Symmetry Phase Research Articles

Engineering of the Fano resonance spectral response with non-Hermitian metasurfaces by navigating between exceptional point and bound states in the continuum conditions.

We address the engineering of Fano resonances and metasurfaces, by placing it in the general context of open non-Hermitian systems composed of coupled antenna-type resonators. We show that eigenfrequency solutions obtained for a particular case of scattering matrix are general and valid for arbitrary antenna radiative rates, thanks to an appropriate transformation of parametric space by simple linear expansion and rotation. We provide evidence that Parity-Time symmetry phase transition path and bound states in continuum (BIC) path represent the natural axis of universal scattering matrix solutions in this parametric coupling-detuning plane and determine the main characteristics of Fano resonance. Specifically, we demonstrate the control of asymmetry and sharpness of Fano resonance through navigation between BIC and PT-symmetric phase transition exceptional point. In particular, we demonstrate a fully symmetric Fano resonance in a system of two coupled bright and dark mode resonators. This result goes beyond current wisdom on this topic and demonstrates the universality of scattering matrix eigenfrequency solutions highlighted in our study. The validity of our approach is corroborated through comparison with experimental and full 3D numerical simulations results published in the literature making it thus possible to grasp a large body of experimental work carried out in this field. The detrimental impact of absorption losses on the contrast of the Fano resonance, which must be two orders of magnitude lower than the radiative losses, is also evidenced.

La2SrSc2O7: A-Site Cation Disorder Induces Ferroelectricity in Ruddlesden-Popper Layered Perovskite Oxide.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A3B2O7, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO6 octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La2SrSc2O7 induced by the disordering of Sr2+/La3+ cations on the A-sites in rocksalt ([Sr/La]Rs = 25/75) and perovskite ([Sr/La]Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A21am structure (a-a-c+ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a-a-c0) at TC ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln2AB2O7 with inequivalent Ln3+ and A2+ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a-a-c+ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a0a0c+ rotations and a-a-c0 tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

Nonlocal Metasurface with Chiral Exceptional Points in the Telecom-Band.

The exceptional point (EP) is the critical phase transition point in parity-time (PT) symmetry systems, offering many unique physical phenomena, such as a chiral response. Achieving chiral EP in practical applications has been challenging due to the delicate balance required between gain and loss and complicated fabrication, limiting both working band and device miniaturization. Here, we proposed a nonlocal metasurface featuring orthogonal gold nanorods, where loss modulation is achieved through rod size and lattice pitch. By tuning the coupling strength, we experimentally observed the PT symmetry phase transition and chiral EP in the telecom-band. The experimental and simulated circular conversion dichroism at EP reach 0.79 and 0.99, respectively. We also demonstrated an abrupt phase flip of a specific component near EP theoretically. This work provides a feasible scheme for exploring EP in polarized space within the telecom-band, which may find applications in polarization control, wavelength division multiplexing, ultrasensitive sensing, imaging, etc.

Non-equilibrium entanglement asymmetry for discrete groups: the example of the XY spin chain

Entanglement asymmetry is a novel quantity that, using entanglement methods, measures how much a symmetry is broken in a part of an extended quantum system. So far, it has only been used to characterise the breaking of continuous Abelian symmetries. In this paper, we extend the concept to cyclic ZN groups. As an application, we consider the XY spin chain, in which the ground state spontaneously breaks the Z2 spin parity symmetry in the ferromagnetic phase. We thoroughly investigate the non-equilibrium dynamics of this symmetry after a global quantum quench, generalising known results for the standard order parameter.

First experiments looking at the symmetry of the plasma formation phase of the dense plasma focus using d-dot probes

The Dense Plasma Focus is being considered as the basis of sophisticated technologies such as an energy producing fusion machine, space propulsion for interplanetary and deep space missions and nanomaterial fabrication. However, there are problems with ensuring a reliable and reproducible operation of the device. It does not operate well immediately after the device is exposed to atmosphere and re-evacuated. A series of training shots is required after which the operation stabilizes until the vacuum chamber is opened again. Our previous work on numerical simulation of the formation phase shows that plasma behaviour on hydrodynamic time scales is difficult to predict because of simultaneous presence of many time scales and scale lengths. A systematic understanding of the role of the training shots in the formation process therefore requires a new experimental approach. This paper presents the first experiments looking at the symmetry of the discharge during its formation phase. The new diagnostic uses 3 d-dot probes symmetrically placed outside the squirrel cage cathode looking at the insulator through gaps between cathode rods. First results are presented and discussed.

