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Department of Physics and Astronomy

Research in Nonreciprocal Optics

Most physical systems obey "time-reversal symmetry"--if the direction of time is reversed, the laws retain the same mathematical form. In optics, this means that the propagation of light waves is the same both forwards and backwards. But in fact a optical channel that only lets light pass one-way is a very useful and important device. Essentially a "diode for photons", such components are vital for suppressing noise and optical communication networks and stabilising lasers. We know how to make these devices out of large benchtop optics-they have existed for a century-but creating a microscopic optical diode or "isolator" for integrated optical circuits is a huge challenge.

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.


I've worked on a number of ways of doing this:

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

Magnetic FLDW waveguides

Nonreciprocal materials and photonic crystals

At optical frequencies, most materials have a very weak magnetic response, and we don't notice the effect. However, a few materials, especially a family of dielectrics based on Yttrium Iron Garnet (YIG) have a response large enough to be useful, and optical device firms have been making standalone centimetre-scale “bulk” devices since the 19th century.

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

Recent news

July 28, 2016

New postdoctoral research position in our group on nonlinear optoacoustics and Brillouin scattering in nanoscale waveguides

Are you an expert in nonlinear optics or phononic/photonic devices? We have a new postdoctoral research fellowship available to work on the theory of the fascinating topic of nonlinear light-sound interactions in nanoscale waveguides, especially semiconductor waveguides. Click here for details on the position and to apply. Applications close July 31 2016.  

September 07, 2015

New arxiv manuscript on a quantum treatment of stimulated Brillouin scattering in nanoscale waveguides

Here's a new piece of work that straddles both areas of current interest in the group: quantum nonlinear optics and opto-acoustics. In a new arxiv paper with long time collaborator John Sipe at the University of Toronto, we take another look at the derivation of equations of motion for the opto-acoustic interaction behind stimulated Brillouin scattering in waveguides. We attack the problem with the techniques of guided wave nonlinear quantum optics. By identifying the right Hamiltonian for the opto-acoustic coupling, the coupled mode equations fall out in a very neat fashion. This also opens the way for thinking about Brillouin interactions at the quantum-classical boundary.  

September 5, 2015

New paper on the implications of acoustic dissipation for stimulated Brillouin scattering in waveguides

One of the things that makes the coupling between light and sound in waveguides complicated is the strong dissipation that phonons suffer. Most last only a few nanoseconds. In a new Scientific Reports paper (lead author Christian Wolff at UTS) we look at how the existence of phonon loss means the amplification of light by stimulate Brillouin scattering can be reduced considerably by the phonon lifetime. It's not a deal-breaker, but one needs to be aware of the issue.  

July 24, 2015

New review paper on rigorous derivation of the coupled mode equations for nonlinear optoacoustics

We have a new (very long) paper out in Physical Review A (lead author Christian Wolff at UTS) that provides a rigorous derivation of the equations of motion for stimulated Brillouin scattering (SBS) in nanoscale waveguides. SBS is the nonlinear interaction between light and high frequency phonons (sound waves) and has tremendous promise as a basis for ultracompact devices for microwave photonics, sensing and narrow linewdith chip-based lasers.  

July 23, 2015

New review paper on femtosecond laser written waveguides and quantum optics

Our new review paper on femtosecond laser written waveguides for quantum optics applications (lead authors Thomas Meany and Markus Gräfe) is out in Laser and Photonics Reviews. This paper is a collaboration with the group of Alexander Szameit (Instite of Applied Optics, Jena, Germany) and provides a detailed survey of the use of laser written circuits for many quantum applications, especially those exploting 3D capabilities.  

April 2, 2015

New paper on efficient characterisation of photon sources with loss

Our new paper on useful connections between quantum and classical nonlinear optical processes in waveguides and how scattering loss can affect these connections has appeared in Optics Letters (lead author Luke Helt).  

January 12, 2015

New paper on symmetry properties of nonlinear opto-acoustic interactions

Symmetry groups of acoustic waveguide modes We have a new paper at Optics Express (lead author Christian Wolff at UTS) detailing how symmetry principles can help to accelerate the computation and design problems of nonlinear opto-acoustics (a phenomenon known as Stimulated Brillouin Scattering).  

November 11, 2014

New paper on Faraday rotation in laser written waveguides

Measured Faraday rotation in a laser written waveguide Our new paper (lead author Qiang (Jocelyn) Liu) on Faraday rotation in femtosecond laser written waveguides is out in Optics Express. Jocelyn's work explores the challenges in developing optical isolators in glass waveguides. She also achieved some of the nicest theory-experiment agreement you're likely to see in an optics experiment!  

October 2, 2014

Former student Dr Thomas Meany wins Maquarie's Research Award!

We are very pleased that our former student Dr Thomas Meany (now at Toshiba Labs, UK) has won the Macquarie University Research Award for Excellence in Higher Degree Research. Congratulations Tom. Perhaps this could be you in a few years if you undertake a PhD with us?!  

September 19, 2014

New arXiv manuscript on 3D tunable quantum circuits

Experimental setup for tunable 3D quantum interference Our new manuscript (lead author PhD student Zachary Chaboyer) on demonstrating tunable quantum interference in a 3D 3-arm Mach-Zehnder interferometer is up on the arXiv. We show that our laser written structure should be able to significantly improve phase estimation sensitivity in the quantum regime.  

September 1, 2014

Congratulations to Qiang Liu on being awarded her PhD

Our PhD student Qiang (Jocelyn) Liu has received her PhD for her thesis work on laser-written waveguides in magneto-optical glasses. Jocelyn's work brings us closer to a dedicated optical isolator for the laser written waveguide platform. Well done!  

July 18, 2014

New arXiv manuscript on lossy biphotons

Lasers Our new manuscript (lead author Luke Helt) on the effect of loss on heralded pair generation by spontaneous parametric down-conversion is up on the arXiv. This work investigates how the loss modifies the generated biphoton state, sometimes for good!  

July 16, 2014

New arXiv manuscript on SBS in nanoscale waveguides

Lasers Our new theory of Stimulated Brillouin Scattering (SBS) in nanoscale waveguides (lead author Christian Wollf at UTS) is up on the arXiv. Christian has found some really neat ways of teasing out the different forces involved in modern SBS systems.  

May 2, 2014

PhD student Tom Meany graduates to Toshiba

We are delighted that our former PhD student Thomas Meany has taken up an exciting postdoc position at Toshiba'sCambridge Labs in the UK. Tom will be continuing his work in the area of integrated quantum photonics in new directions involving solid state single photon sources.  

May 1, 2014

New theory of effective photons

Our work on a theory of effective photons in dissipative structured materials (lead author Dr Alex Judge of Sydney University) has just appeared in the New Journal of Physics.  

April 5, 2014

Media coverage of our hybrid single photon source

Lasers The paper on our hybrid single photon source in Laser and Photonics Reviews has attracted a bit of media attention. Amongst others, we've been picked up by Scatterings in Optics and Photonics News and The Register, UK. Here's the original press release.  

April 2, 2014

Commentary in Nature Photonics

Lasers I've written a commentary on recent work at Caltech that showed non-classical plasmon statistics.  

April 2, 2014

Three and four-way multiplexing in a hybrid integrated device.

Lasers In a major collaboration between Macquarie, the University of Sydney and Université Nice Sophia Antipolis (lead authors, we've shown active multiplexing of three and four single photon channels in a hybrid glass/lithium niobate device. We hope this is the future of single photon sources!