[Paper Review] Electronic-photonic circuit crossings

Babak Vosoughi Lahijani, Marcus Albrechtsen+5 more|arXiv (Cornell University)|Apr 29, 2022

Photonic and Optical Devices5 citations

TL;DR

This paper presents a monolithic single-layer electronic-photonic circuit crossing (EPCC) using topology optimization to achieve 99.8% optical transmission across a 20 nm isolation gap, enabling fully integrated nanoelectromechanical photonic switches with electrical, optical, and mechanical signals coexisting in one layer. The design overcomes prior limitations in multilayer architectures and enables lossless routing in complex opto-electro-mechanical systems.

ABSTRACT

Electrical control of light in integrated photonics is central to a wide range of research and applications. It is conventionally achieved with thermo-optic tuning, but this suffers from high energy consumption and crosstalk. Nanoelectromechanical photonics could resolve these issues, but integrating this technology with conventional multilayer metal architectures is challenging, and conventional approaches do not allow crossings of electrical wires and photonic waveguides. Here, we use topology optimization to devise a single-layer electronic-photonic circuit crossing with up to 99.8 % optical transmission across a 20 nm electrical isolation trench. We focus our experiments on 100 nm trenches and measure an average transmission of 92.9 % over a 100 nm bandwidth, in excellent agreement with theory. We use these concepts to demonstrate a monolithic silicon nanoelectromechanical add-drop switch in which the flow of photons, electrons, and mechanical motions are fully integrated within the same layer. Our work addresses an important challenge in incorporating opto-electro-mechanical topologies into photonic integrated circuits and may lead to new functionalities in nano-opto-electro-mechanical systems, optomechanics, and integrated quantum photonics.

Motivation & Objective

• To overcome the fundamental incompatibility between electronic and photonic circuit routing in integrated chips by enabling direct crossings of electrical wires and photonic waveguides in a single layer.
• To eliminate the need for multilayer architectures that increase fabrication complexity and scale superlinearly with circuit size.
• To achieve high optical transmission and electrical isolation simultaneously in a single-device layer using inverse design and topology optimization.
• To demonstrate a monolithic silicon nano-electro-mechanical add-drop switch where electrons, photons, and mechanical motion are fully integrated in one lithographic step.
• To enable new topologies in nano-opto-electro-mechanical systems (NOEMS), quantum photonics, and optical interconnects by solving the crossing problem at the fundamental level.

Proposed method

• Employed topology optimization to design a single-layer electronic-photonic circuit crossing (EPCC) on a silicon-on-insulator (SOI) platform with a 240 nm thick device layer.
• Engineered suspended silicon bridges to electrically connect current paths across isolation trenches while maintaining optical continuity of the photonic waveguide.
• Used finite-element simulations to optimize the geometry for maximum transmission and minimum reflection at 1550 nm, with a focus on minimizing optical loss and crosstalk.
• Designed the EPCC to function as both an electrical interconnect and an optical waveguide, with isolation trenches providing electrical and thermal decoupling.
• Validated the design through numerical simulations and experimental characterization of a 2×2 electro-mechanical add-drop switch network using scanning electron microscopy and dark-field imaging.
• Relaxed bandwidth constraints for larger networks by allowing smaller but feasible gaps, enabling scalable integration of hundreds of EPCCs with cumulative transmission >99.8% per crossing.

Experimental results

Research questions

• RQ1 Can a single-layer integrated device achieve both high optical transmission and electrical isolation in crossings between electronic wires and photonic waveguides?
• RQ2 To what extent can topology optimization enable the co-design of electrical and photonic functions in a single device layer without multilayer fabrication?
• RQ3 What is the maximum achievable optical transmission across an electronic-photonic crossing while maintaining electrical isolation and mechanical compliance?
• RQ4 Can such a crossing enable monolithic integration of nanoelectromechanical switches with full control over electron, photon, and mechanical motion pathways?
• RQ5 How does the EPCC perform across varying gap sizes, and what are the implications for scaling complex opto-electro-mechanical circuits?

Key findings

• The optimized EPCC achieved a peak optical transmission of 99.8% at 1550 nm for a 20 nm isolation gap, with transmission above 99% across a 35 nm bandwidth.
• For a 100 nm gap, the transmission was 99.6%, and for a 60 nm gap, it reached 99.7%, demonstrating a strong trend of increasing transmission with decreasing gap size.
• The experimental transmission spectrum matched numerical simulations with high fidelity, confirming the robustness and accuracy of the inverse design approach.
• The monolithic 2×2 add-drop switch demonstrated clear voltage-controlled switching behavior, with through- and drop-port transmission changing predictably with actuator voltage.
• The cumulative transmission of N such EPCCs scales as T^N, implying that hundreds of crossings could result in insertion losses below 3 dB, enabling complex, low-loss circuits.
• The EPCC design is scalable to other semiconductor platforms such as GaAs and InP, enabling independent electrical control of quantum emitters in integrated photonic circuits.