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Photonics North (PN)

Optical Bistability in Chip-Scaled Ring Resonators

Giuseppe Brunetti, Caterina Ciminelli

Published May 20, 2025• 0 citations
Publisher

Abstract

Ring resonators are critical in optical systems for their compact size and high selectivity. By leveraging optical bistability-arising from nonlinear effects like the Kerr effect-they enable advanced functionalities, whose performance is strictly correlated to ring resonator ones. Here, a silicon nitride ring resonator, with a groundbreaking Q-factor > 150 million and extinction ratio> 5 dB by gaining from low optical losses and Kerr effect, is presented, unlocking new potentials for data storage, sensing and signal processing technologies.

DOI
10.1109/pn66844.2025.11097149

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Ring resonators are critical in optical systems for their compact size and high selectivity. By leveraging optical bistability-arising from nonlinear effects like the Kerr effect-they enable advanced functionalities, whose performance is strictly correlated to ring resonator ones. Here, a silicon nitride ring resonator, with a groundbreaking Q-factor > 150 million and extinction ratio> 5 dB by gaining from low optical losses and Kerr effect, is presented, unlocking new potentials for data storage, sensing and signal processing technologies.
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We demonstrate, for the first time to our knowledge, optical bistability on a highly integrated silicon device, using a 5-microm-radius ring resonator. The strong light-confinement nature of the resonator induces nonlinear optical response with low pump power. We show that the optical bistability allows all-optical functionalities, such as switching and memory with microsecond time response and a modulation depth of 10 dB, driven by pump power as low as 45 microW. Silicon optical bistability relies on a fast thermal nonlinear optical effect presenting a 500-kHz modulation bandwidth.
We analyze theoretically the nonlinear phenomenon of optical bistability inside a ring resonator formed with a silicon-waveguide nanowire and derive an exact parametric relation connecting the output intensity to the input intensity. Our input-output relation accounts for linear losses, the Kerr nonlinearity, two-photon absorption, free-carrier-induced absorption and dispersion, and thermo-optic effects within the resonator. Based on our study, we generalize the standard definition of effective length to allow for all possible losses within a silicon ring resonator. We also present a simplified version of the bistable phenomenon valid for resonators operating in a regime in which losses resulting from two-photon absorption are relatively small. Our analytical results provide clear insight into the physics behind optical bistability and may be useful for designing silicon-based optical memories.
Optical Kerr nonlinearity is central to optical bistability in high-Q microcavities. Here, we show that optical Kerr nonlinearities in Fabry Perot resonators can also result in self-focusing effects, that can substantially modify the Q-factors of the resonators. For the studied cavities, up to five-fold increase in Q-factors is predicted, attributed to reduction of sidewall leakage, confirmed via simulations performed on curved cavities. We also show that for high Q-factor cavities, the higher-order Kerr effect can become relevant already at modest input intensities scaling inversely with Q-factor for a particular n2/n4 ratio. We investigate numerically the role of higher-order Kerr effect on optical bistability responses and find that its inclusion can shift the bistability threshold to higher input intensities than predicted by the standard n2-only model.
Resonant devices are an integral component of the integrated silicon photonics platform, with applications in filters, switches, modulators, delays, sensors etc. High index contrast SOI waveguides can be used to form compact micro-ring resonators with bend radii on the order of micro-meters. This work describes the application of micro-ring resonators in linear and nonlinear silicon photonics. We describe the use of higher-order coupled resonators for use as ultra-high contrast pass-band filters with close to 100 dB extinction. Using the spontaneous four-wave mixing process, a third-order nonlinear Kerr effect, coupled resonator waveguides are shown to be a useful source of heralded single photons, as well as other unique quantum states of light. We also describe four-wave mixing results in silicon micro-resonators, where nonlinear loss effects are mitigated by reverse biased p-i-n diodes, showing potential for high-speed optical signal processing.
In this work, we experimentally study the optical bistability (BI) behavior in active silicon microring resonators (MRR). A stable hysteresis loop response at the output port is demonstrated at a power level higher than the BI threshold, taking into account the different detuning and free carrier life-times. That hysteresis is observed at low and high BI thresholds, which can be used for all-optical switching and optical routers. The MRR is placed inside a reverse-biased PIN junction and fabricated by CMOS technology. It records a loaded Q-factor, linewidth (3 dB width of the spectrum), and extinction ratio (ER) around105,15.310^{5},15.3105,15.3pm, and 20 dB, respectively.