Directional Amplification Research Articles

Directional amplification across the San Jacinto fault zone, CA

SUMMARY The amplitude, frequency and polarization of ground motion at the surface can be affected by the local geology. While low-velocity sediments and fill can amplify ground motions in certain frequency ranges, the low velocities found in fault zones can also produce prominent wavelets. In this paper, we provide further evidence that polarization of ground motion can be affected by the geological fabric in fault zones that have sustained significant brittle deformation. Aside from the well-known effect of fault-trapped waves in the low-velocity zone with polarization azimuths parallel to the fault strike, the effect of stiffness anisotropy was recently recognized with polarization azimuths at high-angle to the fault strike and orthogonal to the locally predominant fracture field in the fault damage zone. To clarify further such features, we investigate directional amplification effects across the San Jacinto fault zone in Southern California using seismic data recorded by permanent seismic stations and dense across-fault arrays. We observe three main polarization trends. The first trend parallel to the fault strike is ascribed to fault-trapped waves along the low-velocity zone, in agreement with several studies in the last decade in the same region. The second and third trends are orthogonal to the orientation of R and T Riedel planes, respectively. They are related to the stiffness anisotropy in densely fractured rocks in the damage zone, which are more compliant orthogonal to their fractures. At some locations the two effects are superimposed, occurring in different and distinct frequency ranges. Directional amplification at rock sites can be important for expected ground motion and seismic hazard. However, in seismic engineering the current prescriptions of seismic codes do not account for amplification effects at rock sites at frequencies of engineering interest.

Non-Hermitian wave dynamics of odd plates: Microstructure design and theoretical modelling

The concept of odd elasticity was recently introduced to characterize the elastic behavior of solids that consist of active components, exhibiting an asymmetric elastic modulus tensor. In this paper, we propose, for the first time, the microstructure design of an odd plate, which is composed of a lattice plate with a piezoelectric-patch-based sensor–actuator feed-forward system. By leveraging the nonreciprocal coupling between shear forces and bending curvatures, the odd plate constitutive relation is formulated in the low frequency region, which features as four asymmetric coupling parameters known as “odd parameters”. We reveal that the two-dimensional (2D) odd plates can perform directional wave energy amplification and the amplification angle can be determined analytically through the rotation of coordinate system. We also numerically demonstrate the directional wave amplification phenomena that arise from the optimal combination of odd parameters. In addition, we analytically uncover the presence of Stoneley-like interfacial waves between two plates with two odd parameters in opposite signs, which is further characterized by the numerical simulation. Unlike interfacial waves between topological structures, the interfacial waves between odd plates can exist for any working frequency, enabling the design of some novel waveguides. This research on the control of flexural waves in odd plates could shed lights on 2D non-Hermitian systems in elasticity.

Restoration of the non-Hermitian bulk-boundary correspondence via topological amplification

Non-Hermitian (NH) lattice Hamiltonians display a unique kind of energy gap and extreme sensitivity to boundary conditions. Due to the NH skin effect, the separation between edge and bulk states is blurred and the (conventional) bulk-boundary correspondence is lost. Here, we restore the bulk-boundary correspondence for the most paradigmatic class of NH Hamiltonians, namely those with one complex band and without symmetries. We obtain the desired NH Hamiltonian from the mean-field evolution of driven-dissipative cavity arrays, in which NH terms-in the form of non-reciprocal hopping amplitudes, gain and loss-are explicitly modeled via coupling to (engineered and non-engineered) reservoirs. This approach removes the arbitrariness in the definition of the topological invariant, as point-gapped spectra differing by a complex-energy shift are not treated as equivalent; the origin of the complex plane provides a common reference (base point) for the evaluation of the topological invariant. This implies that topologically non-trivial Hamiltonians are only a strict subset of those with a point gap and that the NH skin effect does not have a topological origin. We analyze the NH Hamiltonians so obtained via the singular value decomposition, which allows to express the NH bulk-boundary correspondence in the following simple form: an integer value \nuν of the topological invariant defined in the bulk corresponds to \vert \nu\vert|ν| singular vectors exponentially localized at the system edge under open boundary conditions, in which the sign of \nuν determines which edge. Non-trivial topology manifests as directional amplification of a coherent input with gain exponential in system size. Our work solves an outstanding problem in the theory of NH topological phases and opens up new avenues in topological photonics.

Stable directional amplification in one-dimensional non-Hermitian single-band systems

We investigate the directional amplification of non-Hermitian single-band systems in one dimension by using two models. The first model is the Hatano-Nelson model with added cavity decay and the second model is a linear lattice with fully asymmetric next-nearest-neighbor hopping. Based on the Dyson equation, we calculate the Green's functions of the two models analytically and investigate the directional amplification of a weak probe field. We find that, although the periodic-boundary spectrum of the two models determines the frequency interval in which the probe field can be directionally amplified, the open-boundary spectrum influences not only the bandwidth of the amplification curve but also the shape of the transmission curves. The bandwidth and the gain can be tuned independently by the parameters of the system. We also find that, for lattices with finite size, the frequency interval of the directional amplification shrinks due to the finite-size effects. Our thorough analysis is expected to provide a deeper insight into the directional amplification in non-Hermitian single-band systems and a tool for designing directional amplifiers and sensors.

Directional Amplification at Rock Sites in Fault Damage Zones

Site effects refer to the modification of ground shaking caused by the local geological conditions that can result in the strong amplification of ground motion. The best-known cause for site effects is the presence of superficial soft soil deposits, which are considered in seismic design codes of many countries through the use of scaling factors. Rock sites are assumed to show no local site amplification. However, even at rock sites, seismic waves can be locally amplified at frequencies of engineering interest, with larger motion along one site-specific azimuth on the horizontal plane (the so called “directional site resonance or amplification”). These effects have been related to the presence of large-scale open cracks or microcracks in different geological environments (faults, landslides, volcanic areas) everywhere with a common signature: maximum amplification occurs transverse to the predominant fracture strike. In this paper, we summarize our main results obtained in the last decade with regard to several fault zones with different kinematics, where ground motion is polarized (and amplified) perpendicularly to the predominant fracture field as an effect of the stiffness anisotropy. In order to give a further constraint, we also show some cases where the directional amplification effects were compared with the S-wave splitting analysis method.

Discovery of TIGIT inhibitors by phage display

Abstract TIGIT is an emerging immune checkpoint receptor expressed higher than PD-1 in some tumors especially anti-PD1 resistant tumors. By competing with the co-stimulatory receptor CD226/DNAM-1 for binding to CD155/PVR, TIGIT transduces inhibitory signals and suppresses the immune response. Therefore, we sought to discover TIGIT inhibitors for improving tumor immunotherapy. Through directional cloning, bacterial transformation and phage amplification techniques, we generated a novel genetically encoded, phage-displayed bicyclic peptide library comprised of more than a million peptide sequences. Depending on the two combinations of different disulfide bonds, each peptide backbone is predicted to be folded into either endo-bicyclic or exo-bicyclic structure. We conducted three rounds of phage display panning against recombinant TIGIT IgV domain that enriched high-affinity peptides and identified promising TIGIT binding peptides. Phage ELISA showed binding of phage-presented peptides to recombinant TIGIT IgV and competitive ELISA evaluated the ability of the peptides to disrupt interactions between TIGIT and PVR. This work validated the efficiency of the phage-displayed bicyclic peptide library for discovery of ligands targeting protein receptors and led to identification of novel TIGIT inhibitors. AAI Careers in Immunology Fellowship

Dynamic Modelling and Experimental Testing of a Dynamic Directional Amplification Mechanism for Vibration Mitigation

PurposeInertial amplification of an oscillating mass has been considered by various researchers as a means to introduce enhanced vibration control properties to a dynamic system. In this paper an experimental prototype of a novel inertial amplifier, namely the Dynamic Directional Amplification mechanism (DDA), is developed and its dynamic response is subsequently evaluated. The DDA is realized by imposing kinematic constraints to the degrees of freedom (DoFs) of a simple oscillator, hence inertia is increased by coupling the horizontal and vertical motion of the model.MethodsThe concept and mathematical framework of the amplifier are introduced and then validated with experimental measurements conducted on the vertical shaking table, located in the Dynamics & Acoustics Laboratory, National Technical University of Athens.ResultsAnalysis indicates the beneficial effect of the DDA to the dynamic response of the oscillator when compared to the initial structure, showcasing a decrease in the acceleration values and shift of the resonating frequency in the derived transfer functions.ConclusionsThe key novelty of the DDA lies in its inertial amplification properties, introduced by a simple geometry and easy-to-apply structure. The proposed framework may be incorporated in applications such as sound and vibration isolators, acoustic panels, acoustic and seismic metamaterials and other vibration control devices that aim to explore the DDA’s dynamic amplification properties. The mechanism has been previously applied by the authors to phononic and locally resonant metamaterials aiming to introduce bandgaps within the low-frequency domain.Graphical

Directional amplification and ground motion polarization in Casamicciola area (Ischia volcanic island) after the 21 August 2017 Md 4.0 earthquake

In this paper, we investigated ground motion directional amplification and horizontal polarization using ambient noise measurements performed in the northern sector of Ischia Island which suffered damage (VIII EMS) during the 21 August 2017, Md 4.0 earthquake. Over 70 temporary seismic stations were installed by the INGV EMERSITO task force, whose aim is to monitor site effects after damaging earthquakes in Italy. To investigate ground motion directional amplification effects, we have applied three different techniques, testing their performance: the HVSR calculation by rotating the two horizontal components, the covariance matrix analysis, and time–frequency domain polarization analysis. These techniques resulted in coherent outcomes, highlighting the occurrence of directional amplification and polarization effects in two main sectors of the investigated area. Our results suggest an interesting pattern for ground motion polarization, that is mainly controlled by recent fault activity and hydrothermal fluid circulation characterizing the northern sector of the Ischia Island.

Parametric Mie Resonances and Directional Amplification in Time-Modulated Scatterers

We provide a theoretical description of light scattering by a spherical particle whose permittivity is modulated in time at twice the frequency of the incident light. Such a particle acts as a finite-sized photonic time crystal and, despite its sub-wavelength spatial extent, can host optical parametric amplification. Conditions of parametric Mie resonances in the sphere are derived. We show that time-modulated materials provide a route to tailor directional light amplification, qualitatively different from that in scatterers made from a gain media. We design two characteristic time-modulated spheres that simultaneously exhibit light amplification and desired radiation patterns, including those with zero backward and/or vanishing forward scattering. The latter sphere provides an opportunity for creating shadow-free detectors of incident light.

Optomechanically induced transparency and directional amplification in a non-Hermitian optomechanical lattice

In this paper, we propose a 1-dimensional optomechanical lattice which possesses non-Hermitian property due to its nonreciprocal couplings. We calculated the energy spectrum under periodical boundary condition and open boundary condition, respectively. To investigate the transmission property of the system, we calculate the Green function of the system using non-Bloch band theory. By analyzing the Green function and the periodical boundary condition results, we studied the directional amplification of the system and found the frequency that supports the amplification. By adding probe laser on one site and detect the output of the same site, we found that optomechanically induced transparency (OMIT) can be achieved in our system. Different from the traditional OMIT spectrum, quantum interference due to a large number of modes can be observed in our system. When varying the nonreciprocal and other parameters of the system, the OMIT peak can be effectively modulated or even turned into optomechanically induced amplification. Our system is very promising to act as a one-way signal filter. Our model can also be extended to other non-Hermitian optical systems which may possess topological features and bipolar non-Hermitian skin effect.

Amplification of quantum signals by the non-Hermitian skin effect

The non-Hermitian skin effect (NHSE) is a phenomenon whereby certain non-Hermitian lattice Hamiltonians host an extensive number of eigenmodes condensed to the boundary, called skin modes. Although the NHSE has mostly been studied in the classical or single-particle regime, it can also manifest in interacting quantum systems with boson number nonconserving processes. We show that lattices of coupled nonlinear resonators can function as reciprocal quantum amplifiers. A one-dimensional chain exhibiting the NHSE can perform strong photon amplification aided by the skin modes, which scales exponentially with the chain length and outperforms alternative lattice configurations lacking the NHSE. Moreover, two-dimensional lattices can perform directional photon amplification between different lattice corners, due to the two-dimensional NHSE. These quantum amplifiers are based on experimentally feasible lattice configurations with uniform parametric driving schemes.

