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Presenter: Esmaeil Rahimi Janiabadi
Supervisor:

Date: Tue, February 18, 2020
Time: 11:30:00 - 12:30:00
Place: EOW 430

ABSTRACT

Classical theory presents two contributions to second-harmonic generation in metals: electronic convection and the Lorentz magnetic force. The investigation of these effects in nanoscale split ring resonators showed negligible contribution from the Lorentz magnetic force. This is surprising since these structures are well known for their magnetic resonance. A fundamental question arises about whether the Lorentz force is always weak in metal nanostructures and therefore may be safely neglected. It is well known that metal structures typically provide weak magnetic resonances at high frequencies, but this can be overcome under certain conditions. The analogous consideration for the Lorentz force is whether it can play a non-negligible (or even dominant) role in second harmonic generation.
Typically, magnetic and electric fields are maximized in different places in optics. The simplest example is for cavities formed either by perfect electric conductor or by perfect magnetic conductor mirrors. For the perfect electric conductor cavity, the transverse magnetic field is maximum at the conductor and the electric field is zero there. For a perfect magnetic conductor cavity the situation is reversed. Next we consider resonances for subwavelength structures. Metal nanoparticles, for example, are typically treated in the quasi-static regime where to first order the magnetic field is absent. Apertures in metal films on the other hand are treated as magnetic dipoles and electric fields only arise at higher order. So it seems that quite generally the magnetic field and electric field will not be maximized in the same location and so the Lorentz force is doomed to be small.
In this work, we demonstrate that it is possible to create structures that indeed show both magnetic and electric field enhancements in the same location. We present both theory and experiment of metal nanostructures formed by apertures in metal films that show simultaneous enhanced magnetic and electric field enhancement in the same region and therefore show a dominant Lorentz force contribution to the second harmonic generation.