University of Sydney Photonics Researchers Discuss Recent Developments for Future Integrated Hybrid Plasmonic Applications

Throughout the years, photonics, the science and application of light (photons), has enabled the fabrication of a variety of electronic components, ranging from light-emitting diodes to lasers, imaging tools, displays and modern intercontinental communication. Recently, researchers worldwide have also started exploring the potential of photonic integrated circuits (PICs), which could be a viable alternative to the conventional integrated circuits (ICs) used to fabricate most existing electronic devices.

As silicon devices based on classical ICs are rapidly approaching their limits, both in terms of speed and performance, PICs could ultimately bring about a new wave of innovation in the field of electronics, quantum information processing and sensing—paving the way towards the creation of more advanced technological tools.

Researchers at the University of Sydney’s Nanophotonics and Plasmonics Advancement Lab (NPAL) have been developing nanophotonic and plasmonic technology for several years now. Their research, featured in many renowned scientific journals, could contribute to the advancement and implementation of PICs, as well as other photonics-based components for current electronic-based devices, complementing electronics—if not substituting it.

Earlier this year, a team of scientists at NPAL and the University of Sydney designed an integrated circuit that is based on a hybrid plasmonic waveguide, an optical physical structure that allows researchers to confine light at the nanoscale, therefore mitigating the high loss that pure plasmonic (light-metal interaction) devices exhibit. This work, published in Nature Communications, could help to improve such devices’ efficiency to bring light at the nanoscale via industry-standard photonic waveguides, improving the performance by reducing the size of existing and emerging devices.

The research team (from left to centre): Associate Professor Stefano Palomba and Dr Alessandro Tuniz. Beside them (right) is the director of the Institute of Photonics and Optical Science, Professor Martijn de Sterke. Image Credit: Louise Cooper, the University of Sydney.

To gain a better understanding of how this PIC works and how it could aid the fabrication of electronic devices, the following Q&A covers Electronics Point’s interview with the two researchers who conducted the study: Dr Alessandro Tuniz and Associate Professor Stefano Palomba.

Tuniz completed his PhD studies in Physics at the University of Sydney and subsequently became an Australian Research Council Early Career Fellow. Palomba, director of the NPAL, is an experimental physicist specialised in nanophotonic and plasmonic technologies. Throughout their academic and professional journeys, both Tuniz and Palomba have carried out extensive research related to linear, nonlinear photonics, and plasmonics, nano-optics, and hybrid photonic devices.

Their said Nature Communications paper was written in collaboration with other researchers at the University of Sydney and other major research groups (based in Germany), such as the Abbe Center of Photonics.

Ingrid Fadelli (IF), Electronics Point: Could you briefly introduce yourself and share a little bit about your professional and academic background?

Dr Alessandro Tuniz (AT): I was born in Italy but have always had strong ties to Australia. I completed my PhD at the University of Sydney in 2013, specialising in metamaterial fibres. In 2014 I moved to Germany for an Alexander von Humboldt Fellowship that focused on the topic of fibre plasmonics. I returned to Sydney in 2017 to work on chip-scale highly nonlinear plasmonic waveguides. I recently started an Australian Research Council (ARC) Discovery Early Career Researcher Fellowship at the Sydney Nano Institute and the Institute of Photonics and Optical Science (University of Sydney).

Associate Professor Stefano Palomba (SP): I was born in Italy and obtained an Optoelectronic Engineering degree at the Politecnico in Milan. I earned a PhD degree in Nanoscale Physics from the Nanoscale Physics Research Laboratory at the University of Birmingham, UK in 2007. After working at the Institute of Optics, the University of Rochester in New York; the University of California, Berkeley; and at KLA [Keep Looking Ahead] Corporation in Milpitas, California, I joined the University of Sydney in 2013. Here, I created and now run the Nanophotonic and Plasmonic Advancement Lab (NPAL), which is designed to conduct research in linear, nonlinear and quantum nanophotonics, plasmonics, bionanophotonics [i.e. a field focused on the interactions between light and biological molecules, cells, and organisms at the nanoscale], sensing, and neurophotonics.

IF: Could you tell us how your recent work in Nature Communications came about? What previous research was it based on, and what were its main objectives?

SP: Since I joined the University, my interest has always been to enhance the nonlinearities of integrated devices, such as waveguides. In Berkeley, I started working on a new ‘hybrid plasmonic’ platform, which combines the low loss of dielectric materials—like silicon—with the outstanding capabilities of metals, such as gold, to compress light at the nanoscale and enhance light-matter interactions.

We published some theoretical and experimental work on this before Alessandro joined us. Both of us were—and continue to be—interested in enhancing the nonlinear response of conventional silicon waveguides with the compact and efficient hybrid plasmonics architecture. One key issue we had and wanted to solve was how to efficiently couple light into such small waveguide modes by using a potentially integrated and scalable platform.

In the future, we envision to utilise the Silicon Photonics technology combined with our hybrid plasmonic modules to increase the conversion efficiency in a more compact platform that is seamlessly integrated into a photonic chip. This work was an important step in that direction.

IF: Could you explain the general workings and unique characteristics of the modular approach to attain nonlinear hybrid plasmonic circuits, which you presented in your paper?

AT: Previous work had shown a hybrid plasmonic design that exhibits compact polarisation rotation using a metal film, which completely covered a dielectric waveguide. We also wanted to rotate the mode from the standard TE [transverse electric] in the silicon waveguide to the TM [transverse magnetic] required for a hybrid plasmonic waveguide and then utilise it for some other function—so we partially coated the silicon waveguide to accomplish this goal.

