Bulk Modes Research Articles

Development of a MnOOH Mineral Electrocatalyst for Water Splitting by Controlling the Surface Defects of a Naturally Occurring Ore

Abstract A MnOOH-based mineral electrocatalyst for the oxygen evolution reaction was developed using a natural ore that is typically insulating, simply by applying a ball milling treatment. This material catalytically decomposed water molecules to generate oxygen. Mn K-edge X-ray absorption spectroscopy analyses in the bulk and surface sensitive modes indicated that structural distortion at the surface provided the catalytically active sites. The formation of oxygen vacancies on natural ore surfaces is likely to be the key to developing efficient mineral electrocatalysts.

Electrostatic bulk waves propagation in a slab delay line of metallic nanowire-based hyperbolic metamaterials

As a new planar electrostatic (or, in a better word, quasi-electrostatic) waveguide, the characteristics of electrostatic bulk waves guided by a slab of metallic nanowire-based hyperbolic metamaterials are studied. The dispersion relation of the waves is obtained using the zig-zag ray approach based on the stationary-phase method which is the same as the result obtained from the field analysis. Also, we present a description for the investigation of the group velocity and associated group delay time of electrostatic bulk modes of the system. The analytical results based on the zig-zag ray approach show the group delay is produced partly at the boundaries of the slab and partly inside the slab. The former is not dependent on the order of the mode and can be neglected compared with the latter in the neighborhood of the frequency of resonance where electrostatic wave devices are usually operated.

Ferromagnetic Resonance Modes in the Exchange-Dominated Limit in Cylinders of Finite Length

We analyze the magnetic mode structure of axially magnetized finite-length nanoscopic cylinders in a regime where the exchange interaction dominates, along with simulations of the mode frequencies of the ferrimagnet yttrium iron garnet. For the bulk modes, we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized ``cap modes'' that are ``trapped'' at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semiquantitative explanations are given for some of the modes, in terms of a one-dimensional Schrodinger equation, which is valid in the exchange-dominant case. The assignment of the azimuthal-mode number is carefully discussed, and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

Observation of hybrid higher-order skin-topological effect in non-Hermitian topolectrical circuits

Robust boundary states epitomize how deep physics can give rise to concrete experimental signatures with technological promise. Of late, much attention has focused on two distinct mechanisms for boundary robustness—topological protection, as well as the non-Hermitian skin effect. In this work, we report the experimental realizations of hybrid higher-order skin-topological effect, in which the skin effect selectively acts only on the topological boundary modes, not the bulk modes. Our experiments, which are performed on specially designed non-reciprocal 2D and 3D topolectrical circuit lattices, showcases how non-reciprocal pumping and topological localization dynamically interplays to form various states like 2D skin-topological, 3D skin-topological-topological hybrid states, as well as 2D and 3D higher-order non-Hermitian skin states. Realized through our highly versatile and scalable circuit platform, theses states have no Hermitian nor lower-dimensional analog, and pave the way for applications in topological switching and sensing through the simultaneous non-trivial interplay of skin and topological boundary localizations.

Topological photonic Su-Schrieffer-Heeger-type coupler

We examine the coupling process between the surface modes of a Su-Schrieffer-Heeger lattice both in the linear and the nonlinear regimes. We first develop a coupled-mode theory formalism for the modes of a finite lattice with zero boundary conditions. Our analysis relies on the closed-form expressions for the bulk and the surface eigenmodes of the system. The coupled-mode theory formalism is based on a decomposition of the supermodes into sublattice modes. In the case of the two zero sublattice surface modes, this leads to periodic oscillations between them without the involvement of the bulk modes. We analytically show that launching light only on the waveguide that is located at either edge of the array, can be very effective in successfully exciting the respective surface mode. We extend our analysis, in the case of Kerr nonlinearity, and develop a simplified model that accounts only for the surface modes. By direct numerical simulations, we find that this model can very accurately capture the dynamics of the surface modes when the nonlinearity is small or moderate. On the other hand, in the case of strong nonlinearity, wave mixing leads to the quasi-periodic excitation of the bulk modes, or even to a chaotic behavior where all the modes of the systems are excited, and no prominent signature of the surface modes can be detected.

