Optical coupling structures for heterogeneous integration of quantum photonic devicesLjubotina, Miloš (Avtor) Krč, Janez (Mentor) adiabatic waveguide tapersheterogeneous integrationintegrated optical coupling structuresoptical coupling efficiencyquantum photonic integrated circuitssilicon nitride waveguidessuspended gallium arsenide waveguideswaveguide propagation losseswaveguide sidewall roughnessThis thesis belongs to the general field of integrated photonics. In analogy to integrated electronics, integrated photonics offers several benefits over discrete optical assemblies, including small size and robust assembly, highly scalable functionality and complexity, as well as mass manufacturing at a fraction of the cost of bulk optical devices. However, in contrast to electronics, which is dominated by CMOS fabrication of silicon chips, there exist a plethora of photonic integration platforms and technologies based on a variety of materials such as Si, SiN, InP, GaAs, LiNbO3, glass, polymers, and more. This is due to the competing optical and optoelectronic properties of different materials—no single platform has all the capabilities necessary to enable monolithic fabrication of the variety of active and passive photonic components needed in conjunction to form fully featured devices. Hence, heterogeneous integration of diverse components on a single chip is a very active topic in integrated photonics. Besides enabling advances in more traditional application areas such as optical communications, it opens the door to new opportunities also in other areas, including quantum technologies. In this context, the benefits of photonic integration and importance of photons as quantum information carriers makes quantum photonic integrated circuits (QPICs) critical to the development of future scalable quantum technologies. However, to satisfy the stringent requirements of these technologies, diverse materials and photonic platforms must be heterogeneously integrated. Here, a critical limitation of current integration techniques are optical losses associated with the optical coupling of these platforms, which must be minimised. In this thesis, we focus on design and experimental demonstrations of robust and highly efficient coupling structures used to optically couple heterogeneously integrated quantum photonic devices in QPICs. We particularly focus on the integration of GaAs waveguides with SiN waveguides. The former are used in single-photon sources based on waveguide-embedded InAs quantum dots but suffer from high optical propagation loss. In contrast, the latter are well-known for their low-loss operation and thus present an excellent interposer platform for scalable QPICs. Due to the high losses of GaAs waveguides, which hinder the SiN-GaAs optical coupling efficiency, the research presented in this thesis comes in two parts. First, we systematically investigate the propagation losses of standalone GaAs waveguides to identify their main origins and develop deeper insight into underlying scattering mechanisms. And second, we devise a strategy to design efficient optical couplers for heterogeneous integration considering realistic operating conditions. These conditions include GaAs propagation losses, analysed in the first part of our research, and key structural variations of fabricated devices, which originate from the limitations of current integration techniques. Our study of scattering propagation losses spans two suspended GaAs platforms operating at wavelengths of 930 nm and 1300 nm, corresponding respectively to an established platform used to fabricate high-quality single-photon sources and an emerging one in the telecommunication O-band. This approach allows us to evaluate and compare scattering losses across both mature and exploratory wavelengths relevant to QPICs. Using a combined approach of numerical simulations and experimental measurements, we examine four key waveguide nano- and micro-scale perturbation types: sidewall roughness, top surface roughness, surface particles, and suspension tethers. We decouple their individual contributions to the total propagation loss by employing the same rigorous numerical method in all cases, enabling a well-founded comparative analysis not found in existing literature. We characterise fabricated PIC samples to determine the statistical properties of waveguide perturbations and to directly measure the total scattering propagation loss, also isolating the loss induced by a single suspension tether. By correlating experimental data with simulations, we identify sidewall roughness as the dominant source of scattering loss in both platforms, contributing about 3× more in the 930 nm case than in the 1300 nm case. Secondly, we also identify suspension tethers as significant loss contributors, accounting for roughly ⅓ of the losses at 1300 nm and down to ⅙ (due to higher overall losses) at 930 nm. Through additional simulation analyses, we investigate these losses and propose two design-time avenues for loss reduction. We consider the waveguide width dependence of sidewall scattering and link observed local maxima to the occurrence of weakly-guided higher-order modes, which is an insight enabled by our rigorous numerical approach. By selecting wider waveguide widths corresponding to local minima, we show that sidewall scattering losses can be reduced by factors up to 4–5. To address suspension tether losses, we propose a concrete tether geometry optimisation strategy and demonstrate a simulated loss reduction of ~2.5× while maintaining the overall structure layout. In the second part of our research, we investigate the optimisation of adiabatic taper mode-couplers for high efficiency operation tolerant to fabrication variations and propagation losses. We particularly focus on the integration of the 930 nm suspended GaAs platform with a low-loss SiN interposer through micro-transfer printing (µTP). In this case, the key coupling loss mechanisms are mode coupling and scattering caused by systematic fabrication variations like µTP-induced misalignment and sidewall scattering propagation losses, which are investigated in the scope of the first part of our research. These effects can be minimised by optimising the adiabatic waveguide tapering profile and length. Here, we propose a fully numerical strategy directly incorporating systematic fabrication variations in the taper optimisation procedure based on simulations and a global optimisation algorithm. Moreover, we take a second step and also include sidewall scattering losses determined in the first part of the thesis in the optimisation objective function in addition to losses caused by systematic variations. These are modelled in the same way as in the case of standalone GaAs waveguides and are critically important as they are heavily influenced by the waveguide width variation in the taper. In this way, the optimisation algorithm is guided towards an optimal tapering profile tolerant to explicitly defined variations and propagation losses. We apply our devised strategy to design two adiabatic SiN-GaAs couplers considering: (1) only systematic variations, and (2) both systematic variations and propagation losses. We verify their performance through experiment (and simulations). Our measurements show increased coupling efficiency in the second case (-0.4 dB) compared to the first (-0.6 dB) and a substantial improvement compared to a reference established semi-analytical coupler design (-1.0 dB). In relation to propagation losses, our combined simulation and experimental approach provides deeper insight into the underlying scattering loss mechanisms, which remain a fundamental challenge across all photonic integration technologies—including heterogeneous approaches. Our adiabatic coupler design approach builds on this and incorporates a waveguide width dependent propagation loss model in the taper optimisation procedure for the first time to our knowledge. The devised optimisation strategy enables accurate consideration of explicitly defined systematic fabrication variations and propagation losses, substantially improving coupling performance. We apply this strategy to design adiabatic tapers enabling high efficiency coupling between a low-loss SiN platform and a high loss GaAs platform used to fabricate high quality single-photon sources. Therefore, our findings in both parts of our research advance QPICs both at the monolithic and heterogeneous integration levels. First, the proposed improvements of the standalone GaAs platform offer a practical route to significantly reduce propagation losses in suspended GaAs strip waveguides, representing a valuable step toward enabling more complex QPICs directly within the single-photon source host platform. And second, our adiabatic coupler optimisation strategy and concrete SiN-GaAs coupler designs enable improved coupling of heterogeneously integrated single-photon sources and represent a key step forward for robust high-efficiency adiabatic mode-coupler design, supporting heterogeneous integration of diverse QPIC components.20252026-01-05 13:35:17Doktorsko delo/naloga177727sl