Capturing Ultrafast Electron Spin

I study condensed matter physics, and my research is focused on the mechanisms by which substances exhibit their properties. Elementary particles such as quarks (which could help unlock the mysteries of the universe’s origins) may be the topic du jour in physics, but I prefer to focus on more tangible matter—something you can at least hold in your hand. I use the latest experimental techniques to find out why semiconductors, magnets, and other materials used in the devices so central to our prosperous lifestyles exhibit the properties they exhibit.

This project, Development of Extreme Ultrafast Properties of Spin Currents Using Multidimensional Photoemission Spectroscopy, which has been selected for the Japan Science and Technology Agency’s Fusion Oriented Research for Disruptive Science and Technology program, can best be understood by focusing on three key phrases in the title: multidimensional photoemission spectroscopy, spin currents, and extreme ultrafast properties.

First, spin currents: In quantum mechanics, which has developed into one of the foundations of modern physics, electrons have upward and downward spin, an intrinsic form of angular momentum, in addition to a positive and negative charge. This spin corresponds to the electron’s rotation and is the source of magnetism in matter. Thus, electrons act not only as charged particles but also as tiny magnets.

In electronics, devices function by controlling the flow of charge within semiconductor elements, and using that flow-or-no-flow state as information. In other words, electronics uses electrons as charged particles and controls their behavior (i.e., electric current). In contrast, spintronics, now a major field of study worldwide, treats electrons as tiny magnets as well as charged particles, and our quest is to control not only the flow of electric charge, but also the flow of electron spin (“spin current”). Expectations regarding devices that work based on spintronics are high because of their potential to be smaller, more energy efficient, and faster than conventional electronics.

Old and new: Electric current and spin current.

Harnessing the Potential of Nobel-Prize-winning Attosecond-scale Pulsed Laser

Developing such devices would require the ability to observe and control spin currents at ultrahigh speeds, but we don’t yet have usable technologies for this. That’s why one of the aims of my research is to establish basic principles to underpin these observation and control technologies.

Second, extreme ultrafast properties: Spin currents respond to input signals such as voltage mere attoseconds (quintillionths of a second).

The development of technologies for capturing changes in materials in such extremely short times is the focus of much global attention. This is perhaps best illustrated by the 2023 Nobel Prize in Physics, which went to researchers who demonstrated a way to create pulses of light measured in attoseconds as a means of observing such instantaneous physical changes.

Third, multidimensional photoemission spectroscopy: Just as it takes a fast shutter speed to photograph fast-moving people or objects without blurring (e.g., in an auto race photo finish), capturing ultrafast electron motion requires attosecond shutter speeds. Multidimensional photoemission spectroscopy is a new technology that uses attosecond-long laser pulses like the aforementioned camera shutter to observe spin currents. Hiroshima University’s Research Institute for Synchrotron Radiation Science (HiSOR) has the world’s most advanced research environment in terms of experimental technology for capturing electron spin in materials (Spin- and angle-resolved photoemission spectroscopy), and I intend that my JST-supported project will achieve multidimensional photoemission spectroscopy by incorporating ultra-short pulsed laser into that technology.

The spin-resolved angle-resolved photoemission spectrometer developed at HiSOR.

Capturing the direction of spin (red=upward, blue=downward) and electron motion allows for the observation of spin current.

Just Like Art: Experiment as an Expression of the World

Beyond spin current, the ultrafast phenomena of substances remain largely undiscovered. For instance, we know that irradiating a substance with a powerful laser generates heat that causes substance to shear or break, and this phenomenon is used for industrial applications. We know that the heat generated by light irradiation occurs within picoseconds (one trillionth of a second) after irradiation; what we don’t fully understood is what’s happening much earlier than that in the attosecond timeframe. To give an example, it is common knowledge that, when processing a substance, ultrashort attosecond pulses of powerful laser result in much cleaner surface finishing  than continuous blasts, but the underlying principle about why this is the case remains a mystery. Thus, the science of “ultrafast” has untold potential, not just for basic science, but for new industrial technologies, too.

As I see it, research (my own included) is art. Just as paintings, music, and other works forms of art can change people’s worldviews and methods of expression, I hope to enrich people’s worldviews through experiments that isolate and express ultrafast phenomena that cannot yet be depicted because they are still undiscovered. In other words, to develop technologies for experiments is to create new methods of expression, and research outcomes are indeed ideas for new worldviews.

When I was a student, I remember having my mind blown and my world enriched by a paper on ultrafast phenomena. After getting my degree, I went to Germany—the forefront of scientific study into ultrafast phenomena—and that world-enriching experience has continued to inspire me and helped lead to the idea for this FOREST project. It is my fervent hope that, with the JST’s support, I can harness the potential of multidimensional photoemission spectroscopy to depict the as-yet undepicted and, in doing so, create something that offers people a new worldview.