Edge enhancement in three-dimensional vortex imaging based on FINCH by Bessel-like spiral phase modulation.

Edge enhancement, as an important part of image processing, has played an essential role in amplitude-contrast and phase-contrast object imaging. The edge enhancement of three-dimensional (3D) vortex imaging has been successfully implemented by Fresnel incoherent correlation holography (FINCH), but the background noise and image contrast effects are still not satisfactory. To solve these issues, the edge enhancement of FINCH by employing Bessel-like spiral phase modulation is proposed and demonstrated. Compared with the conventional spiral phase modulated FINCH, the proposed technique can achieve high-quality edge enhancement 3D vortex imaging with lower background noise, higher contrast and resolution. The significantly improved imaging quality is mainly attributed to the effective sidelobes' suppression in the generated optical vortices with the Bessel-like modulation technique. Experimental results of the small circular aperture, resolution target, and the Drosophila melanogaster verify its excellent imaging performance. Moreover, we also proposed a new method for selective edge enhancement of 3D vortex imaging by breaking the symmetry of the spiral phase in the algorithmic model of isotropic edge enhancement. The reconstructed images of the circular aperture show that the proposed method is able to enhance the edges of the given objects selectively in any desired direction.

Parity-time symmetry characterization and dynamics of periodically modulated four-channel optical waveguides

The control of parity-time (PT) symmetry in cosmic-time PT symmetry system is of great significance, but the experimental realization of such an optical configuration using current technology faces enormous challenges. On the contrary, the periodic modulation method is a more feasible alternative. It is worth noting that periodic modulation in optical system is mainly performed through the cyclic change of complex refractive index materials. Unlike the traditional method of aligning periodically modulated waveguides in parallel to gain-dissipative waveguides to satisfy PT symmetry, an innovative physical model introduced in this work, features the cross-placement of these waveguides, marking it the first instance to use this configuration to manipulate PT symmetry. In this work, the influence of periodic modulation on the energy spectrum of the system in the high-frequency approximation is studied, and the dynamical evolution of light in a non-Hermitian four-channel optical waveguide is elucidated through a synergistic method of combining analytical method and numerical method. Adjusting the modulation parameter <i>A/ω</i> reveals a dual capability: it modulates the range of the real energy spectrum and precisely controls the PT symmetry of the system. Notably, at <i>A/ω</i>=0, this structure exhibits a completely real energy spectrum, which is different from the traditional parallel four-channel waveguide configuration. Furthermore, as <i>A/ω</i> varies from 0 to 2.4, the relative intensity and optical periodicity in each waveguide exhibit enhanced stability compared with their traditionally arranged counterparts. Furthermore, our examination of PT symmetry’s effect on light tunneling dynamics in individual waveguide reveals that in the unbroken PT symmetry phase, light oscillates periodically between waveguides, whereas in the broken PT symmetry phase, light propagation in each waveguide becomes stable. In the presence of waveguide coupling, it is observed that each waveguide in the system can obtain steady-state light regardless of the initial light injection point. Furthermore, under weak coupling between the gain-dissipative two-channel waveguide and the neutral waveguide, light, regardless of its entry point, will localize in the gain waveguide with propagation distance, disappear from other waveguides, and ultimately reach a steady-state configuration. The findings reveal that unlike the scenario of traditional four-channel optical waveguide system, the periodic modulation not only narrows the range of existence for the fully real energy spectrum but also enables its earlier observation. Furthermore, the relative light intensity and optical periodicity in the four-channel waveguide exhibit greater stability against variations of modulation parameters. Hence, this theoretical exploration not only profoundly summarizes the universal principle of PT-symmetric tetramers, but also elucidates that spontaneous PT symmetry breaking greatly changes the optical transmission characteristics, transforming periodic light propagation into steady-state illumination, and providing an enhanced and more robust configuration for the manipulation of PT symmetry.

