Nonlinear Optical Properties in Silicon Waveguides

Nonlinear optical phenomena in silicon waveguides arise from intensity-dependent refractive index changes and absorption processes that become pronounced at telecommunication wavelengths. They enable all-optical signal processing, frequency conversion, parametric amplification and soliton dynamics on a chip. Silicon photonics combines high refractive index contrast with CMOS compatibility to confine light tightly and enhance third-order nonlinearities such as the Kerr effect, yet intrinsic two-photon absorption and free-carrier absorption have historically limited performance. Advances in materials—most notably silicon-rich nitride and ultra-silicon-rich nitride—have mitigated these losses by engineering the optical bandgap beyond the two-photon absorption edge, thereby raising the nonlinear figure of merit. Dispersion engineering in waveguide geometries permits phase matching for efficient four-wave mixing and the formation of gap solitons. These developments underpin applications in high-speed communications, on-chip spectroscopy, quantum light sources and optical buffering, demonstrating the global significance of integrated nonlinear photonics.

Research from Nature Portfolio

Recent studies have demonstrated high-gain optical parametric amplification on a CMOS-compatible platform employing ultra-silicon-rich nitride engineered to operate above the two-photon absorption edge. By tailoring the material composition to boost the Kerr nonlinearity while suppressing nonlinear losses, parametric gains exceeding 40 dB and cascaded four-wave mixing across multiple idlers have been achieved. Another seminal report introduced silicon-rich silicon nitride waveguides with carefully controlled deposition parameters, yielding low linear loss around 1.5 dB cm−1 alongside an enhanced nonlinear parameter that enables π-phase shifts at moderate power levels. These platforms overcome intrinsic two-photon absorption limitations and pave the way for practical all-optical signal processing devices compatible with existing fabrication processes.

Research from all publishers

A recent experiment on an integrated chip utilising a nonlinear Bragg grating in ultra-silicon-rich nitride revealed the first on-chip observation of gap soliton propagation. This work achieved slow-light group velocity reductions to approximately 35–40 % of the speed of light, intensity-dependent transmission and temporal compression up to 2.7×, demonstrating on-chip optical buffering and delay. In parallel, investigations into silicon-rich nitride waveguides fabricated by plasma-enhanced chemical vapour deposition have characterised the dynamics of nonlinear losses arising from free-carrier absorption. Detailed measurements of free-carrier generation and recombination lifetimes indicate that reducing pulse durations below the carrier generation lifetime can effectively suppress nonlinear losses, thereby unlocking the full potential of large intrinsic Kerr responses for high-speed photonic applications.

Nonlinear Optical Properties in Silicon Waveguides publication trend

The graph below shows the total number of articles in nonlinear optical properties in silicon waveguides across all publications each year (not limited to Nature Index journals).

Technical terms

Kerr nonlinearity: A third-order optical effect in which the refractive index changes proportionally to the intensity of light, enabling phenomena such as self-phase modulation and four-wave mixing.

Two-photon absorption (TPA): A nonlinear absorption process in which two photons are simultaneously absorbed to excite an electron across the bandgap, leading to loss at high intensities.

Free-carrier absorption (FCA): A loss mechanism arising from the absorption of photons by charge carriers generated through nonlinear processes such as two-photon absorption.

Optical parametric amplification (OPA): A nonlinear process in which a pump photon amplifies a signal photon by generating an idler photon, facilitated by the third-order nonlinearity.

Gap soliton: A self-localised optical pulse that propagates within the bandgap of a periodic medium, characterised by a balance of dispersion and nonlinearity.

References

1. Gap solitons on an integrated CMOS chip. Nanophotonics (2023).
2. Dynamics of Nonlinear Optical Losses in Silicon‐Rich Nitride Nano‐Waveguides. Advanced Optical Materials (2024).
3. Pushing the limits of CMOS optical parametric amplifiers with USRN:Si7N3 above the two-photon absorption edge. Nature Communications (2017).
4. Si-rich Silicon Nitride for Nonlinear Signal Processing Applications. Scientific Reports (2017).
5. Optical bandgap engineering in nonlinear silicon nitride waveguides.. Optics Express (2017).