We called this section of the device, which rotates the mode from TE to TM, the ‘rotator’. After the light travels through the rotator module, the waveguide mode is not only polarised TM but it is also called hybrid plasmonic mode, because it is a ‘hybrid’ between the original photonic mode (low loss but diffracted limited) and a pure plasmonic mode (lossy but tightly confined at the interface between a metal and a dielectric). Indeed, the mode is very well confined into the space between the metal and the silicon waveguide. And while this mode has lower losses than a pure plasmonic mode, it is more confined than a pure photonic mode. Therefore, once the rotator operates the rotation, the hybrid plasmonic mode is efficiently excited and ready to be utilised in various ways, such as enhancing nonlinearity or nanofocusing.

There has been some work in the literature on nanofocusing using plasmonic structures. However, these demonstrations implement a very specific design. We thought that, to help make this technology more practical, we would need to start from an existing dielectric-based waveguide—for example, one that is silicon-based—before enhancing the light-matter interactions with several plasmonic modules back-to-back, so as to reach the end goal.

IF: What do you feel are the most meaningful findings of your recent work, and what are their practical implications?

AT: The most meaningful result that we have observed shows that we can take a conventional waveguide (in this case a silicon waveguide that guides a TE mode) and transform it, by a standard lithography process, into a device with completely new, and somewhat more complex, functions—such as polarisation rotation and nanofocusing. And in turn, use this to enhance the nonlinearity at the nanofocus, after just a few micrometres of propagation.

This could be used for nonlinear nanophotonics, sensing, nano-interconnects, nanoscale terahertz sources and detectors, as well as, potentially, future quantum information processing.

IF: How do you feel that light could be used for different electronic applications? For instance, how could it impact the efficiency and performance of transistors, circuits, and computer processing at large?

SP: One of the advantages of plasmonic devices is that they are made of metal, hence they are the perfect bridge to interface with electronics. Photonics is becoming increasingly dominant for information processing architectures: starting from optical fibres, we are moving optical interconnects between servers, to motherboards, and so on. 

This trend could continue if photonics were to be brought into a given CPU, as well as between that CPU’s cores—thus one day potentially substituting electronic transistors with an equivalent optical or a hybrid photonic-electronic processor. Having said that, it is most likely that electronics and photonics will complement each other in the future, because, in their very essence, light-matter interactions are light-electron interactions. Furthermore, in the very new field of quantum information processing, photonics is one of the promising platforms.

IF: Are there any challenges to the use of light in the development of electronic components? If so, what are the major obstacles and how could they be overcome?

SP: Although photons and electrons are two fundamentally different particles, making them interact is both possible and achieved routinely. We have many technologies where this currently happens, such as detectors and electrically pumped light sources. However, there is still a great need to make such devices smaller, more energy-efficient, and able to have a larger bandwidth.

Coupling light to the ‘free’ electrons at the interface between a metal and a dielectric (i.e. plasmonics) allows light to overcome its intrinsic diffraction limit, to be compressed down to nanoscale volumes and enhance light-matter interactions—such as we achieved in our recent work. Similar platforms could thus form the basis for the next generation of highly compact, efficient, and low-power consumption optoelectronic devices.

IF: On the other hand, what are the key benefits of using light, and what was the key motivation that inspired you to start experimenting with its use in electronics?

AT: Light has many advantages over electronic systems: it is the fastest particle known, it is not susceptible to electromagnetic interference, it doesn’t dissipate energy, it doesn’t need a physical medium to propagate, and it doesn’t interact with itself. However, some of these are also disadvantages. For instance, to have any sort of optical processing device, photons would need to interact with each other, which happens only when mediated by a medium and at high intensities—i.e. nonlinear optics.

We believe hybrid systems that combine photons and electrons are the way forward, which is what inspired us to go down this research path.

IF: Finally, what are your plans for future research?

AT: We are fundamentally nanophotonic scientists, so we will continue working with light at the nanoscale. My current work aims to use this kind of technology for developing on-chip terahertz applications.

SP: I have various projects that span from nonlinear optics, to nanolasing, quantum nanophotonics, and sensing. However, currently I am mainly focusing on using plasmonic and hybrid plasmonic systems for enhancing nonlinearities on-chip with potential applications that span from quantum information processing to far-UVC light generation.

I am also working on a plasmonic-based universal molecular sensor, which could detect any molecule, like biomarkers, pollutants, etc. in any environment—liquid or air—at very low concentrations, and potentially any airborne pathogen, like the current SARS-CoV-2. This is all thanks to integrated plasmonics, photonics, and nanofluidics combined in synergy. The other main project I am pursuing is the development of a universal neurophotonic interface, which would in the future, allow humans to interface directly and bidirectionally with any bionic device.

Dr Alessandro Tuniz and Associate Professor Stefano Palomba are continuing their development of plasmonic and nanophotonic circuits that could revolutionise the structure of future devices—boosting their performance and efficiency further still.

You can read more about their work and keep updated on their research at atuniz.com/about and sydney.edu.au/science/about/our-people/academic-staff/stefano-palomba.

In the future, the studies carried out by these two researchers and the NPAL Laboratory at the University of Sydney could inform the fabrication and manufacturing of new types of devices, which is both inspired by and utilises the unique properties of light.