Qubit-photon bound states in topological waveguides with long-range hoppings

Quantum emitters interacting with photonic band-gap materials lead to the appearance of qubit-photon bound states that mediate decoherence-free, tunable emitter-emitter interactions. Recently, it has been shown that when these band-gaps have a topological origin, like in the photonic SSH model, these qubit-photon bound states feature chiral shapes and certain robustness to disorder. In this work, we consider a more general situation where the emitters interact with an extended SSH photonic model with longer range hoppings that displays a richer phase diagram than its nearest-neighbour counterpart, e.g., phases with larger winding numbers. In particular, we first study the features of the qubit-photon bound states when the emitters couple to the bulk modes in the different phases, discern its connection with the topological invariant, and show how to further tune their shape through the use of giant atoms, i.e., non-local couplings. Then, we consider the coupling of emitters to the edge modes appearing in the different topological phases. Here, we show that giant atoms dynamics can distinguish between all different topological phases, as compared to the case with local couplings. Finally, we provide a possible experimental implementation of the model based on periodic modulations of circuit QED systems. Our work enriches the understanding of the interplay between topological photonics and quantum optics.

Topological protection in a strongly nonlinear interface lattice

Mechanical topological insulators are well understood for linear and weakly nonlinear systems, however traditional analysis methods break down for strongly nonlinear systems since linear methods can not be applied in that case. We study one such system in the form of a one-dimensional mechanical analog of the Su-Schrieffer-Heeger interface model with strong nonlinearity of the cubic form. The frequency-energy dependence of the nonlinear bulk modes and topologically insulated mode is explored using Numerical continuation of the system's nonlinear normal modes (NNMs), and the linear stability of the NNMs are investigated using Floquet Multipliers (FMs) and Krein signature analysis. We find that the nonlinear topological lattice supports a family of topologically insulated NNMs that are parameterized by the total energy of the system and are stable within a range of frequencies. Next, it is shown that empirical calculations of the geometric Zak Phase can define an energy threshold to predict the excitability of the nonlinear topological mode, and that this threshold coincides with the energy that the topological NNM intersects the linear bulk-spectrum. These predictions are validated with numerical simulations of the nonlinear topological system. These results are also tested for parametric perturbations which preserve and break chirality in the system. Thus, we provide a new method for analyzing and predicting the existence of topologically insulated modes in a strongly nonlinear lattice based on the physical observable of band topology.

Trapping light in a Floquet topological photonic insulator by Floquet defect mode resonance

Floquet topological photonic insulators characterized by periodically varying Hamiltonians are known to exhibit much richer topological behaviors than static systems. In a Floquet insulator, the phase evolution of the Floquet–Bloch modes plays a crucial role in determining its topological behaviors. Here, we show that by perturbing the driving sequence, it is possible to manipulate the cyclic phase change in the system over each evolution period to induce self-interference of a bulk mode, leading to a resonance effect, which can be regarded as a Floquet counterpart of defect-mode resonance in static lattices. This Floquet Defect Mode Resonance (FDMR) is cavity-less since it does not require physical boundaries; its spatial localization pattern is, instead, determined by the driving sequence and is found to be different in topologically trivial and nontrivial lattices. We demonstrated excitation of FDMRs by edge modes in a Floquet octagon lattice on silicon-on-insulator, achieving extrinsic quality factors greater than 104. Imaging of the scattered light pattern directly revealed the hopping sequence of the Floquet system and confirmed the spatial localization of FDMR in a bulk-mode loop. The new Floquet topological resonator could find various applications in lasers, optical filters and switches, nonlinear cavity optics, and quantum optics.

