PROGRAM | Electrical & Computer Engineering

By: Yahui Xiao Chair: Tingyi Gu

ABSTRACT

In the current era, photonic integrated circuits have arisen as a pivotal platform for optical communication and computing components. The rapid expansion of component libraries in silicon photonics, characterized by verified performance through manufacturing, underscores the complexity, functionality, and scalability of the PIC system. Devices based on photonic crystals offer a promising route to exploit the capability to engineer the propagation of the electromagnetic field on a microscopic scale, in addition to enabling the realization of devices with compact footprints. This work directs its attention towards a comprehensive exploration of the various tasks necessary to tackle the challenges and demands in the subsequent sections:

1. Scalable photonic crystal waveguides with 2 dB component loss

Minimizing the insertion loss of individual components is paramount for large-scale photonic integrated circuits. The measured component insertion losses (5- 20dB) of photonic crystal waveguides devices are well beyond numerical predictions (<0.1dB). The additional loss is the sum of the coupling loss and propagation loss in photonic crystal waveguides. Mode mismatch and deformation in suspended structures increase the coupling loss. Nanofabrication-related geometric inhomogeneity increases propagation loss. The fabrication variation and offsets of periodic or gradient nanophotonic structures are foundry dependent and need to be calibrated for each processing line. This work demonstrates the first low-loss photonic crystal waveguides through AIM photonics multi-project-wafer run. For sub-millimeter-long photonic crystal waveguides, 2 dB total component loss and 40 dB extinction ratio are observed across dies. A three-dimensional coupler between the channel and photonic crystal waveguides reduces the insertion loss to less than 1 dB per port. The high extinction ratio is attributed to the excellent geometric homogeneity of the nano-manufactured nanophotonic structures.

1. Engineering the light coupling between metalens and photonic crystal resonators for robust on-chip microsystems

We designed an on-chip transformative optic system with a broadband metalens coupler on a foundry-compatible silicon photonic platform. Adjusting the on-chip metalens’ focusing length and mode dimension reduces the insertion loss between the metalens and the photonic crystal waveguide (PhC WG) structures to 2 dB by matching the mode on the metalens focal plane to the PhC WG mode. Alternatively, the integrated metalens allow direct coupling from a multi-mode WG to the PhC cavity. The on-resonance transmission in a lens–cavity–lens microsystem achieves 60%. These micro-systems do not involve single-mode silicon nanowire WG; even a suspended PhC structure can be mechanically robust against vibrations. The proposed microsystem can be a new platform for miniaturized chemical and biosensor applications operating in air or solution environments.

1. Electrically reconfigurable low-loss topological photonic crystal waveguides

Reducing the insertion loss of photonic crystal waveguide components on the foundry-compatible silicon-on-insulator (SOI) platform is crucial for leveraging their unique dispersion properties toward large-scale integration. Topological photonic crystal waveguides with robustness against backscattering and sharp bending may solve the loss issues currently faced by the photonic crystal society. In conventional photonic crystal waveguides on the SOI platform, low insertion loss can only be achieved through undercut. This work demonstrates that the topological valley photonic crystal waveguide structures can reach low-loss transmission (~2dB for 0.1 mm length) without postprocessing. The system can be electro-optically reconfigurable for a high extinction ratio switch (up to 35 dB at 1550 nm operation wavelength). The integrated metalens design reduces the input and output coupling loss, which is all-foundry compatible.

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