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Enabling Multifunctional Plasmonics on Hybrid Artificial Scale-Integrated Systems

Periodic Reporting for period 1 - EMPHASIS (Enabling Multifunctional Plasmonics on Hybrid Artificial Scale-Integrated Systems)

Reporting period: 2018-09-01 to 2020-08-31

Optical isolation or one-way propagation of light is difficult to achieve because, unlike electrons, external forces such as applied electric or magnetic fields cannot easily control the propagation of electromagnetic waves. On the other hand, optical isolators are necessary in fibre optic communication to prevent back reflections and improve signal-to-noise ratio. To realize optical isolation magneto-optical effects are used. In magneto-optically active materials the interaction of light with the magnetized medium breaks the time-reversal symmetry and gives rise to non-reciprocal optical properties i.e. distinct propagation characteristics to forward and backward propagating waves. The development of on-chip optical communications requires downscaling of optical components, e.g replacing optical fibres with nanoscale waveguides. The miniaturization of optical isolators is therefore a key step towards integrated photonic circuits. This process is limited by the limited magnitude of magneto-optical activity in most know materials.

We approach this challenge by taking advantage of surface plasmon resonances that can squeeze light down to nanoscale dimensions, thus giving rise to enhanced light-matter interaction. We combine plasmonic waveguides with ferroelectric and -magnetic materials that, in turn, break the space-inversion and time reversal symmetries to create non-reciprocal conditions for light propagation. The ferroelectric and magnetic materials provide us with an additional interesting advantage: their optical properties can be adjusted by applying external electric and magnetic fields, enabling active control over light in nanoscale. Our objective is thus to demonstrate a miniaturized device capable of optical isolation that takes advantage of properties of ferroelectric and ferromagnetic materials that break the inversion and time-reversal symmetries.
We performed comprehensive analysis of magneto-optical in plasmonic gratings both in direct excitation and diffraction with the objective to exploit these effects for non-reciprocal optical devices. Our work highlights that the magneto-optical effects in gratings results in significant non-reciprocity, in particular in diffracted light (figure 1).

The second part of the project was initialized with the objective to exploit the electro-optical (Pockels) effects in ferroelectric materials such as Barium Titanate (BaTiO3, BTO). Work was carried out towards developing nanofabrication capabilities to fabricate plasmonic devices on BTO substrates and on integrating plasmonic nanoparticles in BTO layers. Due to the state of emergency declared in Spain from 14.3.2020 the experimental work was severely restricted.

The remaining time of the grant period after the medical emergency caused by the outbreak of Covid-19 in Spain was dedicated to developing simulation and theoretical models to model light propagation in anisotropic, magneto-optically active photonic crystals as this work could be carried out via remote connection. Here, we have attained preliminary results that point towards extraordinary magneto-optical effects in relatively simple photonic crystals built out of magneto-optically active, low loss materials (figure 2).
Our results include thorough documentation of magneto-optical effects in diffractive gratings as well as a video presentation of the experimental setup used to obtain the aforementioned results. The experimental work related to effect of Pockels effect in plasmonic devices that was cut short due to the state of emergency is set to continue in collaboration with the Marie Curie fellow, the hosting group in ICMAB and Dr. Rafael Cichelero in Chalmers University (Sweden). Additionally, we have obtained preliminary results that point towards novel magneto-optical effects in photonic crystals that arise from confined resonances in dielectric nanostructures.
Figure 2:Light propagation through a photonic crystal made from a magneto-optically active material.
Figure 1: Far field optical emission characteristics of a magnetoplasmonic grating