Continuum States Research Articles

Quantum size effect on discrete states of a hydrogenic impurity in a core/shell nanowire

The hydrogenic impurity states in a core/shell nanowire is studied by a variational method combined with a finite-difference algorithm. The quantum size effect on the wavefunction localization and energy levels at the ground state and lowing-excited eigenstates are analyzed by changing the core radius or shell composition. The transition from discrete states (confined) to extended states (continuum) will occur if changing the core radius or shell composition to a certain critical value. The ground-state binding energy of the hydrogenic impurity in the core/shell nanowire shows different variation trends with radius and composition.

Interface state-based bound states in continuum and below-continuum-resonance modes with high-Q factors in the rotational periodic system

Abstract In thispaper, we have introduced a new approach to calculate the orbital angular momentum (OAM) of bound states in the continuum (BICs) and below-continuum-resonance (BCR) modes in the rotational periodic system nested inside and outside by transforming the Bloch wave number from the translational periodic system. We extensively classify and study these BICs and BCR modes, which exhibit high-quality (high-Q) factors, in different regions relative to the interface of system. These BICs and BCR modes with high-Q factor have been studied in detail based on the distinctive structural parameters and scattering theory. Outcomes of this research break the periodic limitation of interface state-based BICs, and realize more and higher symmetry interface state-based BICs and BCR modes. Moreover, we can control the region that light captured by adjusting the frequency, and show that Q factor of BICs is more closely related to the ordinal number of rings and rotational symmetry number of the system.

Chiral photothermoelectric effect driven by high-Q quasi-bound states in the continuum

Bound states in continuum (BIC) have been proposed as a means to efficiently improve light-matter interactions in metasurfaces. While breaking the mirror symmetry of structure and developing BIC into a reachable and observable quasi-BIC, it is usually accompanied by the chiral phenomenon with high quality (Q) factor. Here, we report a spin-sensitive photodetector in the infrared (NIR) region comprising a silicon metasurface with chiral quasi-BIC, a silver layer, and a thermoelectric layer. A chiral quasi-BIC supported by a silicon metasurface can be realized under normal incidence. Based on finite element method simulations, a silicon metasurface with a silver layer shows a high Q-factor of 958.6 with a giant absorption circular dichroism of 0.83. Subsequently, we study the thermal performance of the chiral absorbers by using the heat transfer module of COMSOL. Combined with the thermoelectric material bismuth telluride, we calculate the differential photothermoelectric effects of the system under circular-polarized light irradiation. When the incident flux is 100 W cm−2, the output voltage under right-circular-polarization (left-circular-polarization) light reaches 0.59 mV (0.08 mV), which can be used for polarization detection. Therefore, our designed structure incorporating thermoelectricity broadens the applications of chiral BIC in sensors and detectors.

Wigner time delay revisited

The concept of Wigner time delay was introduced in scattering theory to quantify the delay or advance of an incoming particle in its interaction with the scattering potential. The scattered-wave part of the wave packet, which describes the interacting particle, is delayed/advanced with respect to the non-interacting plane-wave packet. It has been widely assumed that this concept can be transferred to ionization considering it as a half-scattering process. In the present work we show, by analyzing the corresponding wave packets, that this assumption is incorrect. For ionization the outgoing wave packet is a superposition of the continuum states ψk(−)(r,t), which satisfy the incoming-wave boundary condition at large distances r from the origin and which for large time t→∞ form a plane-wave packet. The expansion coefficients in this wave packet are the ionization probability amplitudes. If these amplitudes depend on the scattering phase shift, the plane-wave part of the ionized wave packet may be modified, leading to an ionization analog of the Wigner time delay. However, in most cases such a Wigner time delay is absent for ionization. We illustrate this with two examples: (i) photoionization by a weak field of an atom with the electron bound by a zero-range potential and (ii) strong-field ionization.

