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INSIGHTS | PERSPECTIVES

a near-field antenna scanned the top sur- than on the surface of a 3D Hermitian sys- tant advances in topological photonics for
face to map out the bulk and surface modes tem, and originates from the non-Hermiticity many reasons. First, the demonstration of
while an excitation source was placed ei- of the system, rather than from the pres- ideal Weyl points and establishing the con-
ther at the center of the bottom layer of the ence of Weyl points. Similar to the surface nection between topological concepts and
meta-crystal stack or at the edge of the top Fermi arc connecting Weyl points of opposite non-Hermiticity open a broader avenue
surface. These measurements revealed the charges (±1), this bulk Fermi arc connects a for fundamental research of the emergent
presence of two helicoidal surface Fermi pair of EPs with opposite half-charges (±1/2). fields of topological physics and photonics.
arcs connecting bulk states with opposite The EPs in the study of Zhou et al. were Second, they hold the promise for practi-
topological charges. formed via radiative losses on a 2D periodic cal applications. For example, ideal Weyl
The concepts of topologically nontrivial photonic crystal with a rhombic lattice hav- points may lead to enhancement of trans-
phases have been developed for closed ing a Dirac point with nontrivial Berry phase. port induced by chiral anomaly and are ex-
(Hermitian) physical systems, but in pho- The losses transform the Dirac points into pected to be useful for invisibility cloaking
tonics, systems are open, and the pertur- EPs. Zhou et al. characterized the sample us- and 3D-imaging, as well as provide better
bations can cause loss or gain of signal. ing angle-resolved scattering measurements angular and frequency selectivity (1, 9, 11,
Such non-Hermitian systems may have ex- in which a tunable laser illuminated the sam- 12). Similarly, as suggested by Zhou et al.,
ceptional point (EP) degeneracies—where ple and a charge-coupled device camera col- half topological charges in polarization can
not only the eigenvalues but also the cor- lected the scattered light. This direct probing be used for the generation of half-integer
responding eigenvectors coalesce (13)—that revealed open-ended isofrequency contours vector-vortex beams. Third, observation of
are different from Hermitian degeneracies. connecting a pair of EPs, similar to surface a bulk Fermi arc and topological charge
Non-Hermiticity will likely provide a richer Fermi arcs connecting the Weyl points. Far- in an open system begins to establish and
topological landscape with no counterpart field polarization measurements showed that explore the connection between topologi-
in Hermitian systems (14). the polarization vector experiences a 180° cal concepts and non-Hermitian systems.
Zhou et al. report the observation of a winding around the Fermi arc, correspond- Fourth, the platforms developed will allow
Fermi arc and its relation to the topological ing to a half-integer topological charge that is for the exploitation of distinctive properties Downloaded from
features of a non-Hermitian system. Different a manifestation of the ±1/2 topological index of Weyl points in Hermitian systems and
than the Fermi arc reported by Yang et al., the of the EPs in the system. topological features in non-Hermitian sys-
Fermi arc in this study resides in the bulk dis- The observations reported in this issue tems to their full extent.
persion of a 2D non-Hermitian system, rather by Yang et al. and Zhou et al. are impor- Although it remains to be seen how the
reported observations will lead to novel
devices and technological advances, they
Creating Weyl and exceptional will surely help to explore photonics in new
E
points from a Dirac point regimes—for example, by including nonlin- http://science.sciencemag.org/
earities and quantum effects—and lead to
By breaking parity (P) or time-reversal (T) symmetry, better ways of controlling and engineer-
a Dirac point (DP) splits into two or four Weyl ing light and its interaction with matter.
points (WPs) of opposite chiralties (± topological
charges, blue or red). By adding loss, a DP splits There are still many issues to address. For
into two exceptional points (EP) of opposite k instance, how can one realize ideal Weyl
chiralities (yellow and orange). x points in optical frequencies? Is there a
connection between Weyl points and EPs?
Last, how do non-Hermitian perturbations
E on March 1, 2018
k y in the form of induced gain or loss affect
– WP + Weyl points, topological charges, and sur-
face Fermi arcs? j
Fermi arc
Breaking WP + – REFERENCES AND NOTES
P-symmetry
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(2016).
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connected by surface Fermi arcs. Ideal WPs exist 3. B. Yang et al., Nat. Commun. 8, 97 (2017).
at the same energy. 4. J. Noh et al., Nat. Phys. 13, 611 (2017).
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k
k y x (2014).
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DP 14. D. Leykam, K. Y. Bliokh, C. Huang, Y. D. Chong, F. Nori, Phys.
k y Rev. Lett. 118, 040401 (2017).
– EP ACKNOWLEDGMENTS
Bulk Fermi arc
Dirac point EP + S.K.O. is supported by ARO grant No. W911NF-18-1-0043 and GRAPHIC: N. CARY/SCIENCE
Pennsylvania State University, Materials Research Institute.
Dispersion relations show energy (E) as a
function of particle momentum in the x Exceptional points
and y directions, k and k . EPs are connected by bulk Fermi arcs. 10.1126/science.aar8210
x
y
996 2 MARCH 2018 • VOL 359 ISSUE 6379 sciencemag.org SCIENCE
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