Researchers reveal how disorder modifies light states in photonic chips, key for new more robust and sustainable devices

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The Madrid Institute of Materials Science (ICMM-CSIC), of the Spanish National Research Council (CSIC), leads an international research that has made it possible to characterize how structural disorder modifies the distribution of light states in photonic crystals. The work, published in Physical Review Research, questions previous interpretations given to these phenomena.

Photonic chips, which use light instead of electrons to transmit and process information, are considered one of the most promising ways to overcome some of the limitations of current electronics, especially those related to operating speed and heat dissipation. "With information we need two mechanisms: to process it and to transmit it. Light is very useful for transmitting data, as already happens with fiber optics, but we are still developing efficient ways to use it also for processing," says Pedro David García, CSIC researcher at ICMM, lead author of the study and leader of the European project NEUROPIC, within whose framework this work is being developed.

García explains that light is very sensitive to imperfections in the photonic structures through which it propagates, as occurs in real photonic chips. These imperfections, or structural disorder, can generate optical states localized near the edge of the photonic band, something like the limit up to which light can propagate. The distribution of these states is related to the so-called photonic density of states, a fundamental magnitude for understanding the optical properties of the system and designing more controlled devices.

Until now, these states had usually been interpreted as 'Lifshitz tails', in reference to the work of physicist Ilya Lifshitz (1917-1982) on disordered electronic systems in the early 1960s. That theory describes "rare" states that appear deep within an energy gap. However, the new work shows that, in the regime they have been able to access experimentally, the data follow a different behavior: that of an Urbach tail, a law originally identified in the 1950s in the study of optical absorption of solids.

To verify their results, the team, with Marcos Menéndez, ICMM-CSIC researcher, as first author of the study, and in collaboration with the University of Delaware (USA), combined optical measurements in photonic crystal waveguides fabricated in gallium arsenide (GaAs) with full electromagnetic simulations. "The quantitative agreement between experiments and simulations allows us to confirm that the observed distribution responds to an Urbach law and that, furthermore, the Urbach energy contains very relevant information about the type of disorder," continues García. "Specifically, the Urbach energy reveals how much disorder there is in the structure and in which direction, which makes it a practical tool for characterizing fabrication."

Results beyond fundamental physics

The implications of this work go beyond fundamental physics. Understanding how disorder affects light transport in photonic media is important for the development of integrated photonic circuits and future optical information processing systems. In such technologies, speed, energy efficiency and precise control of light propagation are key factors. "Light allows information to be transmitted extremely efficiently, much more than conventional electronic systems. The challenge is to learn to control light within these devices so that it not only transmits information, but can also help process it," the researcher points out.

For García, these results also show how science advances by revising accepted interpretations. "This phenomenon has been described for years as a Lifshitz tail; I myself did so in previous works. But when comparing that interpretation with the data, we saw that it did not fit with what we were observing," says the researcher. "When an explanation carries such an important name as Lifshitz, it is easy to assume that it is the correct interpretation. But the data forced us to review whether that theory was being applied to the appropriate regime."

"It was when analyzing the problem in more detail that we saw that the Urbach law, described decades ago in other systems, explained our results much better," adds García. For the researcher, this conceptual correction is a natural part of the scientific process. "Scientists are not infallible. Sometimes we too are carried away by inertia or inherited interpretations. But in science, in the end, the data always rules: if they contradict an accepted explanation, the explanation must be revised."

Thanks to this work, the team has not only been able to more precisely characterize the distribution of disorder-induced optical states, but also to better understand how light behaves in real photonic structures. This knowledge is a relevant step towards the more controlled design of photonic devices and shows that disorder, when understood, does not have to be just a limitation: it can also become a tool for generating new optical functionalities.

Reference: 

M. Menéndez, Lan Hoang Mai, Nazifa Tasnim Arony, Henry Carfagno, Lauren N McCabe, Joshua M. O. Zide, Cefe López, Matthew F. Doty, and P. D. García*. Anisotropy of the Photonic Urbach Tail. Physical Review Research, DOI: https://doi.org/10.1103/757x-877m