Research Highlights

STROBE scientists solved a major imaging challenge by demonstrating high fidelity short wavelength imaging of highly-periodic structures for the first time, using engineered illumination via high harmonic extreme UV (EUV) beams carrying orbital angular momentum (OAM). This enables high-fidelity imaging and inspection of highly periodic structures for next-generation nano, energy and photonic devices.

Lensless imaging based on coherent diffractive imaging (CDI) enables near-perfect diffraction-limited microscopy at short wavelengths, overcoming the limits of imperfect and lossy optics. However, high fidelity imaging of highly periodic structures has been challenging. In CDI, a beam is scanned across a sample, and the scattered light is collected by a detector. A computer algorithm is then used to reconstruct an image of the sample. However, to retrieve high-fidelity images, the scatter patterns must change as the beam is scanned – which is not the case for highly periodic samples.

Graduate students Bin Wang and Nathan Brooks, working with Henry Kapteyn and Margaret Murnane, solved this long-standing challenge by using high harmonic beams carrying OAM. The high divergence and peak-intensity near their edges introduces strong interference fringes between adjacent diffraction orders in the far-field. These encode phase information into the scattered light as the beam is scanned, significantly enhancing diversity in the diffraction patterns so that the phase can be reliably retrieved. Moreover, defects in otherwise periodic grids can be more sensitively detected with improved signal-to-noise ratio > 100x, and with lower dose and sample damage than for scanning electron microscopy.

Visible laser beams carrying OAM (i.e. donut-shaped) beams revolutionized visible super-resolution microscopy. Now there is a path forward for bringing these powerful capabilities to shorter wavelengths.

STROBE Director of Outreach and Broadening Participation, Dr. Sarah Schreiner, worked with the NSF and other PREM (NSF Partnership for Research and Education in Materials) faculty to develop and organize an MRS PREM Research Scholars Symposium at the 2023 MRS Spring Meeting in San Francisco. This symposium hosted over 100 undergraduate PREM Research Scholars from around the US for two days to participate in professional development and networking activities. Dr. Schreiner ran two workshops at the symposium on Networking at Conferences and Turning Your Science into a Story. The symposium ended with a poster session for all participants. The STROBE-PREM partnership, PEAQS, had 10 students from Fort Lewis College and Norfolk State University participating in the symposium.

Circularly polarized x-rays have a number of microscopy applications that leverage the rotational nature of some physical systems, one of the most common applications being magnetic dichroism of nanoscale magnetic devices. Generating circularly polarized x-rays, particularly coherent x-rays with ultrafast pulse durations is practically difficult and inefficient. One of the major successes of STROBE was the development of not just one, but two mechanisms for generating circularly polarized x-rays using light at moderate intensities in the strong field regime. In this work, we lay the foundation for scaling analogous mechanisms into the relativistic regime. In this regime, the energy cutoff can be much higher than in the strong field. Performing numerical simulations, we confirm that despite the physical mechanism being completely different at relativistic intensities, the same conservation laws observed in the earlier STROBE work are still valid in the relativistic regime. Experiments are being planned to demonstrate this mechanism in the lab, which can leverage new facilities such as the NSF ZEUS which would enable high single shot flux in dichroism experiments.

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible, in principle, to engineer their thermal properties. However, this is currently not feasible because there is no general description for heat flow in 3D nanostructured semiconductors. In recent research, STROBE scientists used short wavelength extreme ultraviolet beams to study heat transport in a silicon metalattice with deep nanoscale features. They observed dramatically reduced thermal conductivity relative to bulk—about x50 times less than current model predictions. To explain this, they developed a new predictive theory that incorporates the idea that heat-carrying lattice vibrations can behave like a fluid—spreading out instead of just moving ballistically in straight lines. Moreover, this new theory of heat transport can be used to predict and engineer phonon transport in many other 3D nanosystems including nanowires and nanomeshes, that are of great interest for next-generation energy-efficient devices.