Exploration and Realization of Novel High-Q Bulk Modes Using Support Transducer Topology

This work reports an exploratory study of novel high quality factor acoustic modes that can be excited using the support transducer topology. The removal of the piezoelectric transducer layer from the high energy confinement mode eliminates the charge cancellation bottleneck. To excite the targeted modes, a frequency match between the thin film piezoelectric on substrate (TPoS) transducer arm and the central Silicon based high quality factor (Q) resonant tank is necessary. The concept of scaling up the frequency without shrinking the device dimensions by exciting higher-order modes has been verified using a third-order Lamé mode. The device exhibits a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> of 21,632 for a resonant frequency of 58.5MHz. Unexplored modes such as the combination of a square extension mode and a radial contour mode operating in an out-of-phase manner, multiple new higher-order modes, etc. exhibiting very high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> have been realized. The average <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> values of these new resonator designs range between 15,000 and 50,000, which is exceptional for piezoelectric transduction-based Microelectromechanical resonators. The idea of excitation of degenerate modes using silicon aligned along non-<100> and non-<110> crystalline axes has also been verified. The proof-of-concept of going beyond the traditional approach of resonance excitation scheme will pave the way to the investigation of unexplored modes that hold immense potential across various sectors of MEMS. [2021-0075]

Symmetry-protected, zero-energy disclination modes and their observation in an acoustic lattice

Building upon the bulk-boundary correspondence in topological phases, disclinations-crystalline topological defects that induce a curvature singularity in a lattice-have been harnessed for trapping modes within the bulk of synthetic lattices. These trapped modes are useful in photonic and sonic crystals because the bulk surrounding a defect generates a bandgap that isolates the mode from the gapless free space. However, in current realizations, these modes are not protected from delocalization because so far, experimentally realized mode-trapping disclinations have broken chiral symmetry. As such, their modes do not lie at mid-gap and can hybridize with bulk modes to form resonances, losing their confinement. Here, we devise a fundamentally new paradigm that allows the protection of modes bound to disclinations that preserve chiral symmetry. These disclination modes are consequently pinned at the mid-gap, which in turn result in their protected maximal confinement. By presenting a protection mechanism that rests on the interplay between the topology of the lattice and the point group symmetry of defects and by experimentally probing these modes in judiciously designed chiral-symmetric acoustic lattices, our work demonstrates the existence of protected modes within the bulk of synthetic lattices.

Thickness-dependent Raman active modes of SnS thin films

Tin sulfide (SnS) thin films have been reported to show strong layer number dependence on their ferroelectricity and Raman spectra. Identifying the number of layers and stacking structures is crucial for optoelectronic device fabrication. Here, we theoretically study the electronic and phononic properties of SnS thin films using first-principles calculations. We identify the characteristic Raman active phonon modes and their dependence on the number of layers and stacking sequences. The clear separation between surface modes and bulk modes is clarified for SnS thin films. In addition, we have clarified the relation between stacking structures and Raman active modes for bilayer SnS. Our results will serve the experimental characterization of such thin monochalcogenide systems through Raman spectra and will expedite their device fabrication.

Disorder-driven phase transition in the second-order non-Hermitian skin effect

The non-Hermitian skin effect exhibits a collapse of extended bulk modes into an extensive number of localized boundary states in the open boundary condition. Here, we demonstrate a disorder-driven phase transition of a trivial non-Hermitian system to the higher-order non-Hermitian skin effect phase. In contrast to clean systems, the disorder-induced boundary modes form an arc in the complex energy plane, which is a manifestation of the disorder-driven energy-dependent phase transition. At the phase transition, localized corner modes and bulk modes characterized by trivial Hamiltonians coexist within a single band, while they remain separated in the complex energy plane. This behavior is analogous to the mobility edge phenomena in disordered Hermitian systems. Using effective medium theory and numerical diagonalizations, we provide a systematic characterization of the disorder-driven phase transition.