Neuromorphic one-shot learning utilizing a phase-transition material

Design of hardware based on biological principles of neuronal computation and plasticity in the brain is a leading approach to realizing energy- and sample-efficient AI and learning machines. An important factor in selection of the hardware building blocks is the identification of candidate materials with physical properties suitable to emulate the large dynamic ranges and varied timescales of neuronal signaling. Previous work has shown that the all-or-none spiking behavior of neurons can be mimicked by threshold switches utilizing material phase transitions. Here, we demonstrate that devices based on a prototypical metal-insulator-transition material, vanadium dioxide (VO2), can be dynamically controlled to access a continuum of intermediate resistance states. Furthermore, the timescale of their intrinsic relaxation can be configured to match a range of biologically relevant timescales from milliseconds to seconds. We exploit these device properties to emulate three aspects of neuronal analog computation: fast (~1 ms) spiking in a neuronal soma compartment, slow (~100 ms) spiking in a dendritic compartment, and ultraslow (~1 s) biochemical signaling involved in temporal credit assignment for a recently discovered biological mechanism of one-shot learning. Simulations show that an artificial neural network using properties of VO2 devices to control an agent navigating a spatial environment can learn an efficient path to a reward in up to fourfold fewer trials than standard methods. The phase relaxations described in our study may be engineered in a variety of materials and can be controlled by thermal, electrical, or optical stimuli, suggesting further opportunities to emulate biological learning in neuromorphic hardware.

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

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

Predissociation dynamics of the hydroxyl radical (OH) based on a five-state spectroscopic model.

Multi-reference configuration interaction potential energy curves (PECs) and spin-orbit couplings for the X 2Π, A 2Σ+, 1 2Σ-, 1 4Σ-, and 1 4Π states of OH are computed and refined against empirical energy levels and transitions to produce a spectroscopic model. Predissociation lifetimes are determined by discretizing continuum states in the variational method nuclear motion calculation by restricting the calculation to a finite range of internuclear separations. Varying this range gives a series of avoided crossings between quasi-bound states associated with the A 2Σ+ and continuum states, from which predissociation lifetimes are extracted. 424 quasi-bound A 2Σ+ state rovibronic energy levels are analyzed, and 374 predissociation lifetimes are produced, offering good coverage of the predissociation region. Agreement with measured lifetimes is satisfactory, and a majority of computed results were within experimental uncertainty. A previously unreported A 2Σ+ state predissociation channel that goes via X 2Π is identified in the calculations. A Python package, binSLT, produced to calculate predissociation lifetimes, associated line broadening parameters, and lifetime uncertainties is made available. The PECs and other curves from this work will be used to produce a rovibronic ExoMol line list and temperature-dependent photodissociation cross sections for the hydroxyl radical.

Single-cell multiomics reveals the interplay of clonal evolution and cellular plasticity in hepatoblastoma

Hepatoblastomas (HB) display heterogeneous cellular phenotypes that influence the clinical outcome, but the underlying mechanisms are poorly understood. Here, we use a single-cell multiomic strategy to unravel the molecular determinants of this plasticity. We identify a continuum of HB cell states between hepatocytic (scH), liver progenitor (scLP) and mesenchymal (scM) differentiation poles, with an intermediate scH/LP population bordering scLP and scH areas in spatial transcriptomics. Chromatin accessibility landscapes reveal the gene regulatory networks of each differentiation pole, and the sequence of transcription factor activations underlying cell state transitions. Single-cell mapping of somatic alterations reveals the clonal architecture of each tumor, showing that each genetic subclone displays its own range of cellular plasticity across differentiation states. The most scLP subclones, overexpressing stem cell and DNA repair genes, proliferate faster after neo-adjuvant chemotherapy. These results highlight how the interplay of clonal evolution and epigenetic plasticity shapes the potential of HB subclones to respond to chemotherapy.

In Vitro Assessment of Cardiac Fibroblast Activation at Physiologic Stiffness.

Cardiac fibroblasts (CF) are an essential cell type in cardiac physiology, playing diverse roles in maintaining structural integrity, extracellular matrix (ECM) synthesis, and tissue repair. Under normal conditions, these cells reside in the interstitium in a quiescent state poised to sense and respond to injury by synthesizing and secreting collagen, vimentin, hyaluronan, and other ECM components. In response to mechanical and chemical stimuli, these "resident" fibroblasts can undergo a transformation through a continuum of activation states into what is commonly known as a "myofibroblast," in a process critical for injury response. Despite progress in understanding the contribution of fibroblasts to cardiac health and disease, much remains unknown about the signaling mediating this activation, in part owing to technical challenges in evaluating CF function and activation status in vitro. Given their role in monitoring the ECM, CFs are acutely sensitive to stiffness and pressure. High basal activation of isolated CFs is common due to the super-physiologic stiffness of traditional cell culture substrates, making assays dependent on quiescent cells challenging. To overcome this problem, cell culture parameters must be tightly controlled, and the use of dishes coated with biocompatible reduced-stiffness substrates, such as 8-kPa polydimethylsiloxane (PDMS), has shown promise in reducing basal activation of fibroblasts. Here, we describe cell culture protocol for maintaining CF quiescence in vitro to enable a dynamic range for the assessment of activation status in response to fibrogenic stimuli using PDMS-coated coverslips. Our protocol provides a cost-effective tool to study fibroblast signaling and activity, allowing researchers to better understand the underlying mechanisms involved in cardiac fibrosis. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Generation of 8-kPa polydimethylsiloxane (PDMS)/gelatin-coated coverslips for cardiac fibroblast cell culture Basic Protocol 2: Isolation of adult cardiac fibroblasts and plating onto PDMS coverslips Basic Protocol 3: Assessment of cardiac fibroblast activation by α smooth muscle actin (αSMA) immunocytochemistry.

