dr. Nicolò Maccaferri to explain us a bit more about his project:

Which was the subject of your thesis?

The research I carried out in nanoGUNE during my PhD led to the demonstration that the excitation of surface plasmons in pure ferromagnetic nanostructures provides a pathway for tuning the magneto-optical response of a system beyond what is offered by the intrinsic material properties. More in detail, my PhD thesis work was focused on the design, fabrication and characterization of plasmonic magnetic nanostructures supporting both localized and propagating plasmon resonances. The final aim was to implement new conceptual designs for an efficient active control of optical modes at the nanoscale and on the exploration of possible applications of magnetoplasmonic nanostructures such as, for instance, ultrasensitive single-molecule detectors.

Why did you choose this subject?

I got close to magnetism during my master studies in Italy. When I was looking for new opportunities in this field to prepare my master thesis outside my country of origin, I had the opportunity to interact with Prof. Vavassori who offered me the chance to work between two very interesting fields in condensed matter physics nowadays, namely magnetism and plasmonics. The prospect to merge different physical properties to realize novel and unexpected phenomena and functionalities for the manipulation of light at the nanoscale, also in view of real life applications, was the main reason which pushed me to study with enthusiasm the properties of nanostructured magnetoplasmonic systems in the last five years.

Which methodology/techniques did you use?

I have used a number of different experimental techniques: Atomic Force Microscopy and Scanning Electron Microscopy for the structural characterization of the samples; MOKE and Faraday spectroscopy to measure the magneto-optical properties of the systems I studied, as well as traditional optical spectroscopy to study the transmission, reflection and absorption properties of nanostructured magnetoplasmonic materials. Moreover, I have learned some bottom-up fabrication techniques to build the systems I studied, such as Hole Mask Colloidal Lithography at Chalmers University of Technology (Sweden), and also top-down methodologies, such as Electron Beam Lithography, here in nanoGUNE and also at Aalto University School of Science (Finland). I have learned how to use many machines, both in the clean-rooms I visited and in the laboratories where I studied the optical properties of my samples. Finally, I expanded analytical theories to account for magneto-optical effects in plasmonic nanostructures, as well as I used numerical methods such as FDTD, FEM and Transfer Matrix methods to simulate the optical and magneto-optical properties of the studied systems.

Which have been the main conclusions?

We explored the influence of the phase of localized surface plasmon resonances on the magneto-optical activity of nanostructured ferromagnetic materials. Based on our findings, we proposed also a novel bio-chemo-sensing methodology, which enables radically improved sensitivity compared with recently reported plasmon-based sensors. Such a high sensitivity is achieved by exploiting the polarization conversion capabilities of both magneto-optically active nanoantennas and noble metal-based anisotropic metasurfaces. We then introduced a novel concept of magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodically arranged magnetic antennas that show anisotropic in-plane plasmonic responses. We demonstrate a significant strengthening of magneto-optical effects due to the coherent excitation of radiative far-field interactions between the nanoantennas as compared to a continuous film or metasurfaces made of randomly distributed nanoantennas. Finally, we have also explored the potential of the synergy between intrinsic magneto-optical mediated polarization conversion and plasmon excitations in ordered systems by substituting localized with propagating surface plasmon polaritons in 2D-magnetoplasmonic crystal. We demonstrated that an enhancement of the Kerr effect is induced when special non-collinear surface plasmon polariton modes, which couple to both p- and s-polarized light, are resonantly excited.

What could be the contribution of your research for present or future nanotechnologies?

We envisage that the concepts and the results presented in my PhD thesis will open up excellent nano-engineering opportunities towards enhanced and generally designed magneto-optical applications. The use of the developed design principles would deliver to an outstanding control over the polarization state of light and pave the way for novel sensing methodologies. Moreover, we envision that the systems studied in this Thesis offer the unique opportunity to explore other potential synergistic effects arising from peculiar static and dynamic magnetic configurations, such as periodic magnetic domains and spin waves, which can be induced and controlled in this kind of system by an external magnetic field, opening new perspectives towards the interplay between the fields of magnonics, spintronics, and plasmonics.

How do you feel now that you have finished the thesis?

I’m really happy that I managed to bring my thesis work to a successful conclusion and to have developed such many research skills. I have to say that I’m also a bit sad, because it also means the end of a terrific period of intense and successful research in the Nanomagnetism Group and also the end of one of the best periods in my life. Currently, I am a Post-doctoral Researcher in the Plasmon Nanotechnologies Group led by Dr. Francesco De Angelis at the Department of Nanophysics at Italian Institute of Technology sited in Genova (Italy). My main research interests continues to be the fundamental and applied aspects of plasmon-based nanotechnologies, in particular plasmonic-based bio-sensing and opto-electronics, and, of course, magnetoplasmonics, since I will keep collaborating with my former collegues in nanoGUNE and from all over the world.

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