Seoul Physicists Build a Photonic Chip That Speeds Up or Slows Down Light on Command

Physicists in South Korea have built an optical chip that can dial the speed of a light pulse up or down after the device has been manufactured, a level of control that previous photonic circuits could not offer.

The work comes from teams at Seoul National University and the University of Seoul, led by professors Namkyoo Park and Sunkyu Yu at SNU and professor Xianji Piao at the University of Seoul. Their results were published in the journal Advanced Science under DOI 10.1002/advs.76378.

Light in a chip does not simply travel at a fixed pace. When a light wave is routed through coupled ring resonators, interference between the paths can dramatically retard the pulse — a phenomenon called coupled-resonator-induced transparency, or CRIT. The catch has always been that the amount of delay is baked into the physical layout of the rings at fabrication time. Building a chip that delays a signal by a different amount meant building a different chip.

The Seoul design breaks that constraint. The researchers added two controllable loop couplers and, critically, reformulated the physics so that the circuit's "bright" and "dark" optical modes are treated as a single tunable parameter rather than two separate ones. That mathematical move turns a fixed structure into a programmable one: a single device can now be reconfigured on the fly for different signal delays, different bandwidths, frequency conversion and dynamic adjustment of pulse speed.

The platform is silicon nitride, a material already standard in commercial photonic foundries, and the team reports that performance holds up under realistic conditions including material losses and thermal crosstalk between neighboring components — the two failure modes that most often kill elegant photonic designs when they leave the simulation and hit the wafer.

"Our study proposes a new design principle that allows the flow of light within photonic integrated circuits to be reconfigured as needed, greatly enhancing design flexibility," Park said.

The practical target is data movement. AI training clusters and hyperscale data centers now burn enormous amounts of power shuttling data between chips, and optical interconnects are the leading candidate to replace copper. Programmable delay lines matter there because signals arriving from different parts of a system must be synchronized, and optical buffering — briefly holding a pulse in place — has been one of the hardest problems in the field. The researchers also point to applications in lidar for autonomous vehicles and in quantum technologies, where photon timing is the whole ballgame.