A Propulsion Neuroprosthesis Improves Overground Walking in Community-Dwelling Individuals After Stroke.

Functional electrical stimulation (FES) is a common neuromotor intervention whereby electrically evoked dorsiflexor muscle contractions assist foot clearance during walking. Plantarflexor neurostimulation has recently emerged to assist and retrain gait propulsion; however, safe and effective coordination of dorsiflexor and plantarflexor neurostimulation during overground walking has been elusive, restricting propulsion neuroprostheses to harnessed treadmill walking. We present an overground propulsion neuroprosthesis that adaptively coordinates, on a step-by-step basis, neurostimulation to the dorsiflexors and plantarflexors. In 10 individuals post-stroke, we evaluate the immediate effects of plantarflexor neurostimulation delivered with different onset timings, and retention to unassisted walking (NCT06459401). Preferred onset timing differed across individuals. Individualized tuning resulted in a significant 10% increase in paretic propulsion peak (Δ: 1.41 ± 1.52%BW) and an 8% increase in paretic plantarflexor power (Δ: 0.27 ± 0.23 W/kg), compared to unassisted walking. Post-session unassisted walking speed, paretic propulsion peak, and propulsion symmetry all significantly improved by 9% (0.14 ± 0.09 m/s), 28% (2.24 ± 3.00%BW), and 12% (4.5 ± 6.0%), respectively, compared to pre-session measurements. Here we show that an overground propulsion neuroprosthesis can improve overground walking speed and propulsion symmetry in the chronic phase of stroke recovery. Future studies should include a control group to examine the efficacy of gait training augmented by the propulsion neuroprosthesis compared to gait training alone.

Plastic phase transitions in tris(hydroxymethyl)aminomethane perchlorate.

Organic crystal materials with metal-free feature and intrinsically low molecular mass are highly desirable for applications in flexible smart devices. Here, we reported a plastic crystal, tris(hydroxymethyl)aminomethane perchlorate (Tris-HClO4), which crystallizes in the R3̄ space group at room temperature and undergoes plastic phase transition at 369 K, showing a large entropy gain of 70.5 J mol-1 K-1, much higher than its fusion entropy gain (12.9 J mol-1 K-1). PXRD measurement indicates that it has cubic lattice symmetry in the high-temperature phase. Moreover, it exhibits excellent dielectric permittivity switching properties and robust cyclic stability. This work could be the pathway for chemical designing multifunctional switchable materials with the motive of combining the idea of symmetry breaking and plastic phase transition.

Topological phase in one-dimensional momentum space lattice of ultracold atoms without chiral symmetry

Symmetry plays a crucial role in understanding topological phases in materials. In one-dimensional systems, such as the Su-Schrieffer-Heeger (SSH) model, chiral symmetry is thought to ensure the quantization of the Zak phase and the nontrivial topological phase. However, our work demonstrates that the one-dimensional lattice system with broken chiral symmetry can still possess quantized Zak phase and nontrivial topological phase. Specifically, we use a Bose-Einstein condensate of <sup>87</sup>Rb atoms in a momentum space lattice of ultracold atoms to effectively simulate a one-dimensional Zigzag model of 26 sites, which intrinsically breaks the chiral symmetry by additional next-nearest-neighbor coupling. To ensure the existence of the nontrivial topological phase, where the Zak phase can be measured from the time-averaged displacement during the system’s evolution, we need to preserve the inversion symmetry by modulating laser power so that all next-nearest-neighbor coupling strengths are equal. Furthermore, by changing the ratio of nearest-neighbor coupling strengths, we observe a topological phase transition from a nontrivial topological phase to a trivial topological phase at the point where the ratio equals 1. Our work demonstrates that the ultracold atom system provides a controllable platform for studying the symmetrical phase and topological phase, with the potential to explore nonlinear topological phenomena by increasing the interactions among atoms. In addition, our system can be used to investigate other interesting topological phenomena with more complex models, such as critical phenomena at the phase transitions and flat band structures in the extended SSH model with long-range coupling. By controlling the coupling strengths, we can also explore the influence of different symmetries on the topological properties of extended SSH models in the future. Moreover, our platform makes it possible to studythe models with more energy bands, such as the Aharonov-Bohm caging model with a three-level structure, which shows peculiar flat-band properties. This work provides opportunities for various studies in the fields of symmetry, topology, and the interaction of controllable quantum systems.