Coupled surface diffusion and mean curvature motion: An axisymmetric system with two grains and a hole

Thin polycrystalline solid state films, which are used in many technological applications, can exhibit various phenomena, such as wetting, dewetting, and hole formation. We focus on a model system containing two contacting grains which surround a hole. For simplicity, the system is assumed to be axisymmetric, to be supported by a planar substrate and to be bounded within an inert semi-infinite cylinder. We assume that the exterior surfaces of the grains evolve by surface diffusion and the grain boundary between the adjacent grains evolve by motion by mean curvature. Boundary conditions are imposed following W.W. Mullins, 1958. Parametric formulas are derived for the steady states, which contain two nodoids describing the exterior surfaces, which are coupled to a catenoid which describes the grain boundary. At steady state, the physical parameters of the system may be prescribed via two angles, β \beta , the angle between the exterior surface and the grain boundary, and θ c \theta _c , the contact angle between the exterior surface and the substrate; additionally, there are two dimensionless geometric parameters which must satisfy certain constraints. We prove that if β ∈ ( π / 2 , π ) \beta \in (\pi /2, \pi ) and θ c = π \theta _c=\pi , then there exists a continuum of steady states. Numerical calculations indicate that steady state profiles can exhibit physical features, such as hillock formation; a fuller numerical study of the steady states and their properties recently appeared in Zigelman and Novick-Cohen [J. Appl. Phys. 134 (2023), 135302], which relies on the formulas and results derived here.

Interference effects in Compton scattering of keV photons at H 2+ : a nonrelativistic analytical approach

In this paper we study interference effects in Compton scattering at molecule by keV photons. As a test case we investigate the molecular system H 2+ at the equilibrium internuclear distance and in the dissociative channel. The initial molecular wavefunction is described as a linear combination of atomic orbitals centered on each nucleus. The final continuum state is a two-centre continuum Coulomb wavefunction. Using these approximations, we obtain an analytical form for the Compton scattering fully differential cross section. We investigate the molecular effects at various internuclear distance for an incident photon energy close to 2.1 keV. The relative contribution of the A⋅P and A 2 coupling terms in the Compton scattering magnitude is discussed (where P is the electron momentum operator and A the vector potential of the field). We focus on Cohen-Fano type interference effects, as the ones observed in the photoionization of molecules.

Efficient and Shape-Sensitive Manipulation of Nanoparticles by Quasi-Bound States in the Continuum Modes in All-Dielectric Metasurfaces.

Current optical tweezering techniques are actively employed in the manipulation of nanoparticles, e.g., biomedical cells. However, there is still huge room for improving the efficiency of manipulating multiple nanoparticles of the same composition but different shapes. In this study, we designed an array of high-index all-dielectric disk antennas, each with an asymmetric open slot for such applications. Compared with the plasmonic counterparts, this all-dielectric metasurface has no dissipation loss and, thus, circumvents the Joule heating problem of plasmonic antennas. Furthermore, the asymmetry-induced excitation of quasi-bound states in continuum (QBIC) mode with a low-power intensity (1 mW/µm2) incidence imposes an optical gradient force of -0.31 pN on 8 nm radius nanospheres, which is four orders of magnitude stronger than that provided by the Fano resonance in plasmonic antenna arrays, and three orders of magnitude stronger than that by the Mie resonance in the same metasurface without any slot, respectively. This asymmetry also leads to the generation of large optical moments. At the QBIC resonance wavelength, a value of 88.3 pN-nm will act on the nanorods to generate a rotational force along the direction within the disk surface but perpendicular to the slot. This will allow only nanospheres but prevent the nanorods from accurately entering into the slots, realizing effective sieving between the nanoparticles of the two shapes.

