image: a, The experimental set-up for DNLS. b, Sketch of the in-plane dispersion around the Dirac nodal ring (DNR) degeneracy (golden). The DNR is four-fold degenerate, consisting of two sets of type-II Weyl nodal rings of TE (blue) and TM (red) polarizations. c and d, Measured transmission spectra for the photonic crystal sample with varied incident angles for the TE and TM plane-wave excitations. The transmission spectra fill the region bounded by the bulk bands (black dashed lines). e and f, Measured reflection spectra for the silver film/photonic crystal sample with varied incident angles for the TE and TM excitations. The black solid lines mark the numerically simulated bowl surface states dispersions. view more
Credit: by Mengying Hu, Ye Zhang, Xi Jiang, Tong Qiao, Qiang Wang, Shining Zhu, Meng Xiao, and Hui Liu
Dirac nodal line semimetals (DNLSs) play a pivotal role in the topological semimetal family featured by various unique properties. They have been observed in both electronic systems and classical waves. However, there is NO photonic DNLS in all previous works. The realization of DNLS is not a trivial work for light since the two polarizations (pseudo-spins) of photons are in general nondegenerate and the mechanisms protecting DNLS do not work for light due to the inherent distinction between fermions (electrons) and bosons (photons).
In a new paper published in Light Science & Application, a team of scientists, led by Professor Meng Xiao from Wuhan University and Professor Hui Liu from Nanjing University, China, and co-workers have provided a stringent photonic realization of type-II DNLS with a ring shape four-fold band degeneracy by exploring symmetries of an AB layered photonic crystal together with the pseudospins of photons. They measured angle-resolved transmission spectra of the DNLS and extracted the dispersions along relevant directions, indubitably evidencing the existence of photonic DNLS. Moreover, when truncated and deposited by a metallic film, the photonic DNLS exhibits a double-bowl state (DBS) comprising two sets of perpendicularly polarized (TE and TM) surface states. In stark contrast to nondegenerate surface states in other photonic systems, here the two sets of surface states are almost degenerate over the whole spectrum range, which were explicitly identified through angle-resolved reflection spectra.
These scientists summarize several advantages of their work:
“We provide a new mechanism to protect the degeneracy between TE and TM polarizations of the Dirac nodal ring, and thus our work points to new possibilities in discovering topological semimetals and may lead to other higher order topological phases.”
“Our system is simple and does not require complex structure, meanwhile the four-fold degeneracy is particularly robust against parameters variation and can be easily shrunk or expanded. These merits of our system enable the experimental realization of a stringent photonic DNLS.”
“The double-bowl surface states for TE and TM polarizations in our system are almost degenerate over the entire spectrum range, which has not been seen in any previous systems. Thus our system can serve as an ideal platform for investigating phenomena that require large field enhancement as well as manipulating light with an arbitrary polarization while studying polarization-sensitive light-matter interactions.”