TY - BOOK

T1 - Heterogeneous integration of GaAs waveguides on low loss substrates for quantum photonic circuits

AU - Shadmani, Atefeh

PY - 2023

Y1 - 2023

N2 - The range of building blocks required to perform quantum photonic tasks demonstrates that existing photonic components based on monolithic material systems may be inadequate. Hybrid quantum photonic circuit is emerging as an exciting field in quantum photonics. The conceptual idea behind it is to combine different building blocks, which can be generally incompatible in terms of their growth and integration conditions, in a functional circuit to form a versatile quantum photonic integrated circuit platform. For example, near-ideal single-photon sources have been realized in III-V platforms like GaAs. However, this platform suffers from a relatively high propagation loss and lacks efficient and low-loss optical components. On the other hand, there are platforms such as silicon nitride (Si3N4) and lithium niobate (LiNbO3), which provide low-loss high-speed optical components like switches, modulators, and filters. By heterogeneous integration, we can take advantage of the strengths of each single platform and create a hybrid platform that can potentially outperform its monolithic constituents. In this thesis, the heterogeneous integration of GaAs on-chip circuits containing self-assembled quantum dots on a Si/SiO2 wafer is demonstrated. Here, we report fabrication techniques developed for this heterogeneous integration based on the adhesive die-to-wafer bonding method. The optical propagation loss for the GaAs waveguides fabricated on the GaAs-on-Si hybrid substrate is measured to be < 7 dB/푚푚, which is comparable with the performance of suspended GaAs circuits. In quantum dot excitation at 푇 < 10 K, anti-bunched emission from individual quantum dots is observed, confirming that the fabrication process does not affect the quantum dot performance. We have also fabricated GaAs waveguides on top of Si3N4 waveguides embedded in a SiO2 layer. A taper-based spot-size converter structure is designed in the GaAs waveguide layer, which enables optical light coupling between the GaAs and Si3N4 waveguide layers. The existing limitations are well understood: for example, the number of counts from quantum dot emission is not high enough, which makes it challenging to measure the auto correlation function between emitted photons and extract the exact value for 푔2 (0). To reach a more precise estimate, a sample with a lower quantum dot density, together with a narrower filtering apparatus will be needed. To be able to measure indistinguishability of the emitted photons, it is required to integrate p-i-n heterostructure GaAs membranes and fabricate electrically contacted devices. We proposed micro-transfer printing as an alternative method to adhesive die-towafer bonding for heterogeneous integration. Double-sided tapered GaAs waveguides have been designed and fabricated as the source wafer for transfer printing. In collaboration with Ghent University, we performed successful transfer printing of standalone nanobeam GaAs waveguides for the first time. In optical measurements, we demonstrated the light transmission with a mean overall loss of 3 dB per device. This thesis reports fabrication techniques and device characterizations for the heterogeneous integration of GaAs waveguides with embedded quantum dots on a SiO2 layer, which is an important step toward the heterogeneous integration of GaAs-based devices with different material systems. This heterogeneous integration can potentially solve the major obstacle in realizing more complex photonic integrated circuits (PICs) using CMOS processes.

AB - The range of building blocks required to perform quantum photonic tasks demonstrates that existing photonic components based on monolithic material systems may be inadequate. Hybrid quantum photonic circuit is emerging as an exciting field in quantum photonics. The conceptual idea behind it is to combine different building blocks, which can be generally incompatible in terms of their growth and integration conditions, in a functional circuit to form a versatile quantum photonic integrated circuit platform. For example, near-ideal single-photon sources have been realized in III-V platforms like GaAs. However, this platform suffers from a relatively high propagation loss and lacks efficient and low-loss optical components. On the other hand, there are platforms such as silicon nitride (Si3N4) and lithium niobate (LiNbO3), which provide low-loss high-speed optical components like switches, modulators, and filters. By heterogeneous integration, we can take advantage of the strengths of each single platform and create a hybrid platform that can potentially outperform its monolithic constituents. In this thesis, the heterogeneous integration of GaAs on-chip circuits containing self-assembled quantum dots on a Si/SiO2 wafer is demonstrated. Here, we report fabrication techniques developed for this heterogeneous integration based on the adhesive die-to-wafer bonding method. The optical propagation loss for the GaAs waveguides fabricated on the GaAs-on-Si hybrid substrate is measured to be < 7 dB/푚푚, which is comparable with the performance of suspended GaAs circuits. In quantum dot excitation at 푇 < 10 K, anti-bunched emission from individual quantum dots is observed, confirming that the fabrication process does not affect the quantum dot performance. We have also fabricated GaAs waveguides on top of Si3N4 waveguides embedded in a SiO2 layer. A taper-based spot-size converter structure is designed in the GaAs waveguide layer, which enables optical light coupling between the GaAs and Si3N4 waveguide layers. The existing limitations are well understood: for example, the number of counts from quantum dot emission is not high enough, which makes it challenging to measure the auto correlation function between emitted photons and extract the exact value for 푔2 (0). To reach a more precise estimate, a sample with a lower quantum dot density, together with a narrower filtering apparatus will be needed. To be able to measure indistinguishability of the emitted photons, it is required to integrate p-i-n heterostructure GaAs membranes and fabricate electrically contacted devices. We proposed micro-transfer printing as an alternative method to adhesive die-towafer bonding for heterogeneous integration. Double-sided tapered GaAs waveguides have been designed and fabricated as the source wafer for transfer printing. In collaboration with Ghent University, we performed successful transfer printing of standalone nanobeam GaAs waveguides for the first time. In optical measurements, we demonstrated the light transmission with a mean overall loss of 3 dB per device. This thesis reports fabrication techniques and device characterizations for the heterogeneous integration of GaAs waveguides with embedded quantum dots on a SiO2 layer, which is an important step toward the heterogeneous integration of GaAs-based devices with different material systems. This heterogeneous integration can potentially solve the major obstacle in realizing more complex photonic integrated circuits (PICs) using CMOS processes.

M3 - Ph.D. thesis

BT - Heterogeneous integration of GaAs waveguides on low loss substrates for quantum photonic circuits

PB - The Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -