arXiv subjects

Jingbo Wu

Publications and source records attributed to Jingbo Wu.

16 recordsLinked to original sources

arXiv2025

Experimental signature of transient symmetry breaking in a cavity superconductor

Transient states of matter far from equilibrium may exhibit physical properties beyond those allowed by the equilibrium-state crystalline symmetries. We explore ultrafast and direct electronic excitations of transient states in a cavity superconductor by using time-resolved terahertz-pump terahertz-probe spectroscopy. Our results show that the strong terahertz field can transiently modify the symmetries of the electronic subsystems via the injection of a transient supercurrent, leading to high-order nonlinear dynamical responses that are not compatible with the equilibrium-state symmetries, which evidences for transient symmetry breaking on the picosecond time scale. Our study also finds that the strong coupling of the superconductor to the designed microcavities enables the sensitive detection of the nonlinear responses associated to the transient symmetry breaking.

cond-mat.supr-con↗

arXiv2025

Estimating Vertical Velocity in Convective Updrafts from Temperature, Pressure, and Latent Heating

The vertical velocity in convective clouds ($w_c$) mediates convective anvil development and global moisture transport, influencing Earth's energy budget, but has yet to be estimated globally over long periods due to the absence of spaceborne retrievals. Here, a method for estimating $w_c$ given vertical profiles of in-cloud temperature, pressure, and latent heating rate is presented and assessed. The method relies on analytical models for the approximately linear relationship between $w_c$ and condensation rate ($\dot{q}_{vc}$) in convective clouds, which we derive from steady-state and non-steady-state plume models. We include in our analysis a version of $\dot{q}_{vc}/w_c$ derived from the supersaturation rate in convective clouds, recently presented in Kukulies et al. (2024). We assess the accuracy of $w_c$ estimates against convective cloud simulations run with different model cores and spatial resolutions in both tropical and mid-latitude environments. The velocity estimates exhibit lower uncertainties and higher precision in the tropics than they do in the mid-latitudes. Vertical velocity is estimated to within $\approx1$ m/s for most samples in the tropics. Potential applications, validation against future satellite mission retrievals, and approaches for improving the estimation are discussed.

physics.ao-ph↗

arXiv2025

Photoswitchable exceptional points derived from bound states in the continuum

Bound states in the continuum (BICs) and exceptional points (EPs), as two distinct physical singularities represented by complex frequencies in non-Hermitian systems, have garnered significant attention and clear definitions in their respective fields in recent years. They share overlapping applications in areas such as high-sensitivity sensing and laser emission. However, the transition between the two, inspired by these intersections, remains largely unexplored. In this work, we reveal the transition process in a non-Hermitian two-mode system, evolving from one bound singularity to a two-dimensional exceptional ring, where the EP is the coalescent state of the quasi-Friedrich-Wintgen (FW)-BIC. This phenomenon is experimentally validated through pored dielectric metasurfaces in terahertz band. Furthermore, external pumping induced photocarriers as the dissipative perturbation, facilitates the breaking of degeneracy in the complex eigenfrequency and enables dynamic EP switching. Finally, we experimentally demonstrate a switchable terahertz beam deflection driven by the phase singularities of the EP. These findings are instrumental in advancing the development of compact devices for sensing and wavefront control within non-Hermitian systems.

physics.optics↗

arXiv2024

Frequency-selective terahertz wave amplification by a time-boundary-engineered Huygens metasurface

Ultrafast manipulation of optical resonance can establish the time-boundary effect in time-variant media leading to a new degree of freedom for coherent control of electromagnetic waves. Here, we demonstrate that a free-standing all dielectric Huygens metasurface of degenerate electric and magnetic resonances can prompt the broadband near-unity transmission in its static state, whereas it enables wave amplification in the presence of time boundary. The time boundary is realized by femtosecond laser excitations that transiently inject free carriers into the constituent meta-atoms for dynamic removal of a pre-established two-fold degeneracy. We observe that the transmittance in the photo-excited Huygens metasurface can exceed unity transmittance, i.e., THz wave amplification, by a factor over 20% in intensity at frequencies tunable by varying the arrival of time boundary with respect to that of the seed terahertz pulse. By numerical simulations and analysis with time-dependent coupled mode theory, we show that the wave amplification results from the ultrafast Q-switching and shift in resonant frequencies. This work demonstrates a new approach to achieve tunable amplification in an optical microcavity by exploiting the concept of time-variant media and the unique electromagnetic properties of Huygens metasurface.

