SNU–University Of Seoul Joint Research Team Develops Programmable Photonic Integrated Circuit That Slows Light On Demand

• Enables storage, delay, and control of light within a single photonic chip
• Expected applications in low-power optical computing for AI servers and next-generation optical communication technologies
• Published in the prestigious international journal Advanced Science

Seoul National University College of Engineering announced that a joint research team led by Professors Namkyoo Park and Sunkyu Yu of the Department of Electrical and Computer Engineering at SNU, in collaboration with Professor Xianji Piao of the School of Electrical and Computer Engineering at the University of Seoul, has developed a photonic integrated circuit that can slow light on demand.

With the rapid advancement of generative AI and large-scale AI models, computational demands have surged, exposing the limitations of conventional electronic semiconductors, including high power consumption and limited data transmission speeds. As a result, demand for optical computing technologies capable of low-power, ultra-fast processing is increasing. However, due to the inherent property of light traveling at a fixed speed, implementing buffer and memory functions—essential for optical computing—has remained fundamentally challenging.

To address this issue, the joint research team devised a method to freely control both the speed and shape of optical signals using a programmable photonic integrated circuit. Through this approach, the team demonstrated the ability to control “slow light” with a higher degree of freedom than any previously proposed method.

The study was published on June 30 in the renowned international journal Advanced Science.

Photonic integrated circuits are emerging as next-generation technologies capable of processing information rapidly and efficiently using light. In particular, in data centers as well as optical communication and computing systems, it is becoming increasingly important not only to transmit optical signals quickly but also to synchronize signal arrival times and delay signals when needed.

To achieve such functionality, structures based on coupled-resonator-induced transparency (CRIT)^*—which utilize interference among multiple optical resonators^*—have been studied. CRIT is an optical phenomenon that selectively transmits light within a specific frequency range while simultaneously slowing down the propagation speed of optical signals.
^* Coupled-resonator-induced transparency (CRIT): An optical phenomenon that selectively transmits and delays light within a specific frequency range through interference among multiple resonators.
^* Optical resonator: A photonic device that confines or circulates light of a specific frequency for a certain period; used in signal delay, filtering, and modulation.

However, conventional CRIT structures typically have fixed operational characteristics once fabricated, making it difficult to reconfigure them for different functions. For example, achieving longer delays or shifting to different frequency ranges requires designing entirely new photonic devices.

This lack of flexibility has increased the complexity of optical communication equipment and data center systems, leading to higher costs and longer development times when introducing new functionalities. In environments such as AI servers and next-generation data centers, where massive data must be processed in real time, this limitation has been a major obstacle to the advancement of optical computing technologies.

To overcome these limitations, the research team proposed a new approach that treats two optical states within CRIT systems—the bright mode and dark mode—as a single unified degree of freedom, and introduced two controllable loop couplers. This enabled the development of a new design principle for programmable photonic integrated circuits, allowing resonator structures that were previously fixed after fabrication to be reconfigured as needed.

The team demonstrated that the flow of light can be delayed and controlled as needed using the newly proposed CRIT structure, and showed that interference between bright and dark modes can be treated as a single integrated design parameter. This significantly increased the design flexibility of photonic resonator circuits, which had previously been constrained by fixed configurations.

In particular, by employing two loop couplers, the researchers theoretically proved that it is possible to control the bandwidth and shape of the passband, as well as the delay and transmission characteristics of signals propagating through the circuit. This implies that the propagation speed and transmission properties of optical signals can be freely reconfigured not only within a single resonator but across entire multi-resonator systems.

Furthermore, the team demonstrated through numerical simulations that the propagation speed of optical pulses^* can be dynamically adjusted in real time while the circuit is in operation. As a result, they confirmed that the delay time of optical signals can be freely controlled while maintaining signal processing performance, and that frequency conversion of light can be achieved without the need for additional specialized components.
^* Optical pulse: A short burst of light used as a basic unit for transmitting information in optical communication and computing systems.

The researchers also verified, through three-dimensional electromagnetic simulations, that the proposed CRIT device can be implemented on a silicon nitride (Si_₃N_₄) photonic integrated circuit platform^*. In addition, they analyzed various practical factors that may arise during fabrication and operation—including material losses, resonator quality variations, backscattering, coupling fluctuations, phase errors in loop couplers, and thermal crosstalk*—and confirmed that the proposed structure can operate reliably in realistic photonic circuit environments.^* Silicon nitride (Si_₃N_₄) photonic integrated circuit: A low-loss and highly stable waveguide platform widely used for optical signal processing and integrated photonic devices.
^* Thermal crosstalk: A phenomenon in which heat generated in one part of a circuit affects neighboring components, potentially altering device performance.

This study presents a new programmable photonic integrated circuit platform that enables real-time control of the temporal and spectral properties of optical signals, overcoming the limitations of conventional fixed optical delay structures. It demonstrates the possibility of integrating key functionalities required for next-generation optical interconnects—such as signal synchronization, variable delay lines, optical buffers, and frequency conversion—within a single photonic circuit architecture.

Moreover, the proposed design methodology can be extended beyond CRIT systems to a wide range of resonator-based photonic circuits, suggesting that it could serve as a foundational technology for next-generation optical signal processing that enables flexible design and control of light propagation.

If commercialized, the programmable photonic integrated circuits developed in this study are expected to allow a single optical chip to perform multiple functions—such as controlling signal speed and switching functionalities—similar to software-defined systems. This could significantly reduce power consumption in data centers and AI servers while improving data processing efficiency.

In addition, the integration of diverse signal processing functions into a single chip could contribute to the miniaturization and cost reduction of optical communication equipment and sensor systems. In the long term, the technology is expected to serve as a key enabling platform for industries requiring ultra-fast information processing, including autonomous driving, next-generation communications, and quantum technologies.

Professor Namkyoo Park, co-corresponding author of the study from Seoul National University, stated, “This research is significant in that it proposes a new design principle that allows the flow of light within photonic integrated circuits to be reconfigured as needed, greatly enhancing design flexibility. We plan to expand this technology toward large-scale programmable photonic integrated circuits based on silicon photonics and photonic AI technologies.”

Co-first authors Dr. Seungkyun Park and Ph.D. student Beomjoon Chae, who led the theoretical framework and numerical analysis, added, “Through this study, we realized that reinterpreting conventional photonic resonator physics from a different perspective can serve as a starting point for discovering new functionalities in photonic integrated circuits. We plan to further develop this research toward practical device implementation and experimental validation.”

Dr. Seungkyun Park is affiliated with the InnoCORE PICORE Center at KAIST and is currently conducting research on photonic AI and quantum optics at the Photonic Systems Laboratory, Seoul National University. Ph.D. student Beomjoon Chae is conducting research on programmable photonic integrated circuits at the Intelligent Wave Systems Laboratory, SNU.

Meanwhile, this research was supported by the Ministry of Science and ICT through the Innovative Research Center (IRC) program, the Basic Research Laboratory (BRL) program, and the Young Researcher Program. Dr. Seungkyun Park also participated in the study with support from the InnoCORE program (PICORE Center).