Though microscopes have been in use for centuries, there is still much that we cannot see at the smallest length scales. Current microscopies range from the simple optical microscopes used in high school science classes, to x-ray microscopes that can image through visibly-opaque objects, to electron microscopes that use electrons instead of light to capture images of vaccines and viruses. However, there is a great need to see beyond the static structure of an object—to be able capture how a nano- or biosystem functions in real time, or to visualize magnetic fields on nanometer scales. A team of researchers from the STROBE Center have been working together to overcome these challenges. STROBE is an NSF Science and Technology Center that is building the microscopes of tomorrow. A large multidisciplinary team from the Miao and Osher groups from UCLA, the Kapteyn-Murnane group at CU Boulder, Ezio Iacocca from CU Colorado Springs, David Shapiro and collaborators at Lawrence Berkley National Laboratory, and the Badding and Crespi groups from Pennsylvania State University. They developed and implemented a new method to use x-ray beams to capture the 3D magnetic texture in a material with very high 10-nanometer spatial resolution for the first time (published in Nature Nanotechnology, see reference below).

Hedgehogs and Anti-Hedgehogs

The team investigated a nanostructured magnetic sample, consisting of tiny spheres of nickel, only ~30nm across, connected together by slender few-nm “necks” of nickel, that together form a structure called a magnetic metalattice. This complex nanostructured magnet is expected to produce swirling magnetic fields with topological spin textures that are far more complex than in a uniform magnet. These are called 3D topological magnetic monopoles – or hedgehogs, due to their spiny shape in magnetic rotation – if the magnetic field points outward. Conversely, they can be thought of anti-hedgehogs if the magnetic field points inward.  However, until recently, there was no experimental method to measure the 3D spin texture at the deep nanoscale. Using advanced algorithms to recover the image, and a microscope at the x-ray synchrotron light source at the Lawrence Berkley National Laboratory, the researchers overcame these challenges.

Imaging spin textures is extremely important, as it can help physicists to better understand magnetism at a fundamental level, and to design more energy-efficient data storage, memory, and nanodevices.  Using electron microscopy, one can capture beautiful 2D images of a static spin-texture, but it is challenging to capture a full 3D image. In the past, other scientists were able to capture a 3D image at a spatial resolution of about 100 nanometers, but they had to make assumptions about the sample to extract the 3D image. With this new technique, researchers do not have to make any assumptions.

Armed with this new visualization technique, the team of researchers is excited to study spin textures further. STROBE is developing tabletop setups and helping with national facilities that can capture the static and dynamic spin texture in materials. All algorithms developed for this data analysis will be open-sourced soon. In this experiment, as with others, they found that collaboration is key for moving scientific progress forward.

Topological magnetic monopoles (TMMs), also known as hedgehogs or Bloch points, are three-dimensional (3D) nonlocal spin textures that are robust to thermal and quantum fluctuations due to their topology. Understanding their properties is of fundamental interest and practical applications. However, it has been difficult to directly observe the 3D magnetization vector field of TMMs and probe their interactions at the nanoscale. Now, a STROBE team from UCLA, CU Boulder, UC Berkeley and LBNL collaborated with the Penn State MRSEC reports the creation of 138 stable TMMs at the specific sites of a ferromagnetic meta-lattice at room temperature. They developed 3D soft x-ray vector ptychography to determine the magnetization vector and emergent magnetic field of the TMMs with a 3D spatial resolution of 10 nm. This spatial resolution is comparable to the magnetic exchange length of transition metals, enabling them to probe monopole-monopole interactions. The team found that the TMM and anti-TMM pairs are separated by 18.3±1.6 nm, while the TMM and TMM, anti-TMM and anti-TMM pairs are stabilized at comparatively longer distances of 36.1±2.4 nm and 43.1±2.0 nm, respectively. They also observed virtual TMMs created by magnetic voids in the meta-lattice. This work demonstrates that ferromagnetic meta-lattices could be used as a new platform to create and investigate the interactions and dynamics of TMMs. Furthermore, it is expected that soft x-ray vector ptychography can be broadly applied to quantitatively image 3D vector fields in magnetic and anisotropic materials at the nanoscale.