Research Highlights

Atomic motion captured, for the first time, in 4D

June 26, 2019

Everyday transformations from one state of matter to another – such as freezing, melting or evaporation – start with a process called “nucleation”, in which tiny particles containing just a few atoms or molecules begin to coalesce. Nucleation plays a critical role in events as diverse as the formation of clouds and the onset of neurodegenerative disease. STROBE Deputy Director Jianwei (John) Miao, led an interdisciplinary team from Lawrence Berkeley Lab, University of Colorado Boulder, University of Buffalo and the University of Nevada Reno, to gain a never-before-seen view of nucleation –– capturing how the atoms rearrange in the tiny seed particles at atomic resolution. Their findings, published in the journal Nature, differ from predictions based on the classical theory of nucleation that has long appeared in textbooks.

J. Zhou, Y. Yang, Y. Yang, D.S. Kim, A. Yuan, X. Tian, C. Ophus, F. Sun, A.K. Schmid, M. Nathanson, H. Heinz, Q. An, H. Zeng, P. Ercius, J. Miao, "Observing crystal nucleation in four dimensions using atomic electron tomography," Nature 570, 500-503 (2019).


Stroboscopic Imaging of Nanoscale Transport

June 14, 2019

The functional properties of photovoltaics and nano-devices for electronics, thermoelectrics and data storage can be enhanced by tuning their structure at the nanoscale. However, at dimensions <100nm, bulk models can no longer accurately predict heat, charge or spin transport, or the mechanical properties of doped or nano-structured materials. STROBE Director Margaret Murnane led a team of researchers from CU Boulder, UC Berkeley, and LBNL to develop a real-time microscope to capture, map and understand nanoscale heat transport in nanostructures. This microscope was then used to validate a very surprising prediction - that an array of closely-spaced nanoscale heat sources can cool more quickly than when spaced far apart.

Karl et al., “Full-Field Functional Imaging of Nanoscale Dynamics Using Tabletop High Harmonics”, Science Advances 4, eaau4295 (2018); Frazer et al., “Engineering nanoscale thermal transport: Size- and spacing-dependent cooling of nanostructures,” Physical Review Applied 11, 024042 (2019).

Electron imaging reveals the conductance watershed between two electrodes

June 13, 2019

Traditional transmission electron microscopy (TEM) excels at determining the physical structure of a sample, but reveals little about the electronic structure. STROBE faculty Chris Regan at UCLA developed a new technique based on secondary electron emission (SE) and electron beam induced current (EBIC) to image the electronic structure of functioning devices with a TEM-like spatial resolution. He used this new SEEBIC technique to map the conductance of a nanodevice containing a thin silicon membrane (brown and green) separating two electrodes (blue).

Hubbard et al, Physical Review Applied. 10 044066 (2018).

De-blurring images of living biological samples

June 12, 2019

Imaging fast moving samples such as living biological samples is challenging because the images can appear blurred if the strobe light is not fast enough. This is an issue for high-quality quantitative phase imaging, which was too slow for many samples (image on right). STROBE faculty Laura Waller from Berkeley led a team to develop a new approach to reduce this blur, that enhances the speed to match the frame rate of the detector, to enable much clearer imaging of many biological systems (image on left).

M. Chen, Z.F. Phillips, L. Waller, Optics Express 26, 32888-32899 (2018).

Real time near-field imaging of biological and nano-systems

June 10, 2019

Label-free chemical nano-imaging in dense molecular environments has remained a long-standing challenge. STROBE faculty member Markus Raschke at Boulder led a team of academic and national laboratory scientists to speed-up scanning near-field optical microscopy (s-SNOM) by a factor of 10! This remarkable achievement allowed the team to image the surface shape and chemistry of biological samples, including mollusk shells, with nanometer spatial resolution.

MS. C. Johnson, E. A. Muller, O. Khatib, E. A. Bonnin, A. C. Gagnon, and M. B. Raschke, Optica 4, 424 (2019).

Demonstration of Electron Ghost Imaging

September 11, 2018

The first demonstration of computational ghost imaging with electrons has been carried out at UCLA. A digital micromirror device is used to directly modulate the photocathode drive laser to control the transverse distribution of a relativistic electron beam incident on a sample. Correlating the structured illumination pattern to the total sample transmission then retrieves the target image, avoiding need for a pixelated detector. In our example, we use a compressed sensing framework to improve the reconstruction quality and reduce the number of shots compared to raster scanning a small beam across the target. Compressed electron ghost imaging can reduce both acquisition time and sample damage in experiments for which spatially resolved detectors are unavailable (e.g. spectroscopy) or in which the experimental architecture precludes full frame direct imaging.

S. Li, F. Cropp, K. Kabra, T. J. Lane, G. Wetzstein, P. Musumeci, and D. Ratner, "Electron Ghost Imaging," Phys. Rev. Lett. 121, 114801 (2018).