Topological corner state localized bound states in continuum in photonic crystals.

In the field of optics, bound states in the continuum (BICs) are of significant practical importance as they can trap electromagnetic waves spatially, even though their frequency lies within the continuous spectrum. Previous research, however, has shown that BICs localized in optical cavities are highly sensitive to geometric and environmental changes. This sensitivity implies that slight variations can lead to the loss of BICs, necessitating extreme precision in manufacturing, which poses a challenge for practical implementation. To overcome this issue, this study employs topological photonic crystals (PhCs) to engineer topological corner states (TCS) within PhCs. By doing so, it establishes a method for creating topological BICs that are inherently robust against disturbances, thereby enhancing their suitability for real-world applications.

Efficient second-harmonic generation of quasi-bound states in the continuum in lithium niobate thin film enhanced by Bloch surface waves

Abstract Nonlinear optics has generated a wide range of applications in the fields of optical communications, biomedicine, and materials science, with nonlinear conversion efficiency serving as a vital metric for its progress. However, the weak nonlinear response of materials, high optical loss, and inhomogeneous distribution of the light field hamper the improvement of the conversion efficiency. We present a composite grating waveguide structure integrated into a Bragg reflector platform. This design achieves high Q in the spectral range by exploiting the unique properties exhibited by the bound states in the Bloch surface wave-enhanced continuum, and efficient second-harmonic generation by close-field amplification with the optical field tightly localized in a nonlinear material. By manipulating the symmetry of the grating, a precise tune over the near field within a designated wavelength range can be achieved. Specifically, we select a photonic crystal configuration supporting surface waves, employing TE polarization conditions and an asymmetry factor of −0.1 between the composite gratings. This configuration resonates at a fundamental wavelength of 783.5 nm, exhibiting an impressive Q-factor of 106. Notably, at an incident light intensity of 1.33 GW/cm2, we achieve a normalized electric field strength of up to 940 at the fundamental frequency and a second-harmonic conversion efficiency of up to 6 × 10−3, significantly amplifying the second-harmonic response. The proposed configuration in this investigation has the potential to be integrated into the field of nonlinear optics for sensing frequency conversion applications.

Mind and body connection in expert meditators: a computational study based on central and peripheral nervous system

A meditative ‘technique’ is conceived as a continuum of different affective states involving mind and body jointly. Meditative practices can involve cognitive effort (e.g., focused attention and open-minded techniques), as well as automatic and implicit practices (e.g., transcendental techniques). The NGALSO tantric self-healing meditation technique is a brief, comprehensive meditation technique relying on mind and body connection. In this study, we aimed to investigate the state and the trait neurophysiological correlates of NGALSO meditation practice. First, 19 EEG channels and a 3-lead ECG signal were recorded from 10 expert meditators (more than 7 years of daily meditation) and 10 healthy inexpert participants (controls) who underwent the same meditative procedure. The neuropsychological profiles of experts and controls were compared. Results showed that expert meditators had significantly higher power spectra on alpha, theta and beta, and a higher sympathetic tone with lower parasympathetic tone after meditation. Conversely, the control group had significantly less power spectra on alpha, theta and beta, and a higher parasympathetic tone with lower sympathetic tone after meditation. A machine learning approach also allowed us to classify experts vs. controls correctly by using only EEG Theta bands before or after meditation. ECG results allowed us to show a significantly higher effort by expert meditators vs. controls, thus suggesting that a higher effort is required for this meditation, in line with the principle ‘no pain, no gain’ in body and mind.

Predissociation dynamics of D2 revealed by variation in fragment anisotropy parameters along the Fano profile.

We conducted a study on the variations of the fragment anisotropy parameters (β) along the Fano profiles for the predissociation of the D2molecule. These variations, known as β profiles, were measured for the D(2l) fragments from the predissociation of the 4pπD'Πu1υ'=1 and 4pσB″Σu+1υ'=2 states. The measured β profiles show significant asymmetry and broader linewidths compared to the corresponding Fano profiles. By fitting the β profiles, we were able to determine the fragment anisotropy parameters associated with the resonance state, continuum state, and the interference effect between them. Additionally, we determined the ratios of the absorption cross sections between the unperturbed and perturbed continuum states interacting with the resonance states although these ratios were found to be very small. Furthermore, we derived approximate formulas to calculate the parameters characterizing the β profile. Despite the linewidths of the four Fano profiles being narrower than our instrumental resolution, we were still able to determine the product of the linewidth with the Fano q parameters. These findings demonstrate the utility of the β profile as an effective tool for studying the predissociation dynamics in diatomic molecules.