physics.optics↗

arXiv2024

Physics-informed Inverse Design of Multi-bit Programmable Metasurfaces

Emerging reconfigurable metasurfaces offer various possibilities in programmatically manipulating electromagnetic waves across spatial, spectral, and temporal domains, showcasing great potential for enhancing terahertz applications. However, they are hindered by limited tunability, particularly evident in relatively small phase tuning over 270o, due to the design constraints with time-intensive forward design methodologies. Here, we demonstrate a multi-bit programmable metasurface capable of terahertz beam steering, facilitated by a developed physics-informed inverse design (PIID) approach. Through integrating a modified coupled mode theory (MCMT) into residual neural networks, our PIID algorithm not only significantly increases the design accuracy compared to conventional neural networks but also elucidates the intricate physical relations between the geometry and the modes. Without decreasing the reflection intensity, our method achieves the enhanced phase tuning as large as 300o. Additionally, we experimentally validate the inverse designed programmable beam steering metasurface, which is adaptable across 1-bit, 2-bit, and tri-state coding schemes, yielding a deflection angle up to 68o and broadened steering coverage. Our demonstration provides a promising pathway for rapidly exploring advanced metasurface devices, with potentially great impact on communication and imaging technologies.

physics.optics↗

arXiv2023

Extremely thin perfect absorber by generalized multipole bianisotropic effect

Symmetry breaking plays a crucial role in understanding the fundamental physics underlying numerous physical phenomena, including the electromagnetic response in resonators, giving rise to intriguing effects such as directional light scattering, supercavity lasing, and topologically protected states. In this work, we demonstrate that adding a small fraction of lossy metal (as low as $1\times10^{-6}$ in volume), to a lossless dielectric resonator breaks inversion symmetry thereby lifting its degeneracy, leading to a strong bianisotropic response. In the case of the metasurface composed of such resonators, this effect leads to unidirectional perfect absorption while maintaining nearly perfect reflection from the opposite direction. We have developed more general Onsager-Casimir relations for the polarizabilities of particle arrays, taking into account the contributions of quadrupoles, which shows that bianisotropy is not solely due to dipoles, but also involves high-order multipoles. Our experimental validation demonstrates an extremely thin terahertz-perfect absorber with a wavelength-to-thickness ratio of up to 25,000, where the material thickness is only 2% of the theoretical minimum thickness dictated by the fundamental limit. Our findings have significant implications for a variety of applications, including energy harvesting, thermal management, single-photon detection, and low-power directional emission.

physics.optics↗

arXiv2022

Manipulating THz Spin Current Dynamics by the Dzyaloshinskii-Moriya Interaction in Antiferromagnetic Hematite

An important vision of modern magnetic research is to use antiferromagnets as controllable and active ultrafast components in spintronic devices. Hematite is a promising model material in this respect because its pronounced Dzyaloshinskii-Moriya interaction leads to the coexistence of antiferromagnetism and weak ferromagnetism. Here, we use femtosecond laser pulses to drive terahertz spin currents from hematite into an adjacent Pt layer. We find two contributions to the generation of the spin current with distinctly different dynamics: the impulsive stimulated Raman scatting that relies on the antiferromagnetic order and the ultrafast spin Seebeck effect that relies on the net magnetization. The total THz spin current dynamics can thus be manipulated by a medium-strength magnetic field. The controllability of the THz spin current achieved in hematite opens the pathway toward controlling the exact spin current dynamics from ultrafast antiferromagnetic spin sources.

cond-mat.mes-hall↗

arXiv2018

Graphene Reflectarray Metasurface for Terahertz Beam Steering and Phase Modulation

We report a THz reflectarray metasurface which uses graphene as active element to achieve beam steering, shaping and broadband phase modulation. This is based on the creation of a voltage controlled reconfigurable phase hologram, which can impart different reflection angles and phases to an incident beam, replacing bulky and fragile rotating mirrors used for terahertz imaging. This can also find applications in other regions of the electromagnetic spectrum, paving the way to versatile optical devices including light radars, adaptive optics, electro-optical modulators and screens.

physics.app-ph↗

arXiv2016

Distributed Nonlinear Observer with Robust Performance - A Circle Criterion Approach

In this paper, we present a distributed version of the KYP-Lemma with the goal to express the strictly positive real-property for a class of physically interconnected systems by a set of local LMI-conditions. The resulting conditions are subsequently used to constructively design distributed circle criterion estimators, which are able to collectively estimate an underlying linear system with a sector bounded nonlinearity.

eess.SY↗

arXiv2015

Cooperative Estimation for Synchronization of Heterogeneous Multi-Agent Systems Using Relative Information

In this paper, we present a distributed estimation setup where local agents estimate their states from relative measurements received from their neighbours. In the case of heterogeneous multi-agent systems, where only relative measurements are available, this is of high relevance. The objective is to improve the scalability of the existing distributed estimation algorithms by restricting the agents to estimating only their local states and those of immediate neighbours. The presented estimation algorithm also guarantees robust performance against model and measurement disturbances. It is shown that it can be integrated into output synchronization algorithms.

