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1. Pseudo-spin switches and Aharonov-Bohm effect for topological boundary modes

   https://doi.org/10.1126/sciadv.adn6095  

   Kawaguchi, Yuma; Smirnova, Daria; Komissarenko, Filipp; Kiriushechkina, Svetlana; Vakulenko, Anton; Li, Mengyao; Alù, Andrea; Khanikaev, Alexander B (April 2024, Science Advances)

   Topological boundary modes in electronic and classical-wave systems exhibit fascinating properties. In photonics, topological nature of boundary modes can make them robust and endows them with an additional internal structure—pseudo-spins. Here, we introduce heterogeneous boundary modes, which are based on mixing two of the most widely used topological photonics platforms—the pseudo-spin–Hall-like and valley-Hall photonic topological insulators. We predict and confirm experimentally that transformation between the two, realized by altering the lattice geometry, enables a continuum of boundary states carrying both pseudo-spin and valley degrees of freedom (DoFs). When applied adiabatically, this leads to conversion between pseudo-spin and valley polarization. We show that such evolution gives rise to a geometrical phase associated with the synthetic gauge fields, which is confirmed via an Aharonov-Bohm type experiment on a silicon chip. Our results unveil a versatile approach to manipulating properties of topological photonic states and envision topological photonics as a powerful platform for devices based on synthetic DoFs. 

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2. Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

   https://doi.org/10.1073/pnas.2120553119  

   Sun, Qiyang; Wei, Bin; Su, Yaokun; Smith, Hillary; Lin, Jiao Y.; Abernathy, Douglas L.; Li, Chen (July 2022, Proceedings of the National Academy of Sciences)

   The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed “geometry-forbidden” neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions. 

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3. On-chip spin-orbit locking of quantum emitters in 2D materials for chiral emission

   https://doi.org/10.1364/OPTICA.463481  

   Ma, Yichen; Zhao, Haoqi; Liu, Na; Gao, Zihe; Mohajerani, Seyed_Sepehr; Xiao, Licheng; Hone, James; Feng, Liang; Strauf, Stefan (August 2022, Optica)

   Light carries both spin angular momentum (SAM) and orbital angular momentum (OAM), which can be used as potential degrees of freedom for quantum information processing. Quantum emitters are ideal candidates towards on-chip control and manipulation of the full SAM–OAM state space. Here, we show coupling of a spin-polarized quantum emitter in a monolayer ${\mathrm{W}\mathrm{S}\mathrm{e}}_2$with the whispering gallery mode of a ${\mathrm{S}\mathrm{i}}_3{\mathrm{N}}_4$ring resonator. The cavity mode carries a transverse SAM of $\mathrm{\sigma <\#comment/>}=\pm <\#comment/>1$in the evanescent regions, with the sign depending on the orbital power flow direction of the light. By tailoring the cavity–emitter interaction, we couple the intrinsic spin state of the quantum emitter to the SAM and propagation direction of the cavity mode, which leads to spin–orbit locking and subsequent chiral single-photon emission. Furthermore, by engineering how light is scattered from the WGM, we create a high-order Bessel beam which opens up the possibility to generate optical vortex carrying OAM states. 

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4. Harnessing negative refraction and evanescent waves toward super-resolution Lamb wave imaging

   https://doi.org/10.1063/5.0152717  

   Danawe, Hrishikesh; Tol, Serife (July 2023, Applied Physics Letters)

   We numerically and experimentally demonstrate super-resolution focusing of the lowest anti-symmetric (A0) mode Lamb waves in a thin aluminum plate. The subwavelength focusing/imaging is achieved by exploiting the anisotropy in phononic crystal (PC) lattices and amplification of evanescent waves. To this end, we embedded a PC flat lens in the aluminum plate, consisting of holes arranged in a square lattice formation. We revealed that the bound slab phonon modes amplify evanescent waves, as previously observed for electromagnetic and acoustic waves. Hence, the slab mode helps propagate subwavelength information through the PC lens to reach the near-field image formed due to negative refraction and result in the high resolution image. 

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5. Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe3

   https://doi.org/10.1038/s41467-022-31612-w  

   Chen, Lebing; Mao, Chengjie; Chung, Jae-Ho; Stone, Matthew B.; Kolesnikov, Alexander I.; Wang, Xiaoping; Murai, Naoki; Gao, Bin; Delaire, Olivier; Dai, Pengcheng (December 2022, Nature Communications)

   Abstract Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe 3 , spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe 3 , suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons. 

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