Silicon photonic chips currently force a trade-off between active and passive components because the silicon layer has a fixed thickness. This project replaces that rigid single-crystal silicon with a polycrystalline silicon platform whose components can be tailored to different dimensions across the same chip. The problem is that active devices—like modulators—work best in tiny waveguides that confine light tightly, while passive devices—like couplers—need larger waveguides to reduce transmission losses. A fixed layer thickness cannot satisfy both. The team will fabricate these variable-sized components using precision etching, deposition, and laser processing, then benchmark them against state-of-the-art systems. If successful, the approach could reduce optical losses and lower energy consumption in photonic integrated circuits. Because the polysilicon is deposited at low temperatures rather than grown epitaxially at high heat, the manufacturing process also becomes more sustainable. The project culminates in a high-speed photonic transmitter built entirely from the new platform, demonstrating that the technology works in practice. This matters for data centre communications, LiDAR in driverless vehicles, and any system that moves data optically.
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Silicon photonics is currently one of the fastest growing areas of research and development. The ability to exploit both the optical and electronic functionality of the semiconductor platform to process and manipulate data offers a route to dramatically increase the speeds, capacities, and efficiencies of optoelectronic systems. Capitalizing on their compact and efficient architectures, silicon photonic chips now underpin several key application areas such as data centre communications, where they are helping to expand internet bandwidths, and LiDAR, where they are enabling 4D vision in driverless vehicles. However, there are challenges associated in realizing the full suite of optoelectronic functionality using single-crystal silicon platforms due to their fixed silicon layer thickness. This is because the active and passive devices that make up the silicon circuits have different size requirements. For example, active devices are more efficient when the light is tightly confined in small waveguides, while passive devices are best when formed from larger waveguides as this helps to reduce the coupling and transmission losses. Moreover, the fixed silicon height can also make it difficult to fully optimise and integrate various components for operation across broad bandwidths or non-standard wavelength regions. The work in this proposal aims to tackle the integration challenge by developing a low cost and versatile polycrystalline silicon (polysilicon) platform technology that allows for the formation of components with a range of tailored dimensions across the photonic chip. The components will be fabricated via a combination of precision etching, deposition, and laser materials processing to achieve high-quality polysilicon waveguide devices. Importantly, as well as boosting the efficiency of the individual components through optimised designs, the improved coupling between components of different dimensions will reduce optical losses, resulting in lower energy consumption within the photonic integrated circuits. Furthermore, the transition from using single-crystal waveguide materials that require high temperature epitaxial growth to low temperature deposited polysilicon opens a route to a more sustainable manufacturing of silicon photonic systems. To explore this new platform technology, the team combines leading UK experts in the areas of semiconductor materials optimisation and integration, as well as active and passive silicon photonic component design and systems level engineering. Although much of the programme is focused on development of the fabrication procedures, a number of passive and active devices will be constructed, including couplers and modulators that will be used to benchmark the performance against state-of-the-art systems. The work will culminate in the demonstration of a high-speed photonic transmitter circuit constructed entirely from our polysilicon platform, illustrating the versatility and practicality of this approach. Thus, this project will take a transformative step towards unlocking the rich potential of integrated polysilicon optoelectronic systems.
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