Integrated gallium phosphide nonlinear photonics (1808.03554v2)

Abstract: Gallium phosphide (GaP) is an indirect bandgap semiconductor used widely in solid-state lighting. Despite numerous intriguing optical properties---including large $\chi^{(2)}$ and $\chi^{(3)}$ coefficients, a high refractive index ($>3$), and transparency from visible to long-infrared wavelengths ($0.55-11\,\mu$m)---its application as an integrated photonics material has been little studied. Here we introduce GaP-on-insulator as a platform for nonlinear photonics, exploiting a direct wafer bonding approach to realize integrated waveguides with 1.2 dB/cm loss in the telecommunications C-band (on par with Si-on-insulator). High quality $(Q> 10^5)$, grating-coupled ring resonators are fabricated and studied. Employing a modulation transfer approach, we obtain a direct experimental estimate of the nonlinear index of GaP at telecommunication wavelengths: $n_2=1.2(5)\times 10^{-17}\,\text{m}^2/\text{W}$. We also observe Kerr frequency comb generation in resonators with engineered dispersion. Parametric threshold powers as low as 3 mW are realized, followed by broadband ($>100$ nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing. These results signal the emergence of GaP-on-insulator as a novel platform for integrated nonlinear photonics.

• The paper demonstrates a GaP-on-insulator platform achieving low waveguide losses (1.2 dB/cm) through a direct wafer bonding process.
• The paper details engineered waveguides with anomalous dispersion that enable Kerr frequency comb generation at a threshold power of just 3 mW.
• The paper highlights GaP’s multifunctionality with observed frequency-doubled and Raman-shifted combs, paving the way for advanced photonic applications.

Insights into Integrated Gallium Phosphide Nonlinear Photonics

The paper presents a detailed investigation into the utilization of gallium phosphide (GaP) on insulator as a potential platform for integrated nonlinear photonics. GaP, a material with significant optical properties, provides enticing opportunities due to its considerable nonlinear refractive index, high-quality waveguides, and compatibility with current semiconductor technologies. The authors explore the alignment of GaP for various nonlinear photonic applications, including frequency comb generation and Raman lasing, with meticulous attention to material integration and fabrication techniques.

The paper highlights GaP's advantageous characteristics, such as a substantial nonlinear coefficient, $\chi^{(3)}$, a high refractive index, and a wide transparency range from visible to infrared wavelengths. These properties intrinsically support strong optical confinement and energy-efficient operation, making GaP an attractive medium for both three-wave-mixing and four-wave-mixing processes due to its non-zero $\chi^{(2)}$ and $\chi^{(3)}$ susceptibilities. The paper discusses the technological challenges associated with integrating crystalline GaP onto low-index substrates while maintaining minimal propagation losses. The authors leverage a direct wafer bonding process to combine GaP with SiO2, achieving waveguide losses as low as 1.2 dB/cm—a significant milestone for GaP-based photonic devices.

A core component of the research involves the fabrication of GaP waveguides and resonators with finely controlled dispersion properties. By engineering the geometry of the waveguides, particularly their width, they achieve the necessary anomalous dispersion required for efficient frequency comb generation via Kerr nonlinearity. The report of achieving Kerr frequency combs in GaP resonators at a threshold power of only 3 mW with parametric oscillation marks a notable advance. Additionally, leveraging the high nonlinear index $$n_2=1.2(5)\times 10^{-17}\,\mathrm{m}^{2}/\mathrm{W}$$ at telecommunications wavelengths, emphasized through direct experimental estimates and analytical methods, is noteworthy given the context of existing materials in the field.

The synthesis of broadband frequency combs, alongside exploration into second-order nonlinear processes such as frequency doubling and Raman scattering in GaP, indicates a matured understanding of the material's nonlinear capabilities. The efficient generation of frequency combs with engineered dispersion and optical isolation strategies implicitly suggests potential enhancements in integrated photonics, computing frameworks, quantum information science, and metrology applications. Furthermore, the presence of frequency-doubled and Raman-shifted combs illustrates GaP's intrinsic multifunctionality, a cornerstone for future photonics research in a variety of frequency ranges.

This research theoretically and experimentally substantiates GaP-on-insulator as a formidable platform for enabling high-functionality photonic circuits. It pioneers pathways for future advancements where reduction in sidewall roughness and precise dispersion engineering could realize applications such as soliton combs and ultra-broadband supercontinuum generation. The insights drawn offer a roadmap not only for integrating GaP into existing photonic infrastructure but also for expanding its utility across novel photonic devices and systems.