Inventing a radical new way of imaging magnetic motion at the nanoscale may sound daunting, but for Gregory D. Fuchs, Applied and Engineering Physics, it’s all part of the job. The Fuchs Group, which focuses on understanding the dynamics of magnetic spin systems, wants a microscope that can image both at very high spatial resolution and very high temporal resolution. This is a tall order because the electrons that produce magnetic spin, an intrinsic form of angular momentum, are moving at gigahertz frequencies.
“Quite a few magnetic microscopies can image spin at either very high spatial resolution or very high temporal resolution,” says Fuchs, “but right now, the main way to get both at the same time is to go to a facility that produces pulsed coherent x-rays, like the Advanced Light Source at the University of California, Berkeley. As beautiful as those measurements are and as lovely as Berkeley is, I want my own tabletop microscope that can do it all and live in my own lab down in the basement.”
First Step—a Microscope That Images at Extremely High Temporal Resolution
Engineering and building this new type of microscope isn’t easy, but that hasn’t stopped the Fuchs Group. “It’s a really hard problem, but we fearlessly went for it anyway,” Fuchs says with a laugh. He took stock of the microscopies that already exist for measuring at nanoscale. They fell into two camps. For very high spatial resolution, electron microscopes use extremely small wavelengths that can measure resolution of around a nanometer. These microscopes, however, can’t measure short time slices such as those that are picoseconds (one trillionth of a second) in length. For very high temporal resolution, on the other hand, optical microscopes use lasers that emit pulses as short as a femtosecond (one quadrillionth of a second), but optical diffraction limits the spatial resolution.
“As beautiful as those measurements are and as lovely as Berkeley is, I want my own tabletop microscope that can do it all and live in my own lab down in the basement.”
To create a new microscope, Fuchs decided to focus on the question of high temporal resolution first. He looked at the laser used in optical microscopes. “Optical pulses produce heat as well as light,” he says. “I thought, what if I use the heat? If I can find a heat-magnetic interaction, that can be the basis of my microscope, and in principle, heat doesn’t have a diffraction limit, which means it won’t interfere with achieving high spatial resolution.”
Fuchs and his collaborators then built a prototype microscope that uses a pulsed laser to heat a sample material, such as a magnetic cobalt film, ultimately measuring a projection of the magnetic moment (the orientation in which the electron spins are aligned), which results in temporal imaging. “It works shockingly well on every material we try it on,” Fuchs says. “It’s extremely sensitive. I would put it up favorably against any type of microscopy for measuring in-plane magnetization. We’ve used it to study spin orbit effects and for imaging magnetic resonance. We’ve generalized the technique, so we can also use it to study magnetic insulators. These are usually very hard to measure, but our new microscope can do it very effectively.”
While the researchers have succeeded in inventing a new tool with high temporal resolution and plenty of applications, they haven’t stopped there. Fuchs still wants to achieve his initial goal of combining high temporal resolution with high spatial resolution. “After years of work, we’re finally getting to the point where we can test this goal of having both at the same time,” he says. “Results are looking very promising. That’s extremely exciting and gratifying.”
Diamonds Are for Technology
The Fuchs Group has two main areas of focus. One side works on condensed matter solid state physics experiments in the fields of magnetism and spintronics—areas that benefit from Fuchs’ new microscope. The other side looks at solid state quantum systems and their application to quantum information science. “We’re interested in studying quantum properties, such as superpositions and entanglement, in solid state quantum systems by looking at defects in solid insulators,” Fuchs says. In a series of experiments, the researchers are looking at the nitrogen-vacancy (NV) center—known as a point defect or color center—in diamond, which is an especially good insulator.
“A crystalline solid like diamond has atoms arranged in a regular, repeating pattern—a lattice,” Fuchs explains. “A point defect is a small, local rearrangement in the atoms in that lattice. You make a defect in the lattice by plucking out one atom or by replacing a certain atom with a different kind. In the diamond NV center, there’s a nitrogen atom sitting on a carbon site, and on the neighboring carbon site, no atom.” Diamonds with a large number of NV centers appear pink. Pink diamonds occur naturally, which is why the diamond NV center was one of the first defect states to be studied by scientists and gemologists who saw it as desirable for the gem market.
Rather than coloring gems, the Fuchs Group wants to use NV centers for applications in quantum technology. They begin with manufactured diamonds that are highly pure, made up of only carbon, and insert a controlled amount of NV centers. Then they work out ways to measure and manipulate the resulting properties. “Spin encodes the quantum state that we’re interested in controlling and using,” Fuchs says. “So we created a mechanical device out of the diamond itself and caused the crystal to vibrate. We discovered for the first time that you can manipulate the spin using these vibrations. By bringing the spin into the same frequency of resonance as the vibration of the crystal, you can make the spin last longer.” Because quantum states are very fragile, persistence of spin is an important step toward using the NV center in an application—for instance, as an inertial sensor in a gyroscope.
These findings also have application in quantum information science. While building a viable quantum computer is still a distance in the future, the NV centers may help make that a reality. “NV centers have a coherent optical transition, which gives them good connectivity,” Fuchs says. “Other researchers have demonstrated that you can entangle the spin with the photon it emits, and that’s a way of getting the quantum information from a local to a flying qubit. We’re exploring ways to make that optical transition better by making it less sensitive.”
At the end of the day, Fuchs’ mission is to train students to be both scientists and engineers. “In my group we develop instruments, but we also use the instruments that we develop to study fundamental physics,” Fuchs says. “Even on the quantum side, engineering is a key part. You can’t separate engineering and physics. They are inextricably linked. I try to train my students in that point of view because if you want to do new things, if you want to see new things, one of the ways to do that is to look in a new way.”