Functionalizing waveguides with 2D materials

Concept schematic of the MoS2 crystal (green) grown over a Si waveguide structure (pink) via the CVD process. Reproduced from licensed under the Creative Commons Attribution 4.0 International License.

New paper published this week! Congratulations to the talented lead author Athira Kuppadakkath, a collaboration with Falk Eilenberger’s group at Fraunhofer IOF (Jena).

2D materials can be used to to functionalize optical waveguides and nanostructures, but getting the materials on there normally requires manual transfer. In this paper we showed that crystalline monolayer MoS2 can grow directly on photonic nanostructures with good quality in a scalable process, and what to pay attention to when doing so, and ensure they continue performing as you’d want.

This work emerged from a visit to Falk way back in 2019, and I clearly remember discussing the first results when COVID lockdown started! I also did some of the calculations and some follow-up experiments at the Nanoplasmonics Lab at the University of Sydney.

Terahertz light cages!

New paper from our Sydney Terahertz Lab! Light cages can provide diffractionless propagation in free space, so we 3D printed a few modules with various materials (stiff and flexible resins, ceramics) and tested out what they can do.

We took some near-field images of the straight and bent waveguides with our new fiber-coupled microprobes, and I was stoked to see the conformal map model of bent waveguides come to life. We heated up a ceramic light cage beyond what polymers can withstand, and showed direct-in-core sensing. We also introduced two new Figures of Merit which compare light cages with free space Gaussian beams (our fiercest competitor in the terahertz range).

Thank you Alessio Stefani, Boris Kuhlmey, Mohammad Mirkhalaf, and Hala Zreiqat for the collaboration. This work couldn’t have been possible without our talented students Benjamin Davies and Zizhen Ding, and the amazing Justin Digweed from ANFF NSW.

For those without access, I include the pre-edited version here (please do not redistribute, and cite the ACS Photonics version):


And to top it all off, a cool near field measurement of the bent waveguide can be seen below, compared with COMSOL simulations:

ABC Elevator Pitch: what is light?

Fun video for the ABC, doing my best to explain what light is – but in a single elevator ride.

As a side note, this is a textbook example of an insufficient explanation for the particle nature of light! (But how to do it in just a fraction of an elevator ride!!!???) As my quantum photonics friends keep telling me, I am cheating by assuming I know the answer. For anyone interested in this fascinating topic, I recommend getting your hands on first few chapters of this wonderful book:

The Quantum Challenge: Modern Research on the Foundations of Quantum Mechanics, by George Greenstein and Arthur G. Zajonc (Jones and Bartlett, 2006)

Plasmonic sensing in waveguides: a non-Hermitian perspective

New paper just published in Photonics Research! Should be useful for anyone working on sensing with plasmonic waveguides and fibers. We discuss in detail what to expect from the experimental spectra of plasmonic waveguide sensors in various regimes.

Because this is a non-Hermitian system, things can be a bit confusing. It’s quite tempting to infer everything from just mode dispersions, but sometimes the propagation constants cross, sometimes they anti-cross. So what to make of it, and how does it all affect an actual sensor? That’s what we discuss here. And the exceptional point.

I put up my “rawest” Python Notebook on github, that reproduces a few key plots, in case anyone wants to adapt it to their own sensor design.

I like to call this my “Sydney lockdown” paper – thanks to Boris Kuhlmey and Markus Schmidt for the collaboration.

Bend losses in polyurethane fibers

Happy to announce that the first experimental paper from our revamped Sydney Terahertz Lab has been published in Optics Express.

Our talented Denison Student Jonathan Skelton had the opportunity to study the world-famous polyurethane fibers of our friend and colleague Alessio Stefani during the summer earlier this year, way back when Sydney was not in lockdown mode. To enable this, we built a fast terahertz 2D imaging setup based on our fiber-coupled Menlo Systems TERAK15. This system makes it very easy to study bend losses in waveguides.

We took a close look at bend losses in two hollow core THz fibers, which are really flexible despite being quite thick. It’s not common to be able to bend terahertz waveguides of this size by this much, and it was interesting to weigh the pros and cons of these kinds of structures. What we learned: in terms of bend losses, these simple tubes are not so bad, unless you bend them a lot. Vice versa: structured tubes are better if you the fibers a lot! Otherwise, you couple to tube cladding modes…

Download the paper for more information:

A. Stefani, J. Skelton, A. Tuniz, “Bend losses in flexible polyurethane antiresonant terahertz waveguides”, Optics Express 29 28692-28703 (2021)

Imaging in Hyperbolic Media: Python Code

In response to an out-of-the-blue email asking for help, I recently dug up my Jupyter Notebook code for reproducing the results of a paper written with Boris Kuhlmey in 2015, “Two-dimensional imaging in hyperbolic media–the role of field components and ordinary waves“.

With a bit of extra love, this can be adapted to reproduce every figure! The beauty of this approach compared to brute-force methods such as FEM and FDTD is that it allows quite rapid calculations via analytical formulae. I seem to remember that, for comparison, COMSOL sometimes struggled to converge, and that the huge slabs considered towards the end of the paper would have impossibly large meshes. However, this code is slowed down by the need for arbitrary-precision computations, which stems from large exponentials that appear when considering fine spatial features… Please cite the paper if you use this code for your research!

Code is here:

Recent Review Papers

Just a quick update to highlight a couple of review papers that have come out in the past two months. both Open Access, so take a look!

“Shortcuts to adiabaticity (STA) in waveguide couplers – theory and implementation”, published last month in Advances in Physics: X, is a review and comparison of various STA techniques. We compared various theoretical STA treatments with full numerical simulations, concentrating on realistic two-waveguide systems which are key building blocks for photonic circuits. Adam Taras and Musawer Bajwa worked on this as part of a third year Interdisciplinary Special Project, co-supervised with Martijn de Sterke and Chris Poulton, in collaboration with Judith Dawes and Vincent Ng.

My first single-author paper, a Review article on “Nanoscale nonlinear plasmonics in photonic waveguides and circuits”, was published just yesterday in the historic journal La Rivista del Nuovo Cimento. I wrote this tutorial-style review to introduce readers to theoretical and experimental aspects of nonlinear plasmonics in the context of optical waveguides with an eye on photonic circuits. I hope it will be a useful entry point for anyone interested in a conceptual toolkit to enter this field.


As the year draws to a close, thought I’d provide a small update with some (non-exhaustive) 2020 highlights: a representative selection of events that played a part in this unusual year.

January/February: Work proceeds as normal. Quite the highlight, given what follows.

March: The course I am co-coordinating, Phys3888 (and its Data3888 partner) moves online with the rest of the world – particularly challenging for an experimental multidisciplinary course such as this. Thanks to the amazing work of colleagues Ben Fulcher and Jean Yang, and our life-saving tutors Alison Wong and Zoe Stawyskyj, I think this transition worked out incredibly well.

May: “Modular nonlinear hybrid plasmonic circuit” is published in Nature Communications – I proposed this work as Oliver Bickerton’s Honours Project all the way back in 2018, great to see it picked up by a few news sites.

June: My DECRA begins, and I become director of the Sydney Terahertz Laboratory – currently installing a few upgrades! For a quick virtual lab tour, I made a silly video.

July: I have the pleasure of giving an MQ Photonics seminar on my recent research – thanks Mikolaj and Judith for the invitation!

August: I give two virtual contributed talks and one poster at CLEO Pac Rim 2020 in its online-only format, on the topics of effective PT-symmetries and fiber plasmonics, the nonlinear coefficient of lossy waveguides, and measurements of the nonlinear response of gold. On a related note, “Pulse length dependent near-infrared ultrafast nonlinearity of gold by self-phase modulation” is published in APL – this work used background-free self-phase modulation to measure the nonlinear susceptibility as a function of pulse duration in the sub-picosecond range.

September: “Establishing the nonlinear coefficient for extremely lossy waveguides” is published in Optics Letters. There are a few analytical expressions for the nonlinear coefficient in lossy plasmonic waveguides, and as part of Gordon Li’s honours project (he’s now PhD student at Caltech), we compared them with full simulations and established which one works best.

I also give a (virtual) invited talk at EOSAM 2020, “Crossing the exceptional point in a fiber-plasmonic waveguide.”

October: “Scalable Functionalization of Optical Fibers Using Atomically Thin Semiconductors” is published in Advanced Materials. The product of a German-Australian collaboration led by Falk Eilenberger from the IAP (Jena), this work shows that semiconducting 2D-Materials can be integrated into microstructured optical fibers in a scalable process. This opens the path towards new applications in quantum enhanced sensing and nonlinear optics. They even made a sexy image that was selected for the inside back cover…

I co-organize and host the 2nd SEMCAN Viral Bytes competition (see the many entries on our Twitter page), and take the opportunity to start a YouTube Channel and upload a few example viral videos with the complicity friend and colleague Andrew Grant.

November: “Omnidirectional field enhancements drive giant nonlinearities in epsilon-near-zero waveguides” is published in Optics Letters. This work followed from the second part of Gordon’s honours project. We took a close look at where the largest nonlinearities are expected for epsilon-near-zero waveguides, to guide future experiments. Some more pleasant news followed – Ben Fulcher and I where honoured with the Faculty of Science Award for Outstanding Early Career Teaching! Sincere thank yous to everyone who played a part.

December: “On-chip Hybrid plasmonics goes modular” was published in OPN’s Optics in 2020 highlights. I also hosted a Sydney Nano Meet the Author online seminar with friends and collaborators Mohammad Valashani and Amandeep Kaur before wrapping up for the year.

And as a final bit of good news for this year, I am happy to have been promoted to Senior Lecturer (Level C).

Ready for 2021.