laser corresponds to intracavity powers > 100 k W.
Consequently, systematic exploration of this laser
and future engineering of practical instruments
will require application of known techniques for
avoiding fiber damage. Furthermore, direct measurements of the 3D field will be an important step
for increased understanding of STML. Ongoing
developments in this direction are promising ( 24).
Investigation of 3D mode-locking will connect
scientifically to areas well beyond short-pulse
generation. The dimensionality and controllable
dissipation and disorder make consideration of
wave turbulence and condensation ( 25, 26) especially interesting. With longer or suitably designed
fibers, disorder can be introduced controllably,
thereby bridging the gap to random lasers and
allowing exploration of the entire parameter space
of MM lasing ( 19, 20). Although disorder is weak
in the present work, recent research indicates that
MM self-organization may be robust to disorder
( 12–16). This will be important for applications,
which will ultimately require environmental stability. In addition, mode-locked lasers are closely
related to coherently driven microresonators, where
the influence of higher-order transverse modes is
starting to be considered ( 27–29). Spatiotemporal
mode-locking and related phenomena should be
possible in these systems and should provide new
features for frequency comb applications.
MM mode-locked lasers have considerable potential for high performance. Initial results obtained with the second cavity (150-nJ and 150-fs
pulses, for ~1-MW peak power and ~10-W average power) already rival the best achieved with
flexible, large-area single-mode fibers ( 30). With
larger fiber-core areas, we expect that, in principle, scaling of pulse energy by more than two
orders of magnitude will be possible (figs. S20 and
S21). At the highest pulse energies, simulations
predict the emergence of a stable MM Gaussian
output beam, which will be useful for applications (fig. S18).
Exploration of the full scope of MM mode-locking will require developments on several
fronts, including measurement capabilities.
The ability to generate high-power and spatiotemporally engineered coherent light fields should
lead to breakthroughs in laser science, as well
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L.G. W. performed experiments and simulations. L.G. W. and F. W. W.
wrote the first drafts of the manuscript. F. W. W. and D.N.C.
provided funding and supervised the project. All authors
contributed to the final manuscript. L.G. W. and F. W. W. have
submitted a patent application for spatiotemporally mode-locked
lasers. This work was supported by Office of Naval Research grant
N00014-13-1-0649 and NSF grant ECCS-1609129. We thank
Z. Ziegler for discussions and contributions to some of the
numerical codes used for MM GNLSE simulations, Z. Liu for
discussions and his early contribution to the work, E. Falco for the
laser schematic in Fig. 1F, and W. Renninger for his critical review
of an early version of the manuscript. All data presented in this
Report and the supplementary materials are available on
reasonable request to L.G. W.
Materials and Methods
Figs. S1 to S33
References ( 31–35)
14 June 2017; accepted 30 August 2017
superstructures at general grain
boundaries in a nickel-bismuth alloy
Zhiyang Yu,1 Patrick R. Cantwell,2 Qin Gao, 3 Denise Yin,1 Yuanyao Zhang, 4
Naixie Zhou, 4 Gregory S. Rohrer, 5 Michael Widom, 3 Jian Luo, 4† Martin P. Harmer1†
The properties of materials change, sometimes catastrophically, as alloying elements and
impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that
regular atomic patterns often arise as a result of this solute segregation at high-symmetry
boundaries, but it is not known whether superstructures exist at general grain boundaries
in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered,
segregation-induced grain boundary superstructures occur at randomly selected general
grain boundaries, and that these reconstructions are driven by the orientation of the
terminating grain surfaces rather than by lattice matching between grains. This discovery
shows that adsorbate-induced superstructures are not limited to special grain boundaries
but may exist at a variety of general grain boundaries, and hence they can affect the
performance of polycrystalline engineering alloys.
Many properties of polycrystalline metals and ceramics are intimately linked to the structure and composition of their grain boundaries (1). Alloying elements, dopants, and impurities are often pre-
sent in higher concentrations at grain bounda-
ries than in grain interiors, an effect known as
grain boundary segregation or adsorption. Seg-
regation can enhance macroscopic properties (2)
but often leads to severe degradation of proper-
ties and performance ( 3, 4). Understanding how
and why this degradation occurs at the atomic
scale is a crucial step toward engineering inno-
vative materials that can resist such deleterious
A recent advance in materials engineering is
the discovery that grain boundaries behave in a
phase-like manner, transitioning from one state
to another as a function of temperature and composition ( 5–8). The term “complexion” has been
introduced to distinguish such interfacial states
from bulk phases ( 9). Complexions have been
discovered at dislocations ( 10), twin boundaries
( 11), and stacking faults ( 12), and they play a role
in nanocrystalline alloys ( 13). However, the exact
structural arrangement of adsorbates within complexions and the resultant impact on properties
are still largely unknown.
SCIENCE sciencemag.org 6 OCTOBER 2017 • VOL 358 ISSUE 6359 97
1Department of Materials Science and Engineering, Lehigh
University, Bethlehem, PA 18015, USA. 2Department of
Mechanical Engineering, Rose-Hulman Institute of Technology,
Terre Haute, IN 47803, USA. 3Department of Physics,
Carnegie Mellon University, Pittsburgh, PA 15213, USA.
4Department of NanoEngineering, Program of Materials
Science and Engineering, University of California, San Diego,
La Jolla, CA 92093, USA. 5Department of Materials Science
and Engineering, Carnegie Mellon University, Pittsburgh, PA
*Present address: School of Materials Science and Engineering,
Xiamen University of Technology, Xiamen 361024, China.
†Corresponding author. Email: firstname.lastname@example.org (M.P.H.); jluo@
RESEARCH | REPORTS