Effect of imaginary potential energy with parity-time symmetry on band structures and edge states of T-graphene

This paper investigates the regulatory effect of non-Hermitian mechanisms on energy spectra and edge states by applying a single- or double-layer imaginary potential with parity-time (PT) symmetry to both sides of the T-graphene ribbon. The findings indicate that the type of imaginary potential applied has a significant modulation effect on the energy band structure and localization of the system. Specifically, when an imaginary potential is applied to the outermost monolayer lattice point of the ribbon, the energy of the edge state appears in the imaginary part. For its probability density distribution, its locality changes from both-sided to one-sided locality, and becomes stronger with the increase of imaginary potential. Additionally, the PT symmetry phase transition occurs in the topologically trivial region. Notably, as the imaginary potential reaches a critical value, new imaginary-energy edge state emerges within the bulk state energy gap and also shows the phenomenon that the localization is on one side of the system. Furthermore, when double-layer imaginary potentials are applied, two different edge states will appear in the system. The first type appears in the top band and the bottom band, localized on one side of the system. The second type emerges in the middle of the second energy band and the third energy band, displaying relatively weak localization and not penetrating the energy gap. This work contributes to understanding the regulatory effect of the edge imaginary potential of PT symmetry on the physical properties of T-graphene structures.

Critical Percolation in the Ordering Kinetics of Twisted Nematic Phases.

I report on the experimental confirmation that critical percolation statistics underlie the ordering kinetics of twisted nematic phases in the Allen-Cahn universality class. Soon after the ordering starts from a homogeneous disordered phase and proceeds toward a broken Z_{2}-symmetry phase, the system seems to be attracted to the random percolation fixed point at a special timescale t_{p}. At this time, exact formulas for crossing probabilities in percolation theory agree with the corresponding probabilities in the experimental data. The ensuing evolution for the number density of hull-enclosed areas is described by an exact expression derived from a percolation model endowed with curvature-driven interface motion. Scaling relation for hull-enclosed areas versus perimeters reveals that the fractal percolation geometry is progressively morphed into a regular geometry up to the order of the classical coarsening length. In view of its universality and experimental possibilities, the study opens a path for exploring percolation keystones in the realm of nonequilibrium, phase-ordering systems.

Investigating the Reaction Mechanism of Iridium Oxides for PEM Water Electrolysis by Operando Soft X-Ray Absorption Spectroscopy

