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1. Hyperbolic shear polaritons in low-symmetry crystals

   https://doi.org/10.1038/s41586-021-04328-y  

   Passler, Nikolai C. ; Ni, Xiang ; Hu, Guangwei ; Matson, Joseph R. ; Carini, Giulia ; Wolf, Martin ; Schubert, Mathias ; Alù, Andrea ; Caldwell, Joshua D. ; Folland, Thomas G. ; et al ( February 2022 , Nature)

   Abstract The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase 1–4 . Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales 5 . Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response 6,7 . Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic 1,3,4 and hexagonal 8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our resultsmore »will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals 10 , many common oxides 11 and organic crystals 12 , greatly expanding the material base and extending design opportunities for compact photonic devices.« less
2. Reconfigurable hyperbolic polaritonics with correlated oxide metasurfaces

   https://doi.org/10.1038/s41467-022-32287-z  

   Aghamiri, Neda Alsadat ; Hu, Guangwei ; Fali, Alireza ; Zhang, Zhen ; Li, Jiahan ; Balendhran, Sivacarendran ; Walia, Sumeet ; Sriram, Sharath ; Edgar, James H. ; Ramanathan, Shriram ; et al ( August 2022 , Nature Communications)

   Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize their practical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonal boron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO₃), transferred on nanoscale oxygen vacancy patterns on the surface of prototypical correlated perovskite oxide, samarium nickel oxide, SmNiO₃(SNO). Using a combination of scanning probe microscopy and infrared nanoimaging techniques, we demonstrate nanoscale reconfigurability of complex hyperbolic phonon polaritons patterned at the nanoscale with high resolution. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale programmable metasurface engineering for reconfigurable nanophotonic applications.
3. Programmable Bloch polaritons in graphene

   https://doi.org/10.1126/sciadv.abe8087  

   Xiong, Lin ; Li, Yutao ; Jung, Minwoo ; Forsythe, Carlos ; Zhang, Shuai ; McLeod, Alexander S. ; Dong, Yinan ; Liu, Song ; Ruta, Frank L. ; Li, Casey ; et al ( May 2021 , Science Advances)

   Efficient control of photons is enabled by hybridizing light with matter. The resulting light-matter quasi-particles can be readily programmed by manipulating either their photonic or matter constituents. Here, we hybridized infrared photons with graphene Dirac electrons to form surface plasmon polaritons (SPPs) and uncovered a previously unexplored means to control SPPs in structures with periodically modulated carrier density. In these periodic structures, common SPPs with continuous dispersion are transformed into Bloch polaritons with attendant discrete bands separated by bandgaps. We explored directional Bloch polaritons and steered their propagation by dialing the proper gate voltage. Fourier analysis of the near-field images corroborates that this on-demand nano-optics functionality is rooted in the polaritonic band structure. Our programmable polaritonic platform paves the way for the much-sought benefits of on-the-chip photonic circuits.
4. Magnetic Dual Chiral Density Wave: A Candidate Quark Matter Phase for the Interior of Neutron Stars

   https://doi.org/doi.org/ 10.3390/universe7120458  

   Ferrer, Efrain J. ; Incera, Vivian ( November 2021 , Universe)

   MDPI (Ed.)

   In this review, we discuss the physical characteristics of the magnetic dual chiral density wave (MDCDW) phase of dense quark matter and argue why it is a promising candidate for the interior matter phase of neutron stars. The MDCDW condensate occurs in the presence of a magnetic field. It is a single-modulated chiral density wave characterized by two dynamically generated parameters: the fermion quasiparticle mass m and the condensate spatial modulation q. The lowest- Landau-level quasiparticle modes in the MDCDW system are asymmetric about the zero energy, a fact that leads to the topological properties and anomalous electric transport exhibited by this phase. The topology makes the MDCDW phase robust against thermal phonon fluctuations, and as such, it does not display the Landau–Peierls instability, a staple feature of single-modulated inhomogeneous chiral condensates in three dimensions. The topology is also reflected in the presence of the electromagnetic chiral anomaly in the effective action and in the formation of hybridized propagating modes known as axion-polaritons. Taking into account that one of the axion-polaritons of this quark phase is gapped, we argue how incident γ-ray photons can be converted into gapped axion-polaritons in the interior of a magnetar star in the MDCDW phasemore »leading the star to collapse, a phenomenon that can serve to explain the so-called missing pulsar problem in the galactic center« less
5. Magnetic Dual Chiral DensityWave: A Candidate Quark Matter Phase for the Interior of Neutron Stars

   Efrain J Ferrer and Vivian de la Incera ( November 2021 , Universe)

   MDPI (Ed.)

   In this review, we discuss the physical characteristics of the magnetic dual chiral density wave (MDCDW) phase of dense quark matter and argue why it is a promising candidate for the interior matter phase of neutron stars. The MDCDW condensate occurs in the presence of a magnetic field. It is a single-modulated chiral density wave characterized by two dynamically generated parameters: the fermion quasiparticle mass m and the condensate spatial modulation q. The lowest-Landau-level quasiparticle modes in the MDCDW system are asymmetric about the zero energy, a fact that leads to the topological properties and anomalous electric transport exhibited by this phase. The topology makes the MDCDW phase robust against thermal phonon fluctuations, and as such, it does not display the Landau–Peierls instability, a staple feature of single-modulated inhomogeneous chiral condensates in three dimensions. The topology is also reflected in the presence of the electromagnetic chiral anomaly in the effective action and in the formation of hybridized propagating modes known as axion-polaritons. Taking into account that one of the axion-polaritons of this quark phase is gapped, we argue how incident g-ray photons can be converted into gapped axion-polaritons in the interior of a magnetar star in the MDCDW phase leadingmore »the star to collapse, a phenomenon that can serve to explain the so-called missing pulsar problem in the galactic center.« less