Removing copper impurities improves photonic integrated circuits

Soliton microcombs are chip-scale optical frequency combs that have transformed photonics over the past decade, with applications spanning optical communications, LiDAR, microwave generation, quantum technologies, and AI interconnects.

But despite rapid progress in the field, one fundamental challenge has persisted: reliably initiating soliton states. Thermal effects, caused by residual absorption in the material, have made soliton generation unpredictable and difficult to scale.

Now, researchers led by Professor Tobias J. Kippenberg at EPFL have achieved a major milestone in integrated photonics by identifying, for the first time, that copper impurities originating from CMOS-grade silicon substrates are responsible for residual thermal absorption in ultra-low loss silicon nitride (Si₃N₄) photonic integrated circuits. By addressing this issue, the team has demonstrated deterministic and repeatable generation of soliton microcombs in Si₃N₄ photonic chips.

Their study, published in Nature, reveals a surprising culprit behind a long-standing obstacle in the field: copper impurities hidden in the very silicon wafers considered industry-standard for “ultra-pure” electronics.

Copper impurities are transition metals that were identified in optical fibers in the 1960s by Charles Kao, whose discovery earned him the 2009 Nobel Prize in Physics.

The EPFL researchers found that copper impurities are also present in silicon nitride photonic chips. They diffuse into waveguides during high-temperature fabrication, accumulating due to silicon nitride’s strong “gettering” effect. Even at trace levels, copper absorption was enough to destabilize soliton formation.

Using two novel substrate preparation techniques, the scientists demonstrated 100% success rates in soliton generation from impurity-free Si₃N₄ microresonators. Their method is compatible with standard commercial foundry processes, ensuring rapid adoption across academia and industry.

“This result removes one of the last barriers to deterministic microcomb operation,” explains Kippenberg. “It means any lab or foundry can now reproducibly generate solitons, opening the door to real-world deployment.”

The implications are far-reaching. Reliable soliton microcombs would accelerate their integration into next-generation technologies, from ultra-high-bandwidth data links and precision metrology to compact spectrometers and quantum light sources. Beyond microcombs, the impurity removal method also benefits a wide range of nonlinear photonic applications, including supercontinuum generation, parametric amplification, and integrated lasers.

“Just as copper once limited the performance of optical fibers, we’ve found it’s now limiting progress in integrated photonics,” says Kippenberg. “Removing these impurities has unlocked a long-standing challenge in the field.” The work represents a crucial advance for both fundamental science and technological innovation, echoing the historic development of ultra-low-loss optical fibers that enabled today’s global internet.

The silicon nitride samples were fabricated in the EPFL Center of MicroNanoTechnology (CMi).