With the increase in fossil energy consumption and the attendant pronounced climate changes, the call for sustainable energy forms has become urgent and has attracted more and more attention.1 Hydrogen (H2), generated by the electricity-derived water electrolysis, is now considered the promising energy carrier for conversion between electricity and chemical energy.2 The most developed water electrolysis technology up to date is the proton-exchange membrane (PEM) water electrolysis, where commercial Pt/C works as the cathodic catalyst for hydrogen evolution reaction (HER) and noble metal-based materials serve as the anodic catalyst for oxygen evolution reaction (OER).3 Considering the sluggish four-electron reaction kinetics of OER and the harsh acidic environment of PEM water electrolysis, the selectivity of high-efficient OER catalysts is narrowed down to Ir-based catalysts because of their superior activity, long-term stability and corrosion resistance against acidic environment.4 However, the scarcity of Ir on earth has impeded the wide application of PEM water electrolysis and has driven the research of cost-efficient OER catalysts. In this study, we thoroughly investigated several amorphous IrOx catalysts with different degree of crystallinity (SA103, SA58, SA14.6 and SA58) and revealed the relation between their structures and OER activities. Besides, BET surface areas, X-ray diffraction (XRD), Transmission electron microscopy (TEM), scanning TEM (STEM), PDF, and XAS were measured to thoroughly understand the catalytic properties.XRD patterns in Figure 1(a) showed that compared with the well-crystallized IrO2, SA58 sample showed slightly broad peak, while the diffraction peaks were much broad of other samples, indicating their amorphous structures. The amorphous structure of IrOx was characterized through atomic pair distribution function (PDF) analyses as shown in Figure 1(b). Different from the typical tetragonal symmetry of crystal IrO2, the combination of orthorhombic symmetry and monoclinic symmetry was found in the amorphous IrOx. Further, OER activities of all samples were investigated in 0.1 M HClO4 and the current density was calculated against the BET surface areas to examine the intrinsic catalytic activity (Figure 1(c)). The crystallized SA58 sample showed the lowest activity, while SA3.5 sample outperformed all other amorphous oxides. From PDF, it was found that the highest OER active SA3.5 sample exhibited the highest monoclinic phase ratio content. The catalytic structure properties were further evidenced by the XAS results. EXAFS results showed different electronic environment of monoclinic phase samples and further fitting results were in good agreement with the PDF analyses. Moreover, O K-edge results showed the formation of µ2-O (O-O) bond at around 528.7 eV, which was previously observed by Nong, H. N. et. al. through operando O-K edge measurements during OER process.5 It was concluded that that the low symmetry of monoclinic phase in amorphous samples resulted in the structure defection, and thus lead to large amount of active electrophilic OI - species, which boosting the OER activity. This finding provided fundamental understanding of the amorphous iridium oxides and could promisingly shed light on the future OER catalyst design. Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Götz, M.; Lefebvre, J.; Mörs, F.; McDaniel Koch, A.; Graf, F.; Bajohr, S.; Reimert, R.; Kolb, T., Renewable Power-to-Gas: A technological and economic review. Renew. Energy 2016, 85, 1371-1390.Dincer, I.; Acar, C., Review and evaluation of hydrogen production methods for better sustainability. Int. J. Hydrogen Energy 2015, 40 (34), 11094-11111.Carmo, M.; Fritz, D. L.; Merge, J.; Stolten, D., A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901-4934.An, L.; Wei, C.; Lu, M.; Liu, H.; Chen, Y.; Scherer, G. G.; Fisher, A. C.; Xi, P.; Xu, Z. J.; Yan, C.-H., Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment. Adv. Mater. 2021, 33 (20), 2006328.Nong, H. N.; Falling, L. J.; Bergmann, A.; Klingenhof, M.; Tran, H. P.; Spöri, C.; Mom, R.; Timoshenko, J.; Zichittella, G.; Knop-Gericke, A.; Piccinin, S.; Pérez-Ramírez, J.; Cuenya, B. R.; Schlögl, R.; Strasser, P.; Teschner, D.; Jones, T. E., Key role of chemistry versus bias in electrocatalytic oxygen evolution. Nature 2020, 587 (7834), 408-413. Figure 1

Chiral symmetry breaking and phase diagram of dual chiral density wave in a rotating quark matter

Chiral symmetry breaking and phase diagram of dual chiral density wave in a rotating quark matter

Experimental investigation and thermodynamic calculation of the Co–Zn–Zr ternary system

The phase equilibria of the Co–Zn–Zr ternary system were systematically studied by combining experimental analysis and thermodynamic modelling. The 450 °C isothermal section at the Zn-rich corner and the 600 °C isothermal section of Co–Zn–Zr system were studied by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Seventeen and ten three-phase regions were obtained at 600 and 450 °C isothermal sections, respectively. Three stable ternary compounds named τ1 (ZrCo2Zn20), τ2 (Zr5Co3Zn12) and τ3 (Zr10Co3Zn7) were found in the 600 °C isothermal section. Among them, the Bravais lattice symmetry of the τ2 phase is a simple tetragonal lattice, and its lattice parameters are a = 8.945 Å, b = 8.945 Å, c = 11.737 Å and α = β = γ = 90°. Based on the comprehensive evaluation of the current experimental results and the data reported in the literature, the thermodynamic description of the system was developed using the CALPHAD technique. A set of self-consistent thermodynamic parameters for the Co–Zn–Zr ternary system was obtained with good agreement between the experimental and calculated results.

Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5

During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators – an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta2Ni(Se,S)5. A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta2NiSe5, but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems.

Parity-time symmetry-enhanced simultaneous magnon and photon blockade in cavity magnonic system

Abstract In this work, we consider a parity-time ( P T ) symmetric cavity magnonic system involving the magnon–photon interaction with small magnon Kerr nonlinearity. Moreover, we investigate the effect of P T -symmetry phase on both the magnon and photon blockade. We show that the P T -symmetry phase, which is achievable by properly selecting the system parameters, can relax the large Kerr nonlinearity requirement for magnon blockade. Consequently, simultaneous perfect magnon and photon blockade can be easily obtained even in the presence of a small value of magnon Kerr nonlinearity. The outstanding feature of the selected scheme is the occurrence of simultaneous perfect magnon and photon blockade with only a small value of magnon Kerr nonlinearity. While photon blockade can be easily distinguished experimentally, the experimental realization of magnon statistics and consequently magnon blockade is still a challenge. The prominent feature of the P T -symmetric cavity magnonic system can relax this challenge by following the magnon blockade criteria via the photon statistics.

Power Conversion Systems with Synchronous Control and Regulation on the Base of Powerful Diode-Clamped Inverters via Pulse Width Modulation

This article provides a brief overview of the development and dissemination of the method of synchronous multi-zone pulse-width modulation (PWM) in relation to the regulation of diode-clamped inverters which are the main executive elements of the medium-power and high-power adjustable speed electric drives. It is shown that the modified schemes and algorithms of synchronous multi-zone PWM, applied for control of diode-clamped inverters, assure providing minimization of the common-mode voltage in systems, as well as continuous synchronization and symmetry of the base phase and linear voltages in drive systems on the base of diode-clamped inverters over the entire control range. There are no even harmonics and subharmonics in the spectra of the phase and line-to-line voltages of three-phase and six-phase drive systems, which helps to reduce losses in systems and increase the efficiency of their operation. As a result of a comparative analysis of the integral spectral characteristics of the main voltage waveforms of adjustable speed drives based on diode-clamped inverters, recommendations are formulated for the rational choice of schemes and algorithms of synchronous multi-zone PWM for the relevant systems and installations, depending on their operating modes.

Transient Loss-Induced Non-Hermitian Degeneracies for Ultrafast Terahertz Metadevices.

Non-Hermitian degeneracies, also known as exceptional points (EPs), have attracted considerable attention due to their unique physical properties. In particular, metasurfaces related to EPs can open the way to unprecedented devices with functionalities such as unidirectional transmission and ultra-sensitive sensing. Herein, an active non-Hermitian metasurface with a loss-induced parity-time symmetry phase transition for ultrafast terahertz metadevices is demonstrated. Specifically, the eigenvalues of the non-Hermitian transmission matrix undergo a phase transition under optical excitation and are degenerate at EPs in parameter space, which is accompanied by the collapse of chiral transmission. Ultrafast EP modulation on a picosecond time scale can be realized through variations in the transient loss at a non-Hermitian metasurface pumped by pulsed excitation. Furthermore, by exploiting the physical characteristics of chiral transmission EPs, a switchable quarter-wave plate based on the photoactive metasurface is designed and experimentally verified and realized the corresponding function of polarization manipulation. This work opens promising possibilities for designing functional terahertz metadevices and fuses EP physics with active metasurfaces.

Bloch oscillations in anti-PT-symmetric electrical circuit resonators

Bloch oscillation is a phenomenon from solid state physics, describing a wave packet in periodic potential undergoing periodic oscillations in space, returning to the initial position after one oscillation cycle, under the action of a weak constant force. This wave phenomenon pertains to many physical systems including acoustic or optical resonators, and optical waveguides. Herein, we demonstrate discrete diffraction and Bloch oscillations, in a non-Hermitian electric circuit chain with anti-parity-time (APT) symmetry. The electric circuit chain consists of an LC circuit with operational amplifiers. We theoretically investigate the time-domain dynamics in different symmetry phases of the system. The time evolution of the envelope of voltage is in excellent agreement with coupled mode theory numerical calculations.