Backscattering protection in integrated photonics is not possible with existing technologies


April 14, 2023

(Nanowerk News) The field of integrated photonics has taken off in recent years. These microchips utilize light particles (photons) in their circuits as opposed to the electronic circuits that, in many ways, form the backbone of our modern age. Offering increased performance, reliability, energy efficiency and new functionalities, integrated photonics has enormous potential and is rapidly becoming part of the infrastructure in data centers and telecommunications systems while becoming a promising competitor for sensors and integrated quantum technologies.

Significant improvements in nanofabrication have made it possible to build photonic circuits with minimal defects, but defects can never be completely avoided, and losses due to interference remain a limiting factor in today’s technology. Minimizing these losses can, for example, reduce energy consumption in communication systems and further increase the sensitivity of sensor technology. And because quantum photonic technology relies on encoding information in fragile quantum states, minimizing loss is critical to scaling quantum photonics to real applications. So the search for new ways to reduce backscattering or even prevent it altogether is under way. The figure shows a scanning electron microscope image of a photonic waveguide The image shows a scanning electron microscope image of one of the photonic waveguides studied by the DTU researchers. The waveguide is formed at the edge between two photonic topological isolators (blue and red) realized in nanoscale silicon membranes. It has been suggested that light propagation in such a topological waveguide is immune to backscattering at structural defects, but this has never been investigated experimentally. For the first time, the DTU team examined this in experiments and found the opposite: strong backscattering. (Image: CA Rosiek)

A one-way street for photons is not possible at the moment

One suggestion for minimizing photon loss in integrated photonic systems is to guide light through the circuit using a topological interface that prevents backscattering by design.

“It would be great if it were possible to reduce losses in this system. But basically, creating a one-way path for photons is a difficult thing to do. In fact, for now, it’s impossible; to do this in the optical domain requires the development of new materials that are none at this time,” said Associate Professor Søren Stobbe, Group Leader at DTU Electro.

Circuits built from topological isolators would, in theory, force photons to continuously move forward, never backward. The fallback channel will not exist. While such effects are well known in the electronics niche and have been demonstrated with microwaves, they have not yet been demonstrated in the optical domain.

But full topological protection is not possible in silicon and all other low loss photonic materials because they are subject to time-reversal symmetry. This means that whenever the waveguide allows light transmission in one direction, a reverse path is also possible. This means that there is no one-way path for photons in conventional materials, but the researchers hypothesize that a two-way path is good enough to prevent backscattering.

“There is a lot of work trying to realize topological waveguides in relevant platforms with integrated photonics. One of the most exciting platforms is silicon photonics, which uses the same materials and technologies that make up today’s ubiquitous computer chips to build photonic systems, and even if interference cannot be completely eliminated, perhaps backscattering could occur.” said Søren Stobbe.

New experimental results from DTU were recently published in Nature Photonics (“Observation of strong backscattering in the Hall-valley photonic topology interface mode”) strongly suggest that with the currently available material this is unlikely to happen.

Sophisticated waveguides offer no protection

Although several previous studies have found that it is possible to prevent backscattering based on various indirect observations, strict measurement of loss and backscattering in topological waveguides has so far been missing. The main experiments conducted at DTU were carried out on a very sophisticated type of silicon waveguide, demonstrating that even in the best available waveguides, topological waveguides exhibit no protection against backscattering.

“We made the best waveguides obtainable with current technology—reporting the smallest loss ever seen and achieving small levels of structural disturbance—but we never saw topological protection against backscattering. If two-way topological isolators protect against backscattering, they it will only be effective at levels of disruption below what is currently possible,” said PhD student Christian Anker Rosiek.

He did most of the fabrication, experimentation and data analysis together with postdoc Guillermo Arregui, both at DTU Electro.

“Measuring the loss alone is very important, but not sufficient, because the loss can also come from radiation from the waveguide. We can see from our experiment that the photons are caught in tiny cavities located randomly in the waveguide as if many tiny mirrors had been placed randomly in the light’s path. Here, light is reflected back and forth, scattering very strongly at the defect. This shows that the power of backscattering is high, even in sophisticated systems, proving that backscattering is a limiting factor,” said Guillermo Arregui.

The waveguide material must break the time-reversal symmetry

This study concluded that, for waveguides to offer protection against backscattering, you need a topological isolator made of a material that breaks the time-reversal symmetry without absorbing light. Such material does not exist today.

“We do not rule out that protection from backscattering may work, and absence of evidence should not be confused with evidence of absence. There is a lot of interesting research to explore in topological physics, but moving forward I believe researchers should be very careful about quantifying losses when presenting new topological waveguide.In doing so, we will get a clearer picture of the true potential of this structure.Suppose someone actually develops a new exotic material that only allows propagation in one direction; our research has established the necessary tests to claim real protection against backscattering,” said Christian Anker Rosiek.


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