Nonlinear control of photonic higher-order topological bound states in the continuum

Higher-order topological insulators (HOTIs) are recently discovered topological phases, possessing symmetry-protected corner states with fractional charges. An unexpected connection between these states and the seemingly unrelated phenomenon of bound states in the continuum (BICs) was recently unveiled. When nonlinearity is added to the HOTI system, a number of fundamentally important questions arise. For example, how does nonlinearity couple higher-order topological BICs with the rest of the system, including continuum states? In fact, thus far BICs in nonlinear HOTIs have remained unexplored. Here we unveil the interplay of nonlinearity, higher-order topology, and BICs in a photonic platform. We observe topological corner states that are also BICs in a laser-written second-order topological lattice and further demonstrate their nonlinear coupling with edge (but not bulk) modes under the proper action of both self-focusing and defocusing nonlinearities. Theoretically, we calculate the eigenvalue spectrum and analog of the Zak phase in the nonlinear regime, illustrating that a topological BIC can be actively tuned by nonlinearity in such a photonic HOTI. Our studies are applicable to other nonlinear HOTI systems, with promising applications in emerging topology-driven devices.

Holographic entanglement entropy in flat limit of the generalized minimal massive gravity model

Previously we have studied the Generalized Minimal Massive Gravity (GMMG) in asymptotically AdS_3 background, and have shown that the theory is free of negative-energy bulk modes. Also we have shown GMMG avoids the aforementioned “bulk-boundary unitarity clash”. Here instead of AdS_3 space we consider asymptotically flat space, and study this model in the flat limit. The dual field theory of GMMG in the flat limit is a BMS_3 invariant field theory, dubbed (BMSFT) and we have BMS algebra asymptotically instead of Virasoro algebra. In fact here we present an evidence for this claim. Entanglement entropy of GMMG is calculated in the background in the flat null infinity. Our evidence for mentioned claim is the result for entanglement entropy in filed theory side and in the bulk (in the gravity side). At first using Cardy formula and Rindler transformation, we calculate entanglement entropy of BMSFT in three different cases. Zero temperature on the plane and on the cylinder, and non-zero temperature case. Then we obtain the entanglement entropy in the bulk. Our results in gravity side are exactly in agreement with field theory calculations.

Measuring bulk and surface acoustic modes in diamond by angle-resolved Brillouin spectroscopy

The acoustic modes of diamond are not only of profound significance for studying its thermal conductivity, mechanical properties, and optical properties, but also play a definite role in the performance of high-frequency and high-power acoustic wave devices. Here, we report on the bulk acoustic waves (BAWs) and surface acoustic waves (SAWs) of single-crystal diamond using angle-resolved Brillouin light scattering (BLS) spectroscopy. We identify two high-speed surface skimming bulk waves (SSBW) with acoustic velocities of 1.277×106 and 1.727×106 cm/s, respectively. Furthermore, we obtain the relationship between the velocity of arbitrary BAWs and that of BAWs propagating along the high-symmetric axis at different incident angles. In the community of diamond-based acoustic studies, our results may provide a valuable reference for fundamental research and device engineering.

Mode delocalization in disordered photonic Chern insulator

In disordered two dimensional Chern insulators, a single bulk extended mode is predicted to exist per band, up to a critical disorder strength; all the other bulk modes are localized. This behavior contrasts strongly with topologically trivial two-dimensional phases, whose modes all become localized in the presence of disorder. Using a tight-binding model of a realistic photonic Chern insulator, we show that delocalized bulk eigenstates can be observed in an experimentally realistic setting. This requires the selective use of resonator losses to suppress topological edge states, and acquiring sufficiently large ensemble sizes using variable resonator detunings.

Photonic non-Bloch quadrupole topological insulators in coupled ring resonators

We investigate the second-order topological phases in a two-dimensional ring resonator array with each plaquette occupied by \ensuremath{\pi} gauge flux and imaginary gauge field. The real and imaginary gauge fields are induced by shifting the displacement and integrating gain or loss into the two half perimeters of the auxiliary rings. The system supports topological corner modes with their emergence being determined by the non-Bloch topological invariant due to skin effects. The bulk modes, exhibiting second-order skin effects in both trivial and nontrivial phases, are accumulated at opposite corners depending on whether clockwise or counterclockwise modes are excited. By introducing an interface with different imaginary gauge fields, we show the bulk modes exist at the interface while the topological corner modes are localized at the physical corners. Furthermore, the skin effects are also presented in the passive ring resonators. The study may find applications in lasers and broadband light trapping.