Memristive effects in YBa2Cu3O7-x devices with transistor-like structure

Cuprate superconductors are highly sensitive to disorder and oxygen stoichiometry; even minute variations of those parameters drastically change their electronic properties. In this work, we exploit this characteristic to engineer a memristive device based on the high-TC superconductor YBa2Cu3O7−x (YBCO), in which local changes of the oxygen content and induced disorder are harnessed to produce memory effects. These effects are electrically triggered in a three-terminal device whose structure is reminiscent of a transistor, consisting of a YBCO channel and an Al gate. The Al/YBCO interface, which controls the gate conductance, displays a large, bipolar, reversible switching across a continuum of non-volatile conductance states spanning over two decades. This phenomenon is controlled by the gate voltage magnitude and is caused by the oxygen exchange between YBCO and Al. Concomitantly, the channel shows a gradual, irreversible superconductor-to-insulator transition that retains a memory of the power dissipated in the device and can be explained by induced bulk disorder. The observed effects, and the understanding of the interplay between the underlying mechanisms, constitute interesting ingredients for the design and realization of novel memristors and switches for superconducting electronics.

Wigner versus Stark: Connecting quantum to classical in a tunnel ionization process

We present a framework designed to illustrate the dynamics of a quantum particle tunneling from a bound state into a continuum of states under the influence of an external field. We concentrate on the question of what is the best classical-level description of the escaping particle. A toy model is constructed and investigated through complementary numerical, analytical, and approximate solutions. Issues related to the location of the apparent exit from the “quantum tunnel” are addressed in the language of Wigner trajectories and discussed in relation to the other types of solutions.

Radiative dynamics of laser-driven Li@C n embedded in quantum plasma

This work considers a guest Lithium atom (Li@C n ) in an endohedral fullerene, embedded in a quantum plasma modeled by the more general exponential cosine screened Coulomb (MGECSC) potential, under the influence of a spherical confinement and laser radiation field. The system is examined in nonrelativistic form and the related wave equation is solved using the tridiagonal matrix method (TMM), thus obtaining the discrete-continuum spectrum and related wave functions. The numerical values of the relevant parameters in this process are physically accessible values. The effects of the plasma, laser field and endohedral cavity on the photoionization cross section (PCS) are analysed in detail. The shielding effect of the plasma medium and the pulsating effect of the laser field modify the effective potential energy of the system, affecting the localizations of the 2s and continuum states, causing various overlapping cases. Considering different values of the endohedral encompassement parameters, which means that different types of fullerenes are taken into account, overlapping cases occur for different spectra and wave functions. Scrutinising these overlappings, the confinement and Cooper resonances of the PCSs are analysed. This analysis provides many details for the radiative dynamics of an artificial system Li@C n . The relevant ranges and critical values of plasma, laser field, and endohedral encapsulation parameters in the formation process of these resonances and PCSs are explained, as well as the cross-section curves, resonance positions, effective photoelectron energy range, and general PCS behavior, which can be important for potential experiments in addition to other theoretical investigations.

Refractive index sensor based on fano-magnetic toroidal quadrupole resonance enabled by bound state in the continuum in all-dielectric metasurface.

For the first time, an all-dielectric metasurface ultra-sensitive refractive index (RI) sensor with very high quality factor (QF) and figure of merit (FOM), with Fano-magnetic toroidal quadrupole (MTQ) resonance enabled by bound state in continuum (BIC) in terahertz (THz) region was designed. Furthermore, the MTQ resonance in the THz due to a distortion of symmetry-protected bound states in the continuum in the designed structure was investigated. Also, to achieve the dark mode, a combination of three methods including (i) breaking the symmetry, (ii) design of complex structures, and (iii) changing the incident angle was utilized. The broken symmetry in the structure caused a new mode to be excited, which is suitable for sensing applications. The designed metasurface was able to sense a wide range of RI in MTQ resonance, where its properties were improved for the value of sensitivity (S) from 217 GHz/RIU to 625 GHz/RIU, for FOM from 197 RIU-1 to 2.21 × 106 RIU-1 and for QF from 872 to 5.7 × 106.