Decimation technique for open quantum systems: A case study with driven-dissipative bosonic chains

The unavoidable coupling of quantum systems to external degrees of freedom leads to dissipative (non-unitary) dynamics, which can be radically different from closed-system scenarios. Such open quantum system dynamics is generally described by Lindblad master equations, whose dynamical and steady-state properties are challenging to obtain, especially in the many-particle regime. Here, we introduce a method to deal with these systems based on the calculation of (dissipative) lattice Green's function with a real-space decimation technique. Compared to other methods, such technique enables obtaining compact analytical expressions for the dynamics and steady-state properties, such as asymptotic decays or correlation lengths. We illustrate the power of this method with several examples of driven-dissipative bosonic chains of increasing complexity, including the Hatano-Nelson model. The latter is especially illustrative because its surface and bulk dissipative behavior are linked due to its non-trivial topology, which manifests in directional amplification.

The conditions of maximum for the superposition field of wave source points system in the Fresnel pattern

The description of the superposition field generated by a system of wave source points with coherent emission is discussed in this paper. The conditions for the problem parameters are identified in a general form, which enable satisfactory description of wave field within the framework of Fresnel and Fraunhofer approximations, respectively. In the presented approach a number of known results of the wave theory are reproduced, which are used for directional amplification. These results are summarized for the Fresnel diffraction case. The possibility to obtain a superposition field with point amplification is considered.

Locally resonant metamaterials utilizing dynamic directional amplification: An application for seismic mitigation

Locally resonant metamaterials (LRMs), with unit-cells exhibiting local resonance, present extraordinary wave propagation properties as a result of their spatial periodicity. However, these structures present certain limitations when wide bandgaps are sought, such as the need of heavy resonating masses. In this paper, a simple dynamic directional amplification (DDA) mechanism is proposed as a means to artificially increase inertia of the resonating mass. The DDA is realized by imposing kinematic constraints to the model's degrees of freedom (DoFs), hence inertia is increased by coupling the horizontal and vertical motion of a reference two-dimensional (2D) mass-in-mass lattice. A discrete element lattice model based on mass, stiffness, and damping elements, is used to establish dispersion behavior and frequency response, while results indicate broader bandgaps and significant improvements compared to the reference LRM. At last, a preliminary design of a DDA-metabarrier is presented as a proof of concept, showcasing the potential application of the mechanism as a seismic wave mitigation measure.

Photon transport mediated by a trimer QED system with PT-symmetry

The photon transport in a pair of parallel waveguides mediated by a parity-time- (PT-) symmetric trimer QED system is investigated. We demonstrate that the transport behaviors of the incident photons transferring between different waveguides can be actively controlled by the PT symmetry. The efficiency of such photon transport can be tuned to be much larger than 100% when the optical gain is introduced, and the transfer intensity is robust against the weak coupling among the atom, the cavity modes, their corresponding coupling mismatch, as well as the atomic dissipation. Furthermore, we demonstrate that when the system is excited by two input fields, the relative phase of the two input signals can serve as a sensitive control parameter for manipulating the photon transport, and controllable directional amplification of the incident signal photons with a fixed frequency can be realized by modulating the relative phase. The obtained results can be useful for designing phase-dependent active nonreciprocal devices, i.e., a phase-sensitive directional amplifier.

Nonreciprocal light transmission via optomechanical parametric interactions.

Nonreciprocal transmission of optical or microwave signals is indispensable in various applications involving sensitive measurements. In this paper, we study optomechanically induced directional amplification and isolation in a generic setup including two cavities and two mechanical oscillators by exclusively using blue-sideband drive tones. The input and output ports defined by the two cavity modes are coupled through coherent and dissipative paths mediated by the two mechanical resonators, respectively. By choosing appropriate transfer phases and strengths of the driving fields, either a directional amplifier or an isolator can be implemented at low thermal temperature, and both of them show bi-directional nonreciprocity working at two mirrored frequencies. The nonreciprocal device can potentially be demonstrated by opto- and electromechanical setups in both optical and microwave domains.

