Optimizing high-speed photodiodes in a nanophotonic membrane platform for terahertz applications
Jasper de Graaf defended his PhD thesis at the Department of Electrical Engineering on March 18th.
As our world becomes more connected, the demand for ultra-fast communication continues to grow. Additionally, sensing applications that detect and track tiny biomolecules are emerging for real-time health monitoring, early disease detection, and faster identification of harmful bacteria or toxins in food, water, and medicine. High speed and efficient devices are crucial to realize the high frequency terahertz signals needed for these applications. Photonics-based solutions have great potential for this as a powerful alternative to complex and costly electronic systems. Jasper de Graaf explores in his PhD research how to optimize uni-traveling carrier photodiodes (UTC-PDs), a key component of photonics-based solutions, in a nanophotonic platform called indium phosphide membrane on silicon (IMOS). The results can be used to enable faster and more cost-effective future communication systems and sensing platforms.
High-frequency signals can be generated more efficiently by using a combination of on-chip lasers and high speed photodetectors. Jasper de Graaf focuses in his PhD research on UTC-PDs, a type of high-speed photodetectors that is capable of converting optical signals into ultra-fast electronic signals. He realized these photodetectors on the platform IMOS. This platform allows for highly efficient photonic circuits that integrate lasers, amplifiers, and other optical components on a single chip, making it ideal for next-generation terahertz technologies.
Improvements on power handling
Within his research Jasper de Graaf developed UTC-PDs of only 6 square micrometers in size to achieve bandwidths beyond 110 GHz while maintaining high conversion efficiency. Though, power handling is a challenge with such small scale high-speed photodetectors. Within his research Jasper de Graaf significantly improved power handling through various approaches including device design, circuit design, and fabrication process enhancements. This led to a two-fold improvement in power handling per device and a five-fold improvement when used in circuits, making these devices far more robust for real-world applications.
Sensing and communications applications on one chip
To push the boundaries of photonic integration, Jasper de Graaf also explored how to combine UTC-PDs with other key components, such as semiconductor optical amplifiers (SOAs), phase shifters, and passive optical elements - all on a single chip. A novel fabrication approach was developed, allowing seamless integration while providing compatibility to the existing processing flow, reducing complexity and cost. The combination of these components on a single chip allows to realize systems suitable for both sensing and communications applications. To showcase the potential of this technology, Jasper de Graaf designed and fabricated a compact Terahertz spectrometer. Measuring just 1.7 x 1.3 mm虏, this small-scale chip demonstrates how integrated photonic circuits can enable high-throughput and high-sensitivity measurements for potential future applications in sensing of biological samples.
Future directions
The research also highlights future directions, including further improvements in power handling through metal plating and heatsinks, as well as bandwidth enhancements beyond 200 GHz using optimized layer stacks. Overall, the results provide a strong foundation for next-generation terahertz communication and sensing technologies, leveraging the advantages of photonic integration over traditional electronic approaches.
Title of PhD thesis: . Promotor: Prof. Kevin Williams. Copromotor: Associate Prof. Yuqing Jiao.
PhD in the picture
What was the most significant finding from your research, and what aspects turned out to be most important to you?
鈥淚 would say that the most significant finding was the flexibility of our nanophotonic membrane platform in which we have realized many different functionalities. By being able to combine both high bandwidth photodetectors and on-chip lasers, many different applications such as high-speed communications and sensing of biological samples can be tackled using integrated photonics.鈥�
What was your motivation to work on this research project?
鈥淢y personal motivation to work on this project was to really do a deep dive into the full process of photonic integrated circuit (PIC) development, starting from the epitaxy design (the layer structure of the devices before doing any processing), all the way to device and circuit characterization in our optical labs. Additionally, I wanted to explore the limits of our high-speed photodetectors and really look for a seamless way to integrate these detectors with on-chip lasers.鈥�
What was the greatest obstacle that you met on the PhD journey?
鈥淲ithin my PhD journey, there were many obstacles, like in any other research project. But the greatest for me was probably the fabrication process. Maintaining focus on each and every step for multiple months in a row can be very challenging, but fortunately our research group together with the cleanroom technicians provide a lot of knowledge and help here!鈥�
What did you learn about yourself during your PhD research journey? Did you develop additional new skills over the course of the PhD research?
鈥淚 mostly learned about myself that I really enjoy performing a lot of different tasks and activities. Within my project, and also outside of that, I was able to work on many different elements of PIC development. And next to that, there were many opportunities to help out with educational activities and promotional work. I think in general, the main skill that I developed is to really be an independent researcher, and to get a feeling of what the direction of a project should be. I also learned how to carry this out and how to make this convincing to both myself and others involved.鈥�
What are your plans for after your PhD research?
鈥淚 recently joined a startup, PhotonIP, which is also located on the 果冻传媒 campus and has close ties to the PhI research group. As an R&D Engineer, I can continue working on a broad scope of tasks all related to PIC development, and it allows me to directly use my knowledge obtained during the PhD. I really enjoy working in the deep-tech industry and I am excited to get more knowledge on how to transfer this technology to market-ready products and solutions.鈥�
