Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators (1909.09778v1)

Abstract: Recent advances in nonlinear optics have revolutionized the area of integrated photonics, providing on-chip solutions to a wide range of new applications. Currently, the state of the art integrated nonlinear photonic devices are mainly based on dielectric material platforms, such as Si3N4 and SiO2. While semiconductor materials hold much higher nonlinear coefficients and convenience in active integration, they suffered in the past from high waveguide losses that prevented the realization of highly efficient nonlinear processes on-chip. Here we challenge this status quo and demonstrate an ultra-low loss AlGaAs-on-insulator (AlGaAsOI) platform with anomalous dispersion and quality (Q) factors beyond 1.5*10^6. Such a high quality factor, combined with the high nonlinear coefficient and the small mode volume, enabled us to demonstrate a record low Kerr frequency comb generation threshold of ~36 uW for a resonator with a 1 THz free spectral range (FSR), ~100 times lower compared to that in previous semiconductor platform. Combs with >250 nm broad span have been generated under a pump power lower than the threshold power of state of the art dielectric micro combs. A soliton-step transition has also been observed for the first time from an AlGaAs resonator. This work is an important step towards ultra-efficient semiconductor-based nonlinear photonics and will lead to fully integrated nonlinear photonic integrated circuits (PICs) in near future.

• The paper presents an AlGaAsOI platform achieving a Kerr frequency comb generation threshold of approximately 36 µW, a 100-fold improvement over prior methods.
• It leverages high Q factors (exceeding 1.5 × 10^6) and tailored dispersion engineering to produce over 250 nm spectral span with coherent soliton combs.
• The research paves the way for scalable, integrated nonlinear photonic circuits with applications in telecommunications, frequency metrology, and quantum technologies.

Ultra-Efficient Frequency Comb Generation in AlGaAs-on-Insulator Microresonators

In the pursuit of advancing photonics technology, the integration of high nonlinear coefficient materials into photonic devices has continually garnered significant interest. The paper under review presents the development of an ultra-low-loss AlGaAs-on-insulator (AlGaAsOI) platform, showcasing its potential for highly efficient nonlinear frequency comb generation. The researchers demonstrate substantial improvements in frequency comb generation efficiency using AlGaAsOI microresonators, highlighting the significant role of high Q factors and material nonlinearity.

The demonstrated AlGaAsOI platform achieves an intrinsic quality factor exceeding 1.5 × 10^6, a notable advancement for semiconductor-based nonlinear optics. This high Q factor, coupled with the advantageous nonlinear properties of AlGaAs and a compact mode volume, yields a Kerr frequency comb generation threshold of approximately 36 µW. This threshold is markedly lower—about 100 times—when compared to preceding semiconductor-based platforms, and exhibits a tenfold reduction relative to the most advanced dielectric micro-resonators. Consequently, the demonstrated platform facilitates the generation of frequency combs with over 250 nm spectral span at a power level beneath the threshold needed for current state-of-the-art dielectric comb generators.

Key to these advancements is the use of heterogeneously integrated AlGaAs, which combines substantial Kerr nonlinear coefficients with the benefit of anomalous group velocity dispersion (GVD) at telecom wavelengths. The flexibility in tailoring the AlGaAs bandgap to prevent two-photon absorption (TPA) further enhances its utility in optical comb applications. Simulations and experimental validation confirm that the high refractive index and unique dispersion qualities of AlGaAsOI enable efficient Kerr-effect processes within the microresonator configurations.

From a practical standpoint, the AlGaAsOI platform reduces the complexity of fabrication processes—an obstacle faced by traditional high-Q dielectrics like silicon nitride. This simplification makes the AlGaAsOI platform attractive for scalable production, facilitating the emerging pathways for fully integrated nonlinear photonic integrated circuits (PICs). The efficacy of AlGaAsOI in soliton comb generation was corroborated by the observed soliton-step transition, suggesting its future role in stable and coherent comb applications.

The long-term implications of this research are manifold. First, the clear demonstration of ultra-low threshold power for Kerr frequency comb generation propels the AlGaAsOI platform as a frontrunner in nonlinear optics, possibly surpassing traditional platforms. Next, its potential integration into photonic circuits promises enhanced performance for telecommunications, frequency metrology, and quantum photonics. Additionally, the inherent capability to facilitate second order frequency conversions positions AlGaAsOI for multi-functional, compact photonic chip applications.

Moving forward, focus areas include further enhancements of the Q factors of larger resonators, more detailed studies on soliton dynamics, and potential hybrid applications that leverage both the high-order nonlinear effects and excellent integration capabilities of the AlGaAsOI platform. Continued optimization in the fabrication methods and dispersion engineering is likely to foster additional breakthroughs in comb generation efficiency and enable broader applications for nonlinear photonics.

Ultimately, this work is a pivotal step in advancing the capabilities of semiconductor nonlinear photonics, offering a versatile and potent alternative to dielectric platforms for a diverse array of applications within the optical communication and quantum technology sectors.