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Research in Nonreciprocal Optics

To create isolators, we need to break time-reversal symmetry and there are very few ways of doing this. It turns out that certain magnetic materials can break time reversal symmetry, but the effect is very weak. In our work we study the use of photonic crystals and other resonant structures to enhance the magnetic effects and make isolators and other "one-way" devices possible. The combination of fundamental physics in breaking time-reversal symmetry and applied optics in designing one-way structures makes this a fascinating topic to study. This fact is not very surprising, but it is often a serious complication for optical scientists and engineers. The problem is that a light ray carrying a signal can be reflected off junctions or random imperfection in the waveguide and scatter into a backwards ray. If the ray is rescattered in the forwards direction it produces a delayed echo that degrades the signal. Back-scattered rays can also couple back into laser sources and induce instabilities.

To get around this problem, we use devices called “optical isolators”, which allow light to only pass one-way. These components, essentially “diodes for photons”, are vital for suppressing noise and optical communication networks and stabilising lasers. The one-way behaviour is termed nonreciprocal propagation. But creating nonreciprocal action requires breaking time-reversal symmetry, so the question is how to do that?

In fact there are very few ways of doing this. One approach is to change the properties of the waveguide material explicitly in time using nonlinear optics. But this requires high powers or multiple lasers and is not straightforward.

The other approach is to use a unique property of magnetic materials: it turns out that any magnetised material breaks time-reversal symmetry, at least weakly. The best-known example of this is Faraday rotation: the plane of polarisation of a light beam is rotated as it passes through the magnetic material, and the rotation has an absolute sense in space. In other words, light travelling forwards rotates with a right-hand rule say, while light travelling backwards rotates with the left-hand rule. (This is different from optical activity, which is the rotation of light passing through a chiral material like sugary water. With optical activity, the sense of rotation depends only on the material, not on the direction of travel.) This extraordinary property is the key to nonreciprocal optics, and combined with a pair of polarisers, it is sufficient to build a Faraday isolator. (Wikipedia has a simple discussion.)
(For the curious, the nonreciprocal behaviour ultimately derives from the fact that a magnetic field B is an axial vector— when a B vector is reflected in a mirror, it still points in the same direction not the reverse.) The combination of fundamental physics in breaking time-reversal symmetry and applied optics in designing one-way structures makes this a fascinating topic to study.

Activities

• FLDW waveguides
• Enhanced magneto optics
• Isolators without magnetic materials

Magnetic FLDW waveguides

Nonreciprocal materials and photonic crystals

Today however, photonics research is driven by the goal of compact integrated optical circuits. We believe that that such circuits will form the underlying technology for the next wave of communications, biophysics, sensing and basic quantum science, just as integrated circuits enabled the computer revolution. These circuits will contain lasers, filters and switches just a few microns across, and they will need to contain nonreciprocal devices such as isolators as well. But as the magnetic response of YIG is weak, the standard designs for traditional benchtop isolators can't be scaled down to micron dimensions. Instead we need to find new strategies for enhancing the nonreciprocal behaviour so that we can achieve the necessary reduction in size.

At Macquarie, we are tackling these issues on several fronts. We are using photonic crystals to exploit bandgap cavity and slow light effects to enhance the magnetic behaviour, and exploring new materials that may provide a useful magnetic response.

Isolators without magnetic materials