Towards Highly Efficient On-chip Frequency Comb Generation in AlGaAs-on-Insulator Microresonators

Research output: Book/ReportPh.D. thesisResearch


Integrated optical microresonators offer a powerful platform for observing nonlinear optical phenomena. They confine light in resonant modes at very small scales and for prolonged periods of time, enhancing the intensity of low-power optical signals. Via the nonlinear interaction between light and matter in these resonators, a single optical frequency can be converted into numerous frequencies coherently, forming frequency combs. These broad and coherent optical signals have found many applications ranging from precision spectroscopy to optical communications. Through chip integration, frequency comb applications benefit from the small footprint and low cost of nano-fabrication techniques. Among many materials used in integrated photonics, Aluminum Gallium Arsenide (AlGaAs) offers itself as an efficient nonlinear material. Its very high Kerr nonlinear parameter and high refractive index are exactly the desired properties for efficient frequency comb generation via parametric frequency conversion. By controlling the Aluminum content in the AlGaAs alloy, the material bandgap can be engineered to eliminate two-photon absorption that would otherwise hinder the use of AlGaAs at telecommunications wavelengths. Though challenging to realize, a breakthrough has been made in fabricating high index-contrast AlGaAs waveguides in the AlGaAs-on-Insulator platform. Using this novel fabrication technique, high-Q AlGaAs resonators have enabled efficient frequency comb generation. However, AlGaAs-on-Insulator devices have not yet reached the potential that the properties of AlGaAs suggest. In this thesis, the obstacles in the way of advancing the AlGaAs-on-Insulator platform are tackled. Coupler loss was identified as a significant source of loss and was mitigated by new coupler designs. Surface roughness in optical microresonators is responsible for coupling the resonator modes, and was found to affect the dispersion significantly in the high-index contrast AlGaAs-on-Insulator platform. The effect of mode coupling was mitigated by including tapered sections that filter out higher-order modes while maintaining anomalous dispers dispersion for the fundamental mode. The interface between III-V materials and dielectrics is known to pose a serious challenge against III-V-based electronic devices. The vast knowledge accumulated by studies on the electronic properties of III-V-oxide interfaces was used in order to evaluate their effect on the optical properties of AlGaAs-on-Insulator waveguides. By numerical simulations and experimental measurements, interface states were found to alter the band diagram significantly in the small AlGaAs waveguides we use. Furthermore, the interface states were found responsible for significant nonlinear losses due to the absorption of single photons. These effects were quantified by combining analytical models, numerical simulations and experimental measurements, and their implications on frequency comb generation were studied. The results in this thesis point to the importance of interface passivation if the performance of AlGaAs-on-Insulator waveguides is to match the promising bulk properties of AlGaAs.
Original languageEnglish
PublisherTechnical University of Denmark
Publication statusPublished - 2019


Highly efficient on-chip frequency comb generation

Kamel, A. N., Rottwitt, K., Pu, M., Thomsen, J. W., Laurynenka, A., De Rossi, A. & Torres-Company, V.



Project: PhD

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