With the fast development and rapid adoption of all-optical data processing, integrated optical switches are expected to have an important impact on enhancing the speed, reducing power consumption, and minimizing the size of telecommunication systems and optical computing devices. Integrated optical switches usually rely on mechanisms like the thermo-optic effect or carrier dispersion effect to alter the refractive index of materials, allowing for switching between two discrete states. Mach-Zehnder interferometer (MZI), microring resonator (MRR), and the hybrid MRR and MZI are the commonly employed structures. Over the last decades, considerable efforts have been made to improve the performance of integrated optical switches in terms of speed, power efficiency, bandwidth, compactness, and so on.
In the field of optical physics, it is known that spontaneous symmetry breaking (SSB) caused by the nonlinear Kerr effect can exhibit two different states as optical switches. In a single optical microresonator, SSB can be observed from the Kerr-interaction between two counter-propagating light waves or polarization states. In the context of bi-directionally pumped microresonators, SSB is caused by an instability of the intracavity power. Beyond the threshold pump power, a small deviation between the counter-propagating light powers results in a large resonance frequency split of the initially degenerate modes in clockwise (CW) and counterclockwise (CCW) directions. This leads to a strong optical nonreciprocity in a symmetrical broken resonator. However, until now, SSB induced switches have not been reported on chips.
Researchers from The Chinese University of Hong Kong (Shenzhen) & the Technical University of Denmark & the Max Planck Institute for the Science of Light proposed to use SSB in high-quality silicon nitride (Si₃N₄) resonators to realize a new type of integrated optical switch. Through bidirectional pumping, the authors observed the SSB phenomenon at an optical power of 3.9 mW. When the nonlinear threshold power is exceeded, the system can be observed to switch between CW or CCW states. The intrinsic quality (Q) factor of the Si₃N₄ resonator is 9.4 × 10⁶. Finally, the hysteresis loop under different pump powers and frequency detuning was experimentally studied, indicating that it is suitable for optical switches. Relevant research results were recently published in Photonics Research, Volume 13, Issue 2, 2025. [Yaojing Zhang, Shuangyou Zhang, Alekhya Ghosh, Arghadeep Pal, George N. Ghalanos, Toby Bi, Haochen Yan, Hao Zhang, Yongyong Zhuang, Lewis Hill, Pascal Del'Haye, "Integrated optical switches based on Kerr symmetry breaking in microresonators," Photonics Res. 13, 360 (2025)]
Fig. 1. Symmetry breaking of counter-propagating light in a high-Q Si3N4 resonator. (a) Schematic of the Kerr-nonlinearity-induced interaction of light in the Si3N4 resonator. Two counter-propagating light waves with the same frequency and power are injected from the left and right sides of the bus waveguide, which can support both TE00 and TM00 modes. (b) For bi-directional pumping, at low pump power, the two counter-propagating waves generate standing waves and the resonant transmission spectra of the two light waves are identical (left panel). At high power, the resonator either enters a state with clockwise or counterclockwise circulating light. In this symmetry-breaking state, the optical modes in different directions have different resonance frequencies (middle panels). Increasing the optical power in the non-dominant direction enables switching between the counter-propagating light states. This switching behavior follows a hysteresis (right panel). (c) Microscope image of the Si3N4 resonator.
Symmetry breaking in a bidirectionally pumped Kerr resonator arises from the difference in the Kerr resonance frequency shift caused by the self-phase modulation of the co-propagating photons and the cross-phase modulation of the counter-propagating photons. If the two counter-propagating light waves have the same frequency but different intracavity powers in the resonator, they will see different frequency detuning relative to the resonant frequency. In theory, for the resonator structure shown in Fig. 1(a), two light fields are injected from the left and right sides of the bus waveguide with the same frequency and power, namely PL and PR. The two light fields propagate in the resonator in the clockwise (CW) and counterclockwise (CCW) directions, respectively, and can interact with each other. As shown in Fig. 1(b), when the power of the two intracavity light fields is below the nonlinear threshold, the mode frequencies in the two directions are in a degenerate state. However, if the input power is increased, the different nonlinear refractive index changes in the two directions will lead to different mode frequencies in the two directions. Practically, there are inevitably small fluctuations in the laser power and laser frequency, which will introduce small differences in power and frequency between the two counter-propagating light waves. If the resonator has a high Q factor, this power difference will be significantly amplified in the resonator through Kerr symmetry breaking. The higher-power resonant mode will experience a smaller frequency shift and be closer to the pump laser frequency, while the weaker-power resonant mode will experience a larger frequency shift due to cross-phase modulation and be pushed away from the pump laser frequency. This resonant frequency difference increases with increasing pump power. When the power of the pump laser fluctuates randomly, the two light waves will randomly select one of the two states, thereby switching between the two light states and following the hysteresis loop shown on the right side of Fig. 1(b), realizing the optical switch function. In addition, based on the achieved symmetry breaking effect, the resonator in Fig. 1 can also be used for optical isolators. In future research, the optical switch achieved in this work can be heterogeneously integrated with semiconductor lasers for optical computing. The on-chip spontaneous symmetry breaking achieved in this work can also be applied to applications such as optical gyroscopes and random number generation.