Super-Resolution Imaging of Clickable Graphene Nanoribbons Decorated with Fluorescent Dyes

July 05, 2018

As part of STROBE’s research into super-resolution visible microscopy, Xu and collaborators are developing techniques to enable super-resolution microscopy for graphene nanoribbons (GNRs) and carbon nanotubes (CNTs). Graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) are important functional materials with great promise in nano-electronics and other applications. However, it has been a challenge to visualize and characterize GNRs and CNTs in a high-throughput manner due to their small physical dimensions.  Temporal fluorescence intensity fluctuations in these samples enabled the reconstruction of SRM images through superresolution optical fluctuation imaging (SOFI) and the related superresolution radial fluctuation (SRRF) analysis methods. They obtained a spatial resolution of ~50 nm on glass substrates (Panel A), as well as device-relevant substrates such as bare Si (Panel B) and Si/SiO2 (Panel C) wafers. This enabled them to characterize the nanoscale structures of these functional nanomaterials in a nondestructive, high-throughput manner.

D. Joshi, M. Hauser, G. Veber, A. Berl, K. Xu, F.R. Fischer, “Super-Resolution Imaging of Clickable Graphene Nanoribbons Decorated with Fluorescent Dyes,” Journal of the American Chemical Society 140, 9574 - 9580 (2018).

Wave-front shaping controls nonlinearity in complex media

May 07, 2018

Wavefront shaping in random media is a topic of great interest with both fundamental implications and many new exciting applications such as imaging through turbid media, looking through fog, and using multimode fibers as ultra-thin endoscopes. A fascinating subfield encompasses the dynamics of propagating modes in multimode fibers, involving intriguing physics and opportunities for application. In particular, nonlinearities in multimode fibers remains a largely unexplored field with opportunities to exploit the multimode degrees of freedom for controlling spatial-spectral-temporal interactions. A University of Colorado team, supported by the STROBE Science and Technology Center, has demonstrated the synergy between wavefront shaping and nonlinearity in complex media such as multimode fibers. The team implemented a new kind of control that enables optimization of highly non-linear interactions through fibers. Tailoring the wave-front shape at the input, the system controls the generation of nonlinear phenomena known as stimulated-Raman-scattering cascades and four-wave-mixing, generating and shaping the different colors of the light pulses at the output of the fiber. This research has implications for the understanding of so-called adaptive systems, optical communications, fiber lasers, and imaging.

O. Tzang, A. Miguel Caravaca Aguirre , Kelvin Wagner, and R. Piestun, “Adaptive wavefront shaping for controlling nonlinear multimode interactions in optical fibres” Nature Photonics 12, 368–374 (2018).

Sub-ångström cryo-EM structure of a prion protofibril reveals a polar clasp

January 15, 2018

Brain dysfunction can be caused by aggregated proteins such as prions. In humans, the prion protein causes an infection by changing its shape and transforming into rope like aggregates. In this way, stable prion aggregates are immune to normal processes of destruction in an organism and bring about neuronal death and ultimately disease. There is no molecular explanation for the variable efficiency of prion spread between species. To investigate this phenomenon, we use cryo electron microscopy to reveal the atomic structures of regions within prion that have a high propensity to form structured aggregates. In an article published this year in Nature Structural and Molecular Biology (Gallagher-Jones et al. 2018), we take an atomic look at structural motifs that may represent the molecular basis for a prion species barrier.

M. Gallagher-Jones, C. Glynn, D.R. Boyer, M.W. Martynowycz, E. Hernandez, J. Miao, C.T. Zee, I.V. Novikova, L. Goldschmidt, H.T. McFarlane, G.F. Helguera, J.E. Evans, M.R. Sawaya, D. Cascio, D.S. Eisenberg, T. Gonen, J.A. Rodriguez, “Sub-ångström cryo-EM structure of a prion protofibril reveals a polar clasp,” Nature Structural & Molecular Biology 25, 131 - 134 (2018).

Sub-wavelength coherent diffractive imaging using a tabletop high harmonic light source

March 20, 2017

Visible microscopes can produce crisp images with a spatial resolution on order of the illuminating wavelength, because of the availability of near-perfect lenses in this region of the spectrum. Extreme ultraviolet (EUV) and soft X-ray (SXR) light has wavelengths 10-100 times shorter than visible light: thus, it should be possible to design a powerful microscope that can image structures that are too small or too opaque to be seen with visible light. However, EUV/SXR lenses are very lossy and imperfect, limiting the advantage of using shorter wavelengths, and blurring the resulting images to >8 times the theoretical limit. Fortunately, new techniques pioneered by STROBE scientists Kapteyn, Murnane and Miao make it possible to build lensless microscopes illuminated by coherent laser-like beams — a capability that is revolutionizing X-ray imaging worldwide. Very recently, the Kapteyn-Murnane group at CU Boulder used tabletop EUV beams at a wavelength of 13nm to achieve sub-wavelength spatial resolution imaging at short wavelengths for the first time – essentially demonstrating the first near-perfect X-ray microscope. Moreover, because the EUV source produces exceedingly short, femtosecond (~10-15 sec), bursts of light, it can now be used to make stroboscopic movies to observe how the nanoworld functions. STROBE graduate student Dennis Gardner received the American Physical Society Division of Laser Science Thesis Award for this work.   

D. Gardner, M. Tanksalvala, E. Shanblatt, X. Zhang, B. Galloway, C. Porter, R. Karl, C. Bevis, D. Adams, H. Kapteyn, M. Murnane, G. Mancini, Nature Photonics 11, 259–263 (2017).