Nonlocal active metamaterial with feedback control for tunable bandgap and broadband nonreciprocity

In this paper, a nonlocal active metamaterial with feedback control is proposed. For a metamaterial with a symmetric feedback signal, the tunable bandgap and negative effective stiffness are discussed. Symmetric displacement feedback control significantly broadens the bandgap, and combined with velocity feedback control can rapidly attenuate the vibration amplitude. In the metamaterial with an asymmetric feedback signal, the spatial symmetries of the control force are broken by the asymmetrical feedback signal. The non-Hermitian skin effects are achieved by asymmetrical feedback signal. The modes of localized are determined by topological properties with winding number, and the asymmetrical feedback control gain can change the topological properties with winding number. The imaginary parts of the wavenumber are also asymmetrically distributed in space. Theoretical analysis shows that nonlocal active metamaterials with asymmetric feedback signals can achieve nonreciprocal wave propagation and directional amplification across a broad frequency range. Numerical results verify that this metamaterial possesses the characteristics of nonreciprocal wave propagation and directional amplification. The nonlocal active metamaterials in this paper can not only achieve superior bandgap tuning, but also nonreciprocal wave propagation and directional amplification. Function switching between bandgap control and nonreciprocity can be realized by switching the feedback control signal.

Exact formulas of the end-to-end Green's functions in non-Hermitian systems

Green's function in non-Hermitian systems has recently been revealed to be capable of directional amplification in some cases. The exact formulas for end-to-end Green's functions are significantly important for studies of both non-Hermitian systems and their applications. In this work, based on the Widom's formula, we derive exact formulas for the end-to-end Green's functions of single-band systems which depend on the roots of a simple algebraic equation. These exact formulas allow direct and accurate comparisons between theoretical results and experimentally measured quantities. In addition, we verify the prior established integral formula in the bulk region to agree with the result in our framework. We also find that the speed at which the Green's functions in the bulk region approach the prior established integral formula is not slower than an exponential decay as the system size increases. The correspondence between the signal amplification and the non-Hermitian skin effect is confirmed.

Correspondence between Non-Hermitian Topology and Directional Amplification in the Presence of Disorder

In order for non-Hermitian (NH) topological effects to be relevant for practical applications, it is necessary to study disordered systems. Without disorder, a class of driven-dissipative cavity arrays displays directional amplification when associated with a nontrivial winding number of the NH dynamic matrix. In this work, we show analytically that the correspondence between NH topology and directional amplification holds also in the presence of disorder. We first show that a NH topological phase is preserved as long as the size of the point gap, i.e., the minimum distance of the disorderless complex spectrum from the origin, is larger than the maximum amount of disorder; the disorder is assumed to be bounded but otherwise general, i.e., complex and both local (on-site disorder) and nonlocal (disordered couplings). We then derive analytic bounds for the probability distribution of the scattering matrix elements, which show that the key features of nontrivial NH topology, namely, that the end-to-end forward (reverse) gain grows (is suppressed) exponentially with system size, are preserved in disordered systems. Our results prove that NH topology in cavity arrays is robust against disorder.

Nonreciprocal photon blockade and directional amplification in a spinning resonator coupled to a two-level atom

We propose a scheme to realize nonreciprocal photon blockade and directional amplification in a spinning resonator with clockwise and counterclockwise traveling modes coupled to a stationary two-level atom. By manipulating the rotational frequency of the resonator, the two-level atom can be resonant with one of the traveling modes and largely detuned from the other one. Nonreciprocal conventional photon blockade is induced by the difference of the detunings between the two-level atom and the optical modes traveling in different directions. Besides that, nonreciprocal unconventional photon blockade (UPB) based on the quantum interference can also be observed in the system. Based on the mechanism for nonreciprocal UPB, directional amplification of the blockaded photons is predicted by driving the spinning resonator and stationary two-level atom simultaneously. Our proposal will be applicable to achieve highly efficient nonreciprocal single-photon devices for applications in chiral quantum information processing.