Anomalous unidirectional excitation of high-k hyperbolic modes using all-electric metasources

The unidirectional excitation of near-field optical modes is a fundamental prerequisite for many photonic applications, such as wireless power transfer and information communications. We experimentally construct all-electric Huygens and spin metasources and demonstrate anomalous unidirectional excitation of high-k hyperbolic modes in two types of hyperbolic metasurfaces. We use a Huygens metasource to study the unidirectional excitation of hyperbolic bulk modes in a planar hyperbolic metamaterial (HMM). Specifically, unidirectional excitation is the same as that in free space in the vertical direction, but opposite to that in free space in the horizontal direction. This anomalous unidirectional excitation is determined by the anisotropic HMM dispersion. In addition, we use a spin metasource to observe the anomalous photonic spin Hall effect in a planar hyperbolic waveguide. For a near-field source with a specific spin, the guide mode with a fixed directional wave vector is excited due to spin-momentum locking. Because the directions of momentum and energy flows in the HMM waveguide are opposite, the unidirectional excitation of hyperbolic guided modes is reversed. Our results not only uncover the sophisticated electromagnetic functionalities of metasources in the near-field but may also provide novel opportunities for the development of integrated optical devices.

Understanding first-order Raman spectra of boron carbides across the homogeneity range

Boron carbide, a lightweight, high temperature material, has various applications as a structural material and as a neutron absorber. The large solubility range of carbon in boron, between $\ensuremath{\approx}9%$ and 20%, has been theoretically explained by some of us by the thermodynamical stability of three icosahedral phases at low temperature, with respective carbon atomic concentrations: 8.7% (${\mathrm{B}}_{10.5}\mathrm{C}$, named ${\mathrm{OPO}}_{1}$), 13.0% (${\mathrm{B}}_{6.7}\mathrm{C}$, named ${\mathrm{OPO}}_{2}$), whose theoretical Raman spectra are still unknown, and 20% (${\mathrm{B}}_{4}\mathrm{C}$), from which the nature of some of the Raman peaks are still debated. We report theoretical and experimental results of the first-order, nonresonant, Raman spectrum of boron carbide. Density functional perturbation theory enables us to obtain the Raman spectra of the ${\mathrm{OPO}}_{1}$ and ${\mathrm{OPO}}_{2}$ phases, which are perfectly ordered structures with however a complex crystalline motif of 414 atoms, due to charge compensation effects. Moreover, for the carbon-rich ${\mathrm{B}}_{4}\mathrm{C}$, with a simpler 15-atom unit cell, we study the influence of the low energy point defects and of their concentrations on the Raman spectrum, in connection with experiments, thus providing insights into the sensitivity of experimental spectra to sample preparation, experimental conditions, and setup. In particular, this enables us to propose a new structure at 19.2% atomic carbon concentration, ${\mathrm{B}}_{4.2}\mathrm{C}$, that, within the local density approximation of density functional theory (DFT-LDA), lies very close to the convex hull of boron carbide, on the carbon-rich side. This new phase, derived from what we name the ``3+1'' defect complex, helps in reconciling the experimentally observed Raman spectrum with the theory around 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$. Finally, we predict the intensity variations induced by the experimental geometry and quantitatively assess the localization of bulk and defect vibrational modes and their character, with an analysis of ``chain'' and ``icosahedral'' modes.

Anharmonic phonon-phonon scattering at the interface between two solids by nonequilibrium Green's function formalism

The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via non-equilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix, a decomposition of the interfacial spectral energy exchange is made into contributions from local and non-local anharmonic interactions, of which the former is shown to be predominant for high-frequency phonons whereas both are important for low-frequency phonons. The anharmonic decay of interfacial phonon modes is revealed to play a crucial role in bridging the bulk modes across the interface. The overall quantitative contribution of anharmonicity to thermal boundary conductance is found to be moderate. The present work promotes a deeper understanding of heat transport at the interface and an intuitive interpretation of anharmonic phonon NEGF formalism.