Topological Light Transport on Chip Achieved

Following the emergence of the concept of topological light, the first images of topological light moving around a microchip came out. Now comes the full measurements of the transmission of light around and through a chip.

The topological transport of light is the photonic equivalent of topological electron flow in certain semiconductors.

In topological electron flows case, the current flows around the edge of the material but not through the bulk. It is Topological in the sense that even if electrons encounter impurities in the material, the electrons will continue to flow without losing energy.

In the photonic version, light does not flow through and around a conventional material. Rather it flows in a meta-material made of an array of tiny glass loops fabricated on a silicon substrate.

If the loops are designed just right, the topological feature appears. Light is sent into the array, and easily circulates around the edge with very little energy loss, even if some of the loops aren’t working, while light taking an interior route suffers loss.

Potential Applications of Topological Light

Researchers at the Joint Quantum Institute, led by Mohammad Hafezi, have published a series of papers concerning topological light. The first paper outlined the potential application of robustness in delay lines and introduces a scheme to implement quantum Hall models in arrays of photonic loops.

In the field of [easyazon_link asin=”0195179463″ locale=”US” new_window=”default” nofollow=”default” tag=”sharonfilms-20″ add_to_cart=”default” cloaking=”default” localization=”default” popups=”default”]photonics[/easyazon_link], signals at times need to be delayed, typically by sending light into a miles long loop of optical fiber.

In an on-chip scheme, these delays could be done on the micro-scale. In addition, the energy-loss reduction made possible by topological robustness would be beneficial.

The second paper reported results from an experiment. Since the tiny loops aren’t perfect, they do let a bit of light escape vertically out of the plane of the array.

This faint light allowed the JQI experimenters to image the course of light. It verified the plan that light persists when it goes around the edge of the array but suffers energy loss when traveling through the bulk.
The third paper, recently published, shows detailed measurements of the transmission and delay for edge-state light and for bulk-route light.

“Apart from the potential photonic-chip applications of this scheme,” said Hafezi, “this photonic platform could allow us to investigate fundamental quantum transport properties.”

Microchip Consistency

resonator arrayAnother measured quality is consistency. Sunil Mittal, first author on the paper, explains that microchip manufacturing is not a perfect process.

“Irregularities in integrated photonic device fabrication usually result in device-to-device performance variations,” Sunil said.

This frequently undercuts the microchip performance. However, with topological protection, meaning photons traveling at the edge of the array are practically invulnerable to impurities, consistency is greatly raised.

For light taking the arrays interior route, the delay and transmission of light can vary greatly. But for light taking the edge route, the amount of energy loss is regularly less and the time delay for signals more consistent.

This is good, since robustness and consistency are vital if you want to integrate such arrays into photonic schemes for processing quantum information.

How Topological Properties Emerge

[easyazon_image add_to_cart=”default” align=”left” asin=”8120320468″ cloaking=”default” height=”160″ localization=”default” locale=”US” nofollow=”default” new_window=”default” src=”” tag=”sharonfilms-20″ width=”121″]How does the topological property surface at the microscopic level? Firstly, we must look at electron topological behavior, an offshoot of the quantum Hall effect.

Electrons, under the influence of an applied magnetic field can execute tiny cyclonic orbits. In some materials, called topological insulators, no external magnetic field is needed since the necessary field is supplied by spin-orbit interactions, in other words, the coupling between the orbital motion of electrons and their spins.

In these materials the conduction regime is topological; the material is conductive around the edge but is an insulator in the interior.

And the photonic equivalent? Light waves do not typically feel magnetic fields, and if they do it is very weak.

In the photonic case, the equivalent of a magnetic field is supplied by a subtle phase shift imposed on the light as it circulates around the loops.

Actually the loops in the array are of two kinds: resonator loops designed to exactly accommodate light at a certain frequency, allowing the waves to circle the loop many times. Link loops, by contrast, are not exactly suited to the waves, and are designed chiefly to pass the light onto the neighboring resonator loop.

Light that circulates around one unit cell of the loop array will go through a minor phase change, an amount signified by the letter phi.

That is, the light signal, in coming around the unit cell, re-arrives where it started advanced or retarded just a bit from its original condition. Just this amount of change imparts the topological robustness to the global transmission of the light in the array.

Documented on-chip light delay and a robust, consistent, low-loss transport of light has now been demonstrated. The transport of light is tunable to a range of frequencies and the chip can be manufactured using standard micro-fabrications techniques.


S. Mittal, J. Fan, S. Faez, A. Migdall, J. M. Taylor, and M. Hafezi.
Topologically Robust Transport of Photons in a Synthetic Gauge Field.
Physical Review Letters, August 2014

Images courtesy: E. Edwards/JQI(top) & Sean Kelley/JQI (bottom)