Kazuhiro Morimoto: “rising star” of nanoscience

Imaging systems are central to a growing body of technology, spanning everything from driverless vehicles to virtual and augmented reality, navigation systems, and more. As part of his PhD research at EPFL’s Advanced Quantum Architecture (AQUA) Laboratory, Morimoto worked on a device that, in 2020, gained critical acclaim as a major advance in 3D-imaging technology. Developed under the supervision of Prof. Edoardo Charbon, the world’s first million-pixel camera set new global benchmarks for speed and resolution. The Japanese engineer, who also holds the world record for the smallest pixel, was recently named one of four “rising stars who are reshaping nanoscience” by Nature.

Being named as a rising star shows just how much attention your work is attracting. Various experts in the field have contacted you about using this technology, and Prof. Charbon said you completed what would normally be a 4-year task in just about half of that time. What’s your secret?

Image sensors are judged according to three essential criteria: spatial resolution, temporal resolution, and single-photon resolution. Our camera is still the only technology that performs perfectly on all three counts. Even the best sensors on the market fall short in one area or another. The secret of our device lies in miniaturization: by shrinking the size of the pixels, we were able to fit a million of them on a single-photon avalanche diode (SPAD), a component that detects single photons and converts them into electrical signals, which are then translated into digital information.

As for my work, I owe a large part of my success to Prof. Charbon. When I contacted him in 2018, I was looking for an opportunity to work on improving SPADs, given the immense promise they hold in so many areas of industry and research. His enthusiasm about the potential applications of this technology really spurred me on.

Did you have a “Eureka” moment as you were working on the device?

Not really. It was more a question of thinking long and hard. We had a goal in mind: achieving a resolution of a million pixels. But we didn’t know how we were going to get there. Before I started my research, Prof. Charbon and I sat down on numerous occasions to thrash out our ideas, jotting down our thoughts on a whiteboard and in notebooks. We talked about theory, design, tensors and so on. To be honest, what we were aiming for seemed pretty much impossible. But somehow, against the odds, we made it happen. I began designing the sensor some 18 months later, after running a battery of successful simulations.

I still remember seeing the image appear on my computer screen: it was the first ever observation of a pulse of light moving in three dimensions.

Why did you choose to complete your PhD at the EPFL School of Engineering ?

I started working for Canon in Kanagawa, Japan, in 2013. The firm runs a program that lets employees take a sabbatical to conduct R&D in another country. I wanted to do something related to single-photon detection, so I started looking around for a suitable host institution. That’s when I came across AQUA: the engineers there are doing cutting-edge research, and they’d already hosted PhD students working on similar subjects over the previous 15 years. EPFL is also regarded highly as an engineering school in Japan. So that’s how I ended up coming to Switzerland in 2017. Since returning home in 2019, I’ve continued authoring papers with my colleagues at the lab.

What’s your fondest memory of your time at EPFL?

I’ll never forget the moment when I got my camera – known as MegaX – working for the first time. Semiconductor image sensors can take months to design, and then many more months to build, so you often end up waiting years before you see the end result. In my case, I received the first test sensor a full two years after starting my research. It felt like an eternity. Early teething problems meant I had to spend several weeks meticulously fixing bugs in the system before the camera finally worked. I still remember seeing the image appear on my computer screen: it was the first ever observation of a pulse of light moving in three dimensions.

What are the next steps for MegaX? And what’s the current focus of your research, now that you’re back working at Canon?

We’ve already used the sensor in our own work, as have colleagues at other institutions. It’s even featured in papers authored by EPFL researchers, for applications such as distance calculations, vehicles and biomedical imaging. I’ve also managed to shrink the pixel even further (down to 2.2 microns), and I currently hold the world record for the smallest SPAD pixel size. More generally, I’m continuing my work as a semiconductor systems engineer, focusing specifically on imaging technology. I enjoy being in industry and collaborating with researchers in different fields, such as software development, optics, and electrical and mechanical engineering. It’s incredibly exciting work.