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.