eess.SY↗

arXiv2015

Distributed Filter Design for Cooperative H-Infinity-Type Estimation

In this paper, we consider the distributed robust filtering problem, where estimator design is based on a set of coupled linear matrix inequalities (LMIs). We separate the problem and show that the method of multipliers can be applied to obtain a solution efficiently and in a decentralized fashion, i.e. all local estimators can compute their filter gains locally and iteratively, with communications restricted to their neighbours. The convergence properties of the iterative algorithm are analyzed and interpreted.

eess.SY↗

arXiv2015

Cooperative H-infinity Estimation for Large-Scale Interconnected Linear Systems

In this paper, a synthesis method for distributed estimation is presented, which is suitable for dealing with large-scale interconnected linear systems with disturbance. The main feature of the proposed method is that local estimators only estimate a reduced set of state variables and their complexity does not increase with the size of the system. Nevertheless, the local estimators are able to deal with lack of local detectability. Moreover, the estimators guarantee H-infinity-performance of the estimates with respect to model and measurement disturbances.

eess.SY↗

arXiv2015

Excitation, detection, and electrostatic manipulation of terahertz-frequency range plasmons in a two-dimensional electron system

Terahertz time domain spectroscopy employing free-space radiation has frequently been used to probe the elementary excitations of low-dimensional systems. The diffraction limit blocks its use for the in-plane study of individual laterally defined nanostructures, however. Here, we demonstrate a planar terahertz-frequency plasmonic circuit in which photoconductive material is monolithically integrated with a two-dimensional electron system. Plasmons with a broad spectral range (up to ~400 GHz) are excited by injecting picosecond-duration pulses, generated and detected by a photoconductive semiconductor, into a high mobility two-dimensional electron system. Using voltage modulation of a Schottky gate overlying the two-dimensional electron system, we form a tuneable plasmonic cavity, and observe electrostatic manipulation of the plasmon resonances. Our technique offers a direct route to access the picosecond dynamics of confined transport in a broad range of lateral nanostructures.

physics.optics↗

arXiv2012

Terahertz Nonlinear Superconducting Metamaterials

We investigate the nonlinear effect of a planar superconducting metamaterial made from niobium nitride (NbN) at terahertz frequencies. As the variation of the incident intense terahertz field alters the intrinsic conductivity in the NbN, a consequent giant amplitude modulation is observed due to the strong nonlinearities. The high sensitivity of the chosen metamaterial even allows observing the nonlinear behaviors at various temperatures, but the resonance modulation induced by the nonlinear effect was distinct from that induced by the heating effect. The presented results illustrate a clever implementation of strongly enhanced nonlinearities and thus may bring nonlinear metamaterials into novel applications.

physics.optics↗

arXiv2010

Low Loss and Magnetic Field-tuned Superconducting THz Metamaterial

Superconducting terahertz (THz) metamaterial (MM) made from superconducting Nb film has been investigated using a continuous-wave THz spectroscopy with a superconducting split-coil magnet. The obtained quality factors of the resonant modes at 132 GHz and 450 GHz are about three times as large as those calculated for a metal THz MM operating at 1 K, which indicates that superconducting THz MM is a very nice candidate to achieve low loss performance. In addition, the magnetic field-tuning on superconducting THz MM is also demonstrated, which offer an alternative tuning method apart from the existed electric, optical and thermal tuning on THz MM.

physics.optics↗

arXiv2010

Bridging Time Scales in Cellular Decision Making with a Stochastic Bistable Switch

Cellular transformations which involve a significant phenotypical change of the cell's state use bistable biochemical switches as underlying decision systems. In this work, we aim at linking cellular decisions taking place on a time scale of years to decades with the biochemical dynamics in signal transduction and gene regulation, occuring on a time scale of minutes to hours. We show that a stochastic bistable switch forms a viable biochemical mechanism to implement decision processes on long time scales. As a case study, the mechanism is applied to model the initiation of follicle growth in mammalian ovaries, where the physiological time scale of follicle pool depletion is on the order of the organism's lifespan. We construct a simple mathematical model for this process based on experimental evidence for the involved genetic mechanisms. Despite the underlying stochasticity, the proposed mechanism turns out to yield reliable behavior in large populations of cells subject to the considered decision process. Our model explains how the physiological time constant may emerge from the intrinsic stochasticity of the underlying gene regulatory network. Apart from ovarian follicles, the proposed mechanism may also be of relevance for other physiological systems where cells take binary decisions over a long time scale.

q-bio.CB↗