Multiband Systems Research Articles

Revealing inverted chirality of hidden domain wall states in multiband systems without topological transition

Chirality, a fundamental concept from biological molecules to advanced materials, is prevalent in nature. Yet, its intricate behavior in specific topological systems remains poorly understood. Here, we investigate the emergence of hidden chiral domain wall states using a double-chain Su-Schrieffer-Heeger model with interchain coupling specifically designed to break chiral symmetry. Our phase diagram reveals single-gap and double-gap phases based on electronic structure, where transitions occur without topological phase changes. In the single-gap phase, we reproduce chiral domain wall states, akin to chiral solitons in the double-chain model, where chirality is encoded in the spectrum and topological charge pumping. In the double-gap phase, we identify hidden chiral domain wall states exhibiting opposite chirality to the domain wall states in the single-gap phase, where the opposite chirality is confirmed through spectrum inversion and charge pumping as the corresponding domain wall slowly moves. By engineering gap structures, we demonstrate control over hidden chiral domain states. Our findings open avenues to investigate novel topological systems with broken chiral symmetry and potential applications in diverse systems.

Design of the dual-band shared-aperture asynchronous zoom optical system using focus tunable lenses.

To meet the increasing demand of the current market and the diversity of application scenarios, combine the zoom system and the multi-band shared-aperture system, and fully harness their respective advantages, this paper proposes a dual-band shared-aperture asynchronous zoom optical system using focus tunable lenses (FTLs). To address the lack of available patents for such systems, we designed a sub-system simultaneous iterative optimization algorithm to calculate the initial structure parameters. This synchronous iterative optimization approach can strengthen the connection between sub-systems and compensate for the shortcomings of current mainstream design methods. The initial structure constructed in this way has a good performance in terms of structural stability and optimization potential. Based on these methods, we successfully designed an optical system that can work in both VIS and NIR bands, and the two sub-systems can zoom independently. The design results possess good performance in terms of distortion control, aberration correction, and volume control.

Engineering low-temperature proximity effect in clean metals by spectral singularities

The present study investigates the behavior of the Cooper pair wave function in a normal metal (NM) near superconductor-NM-junctions, specifically focusing on the ballistic regime at zero temperature. It is widely assumed that the wave function follows a power-law decay, with the decay exponents dependent on the system’s dimensionality. Our work reveals that the multiband nature of a compound significantly influences the damping degree of pair amplitudes in an NM, rendering it sensitive to the position of the Fermi level. To explore this phenomenon, we employ the numerical method of self-consistent Bogoliubov–de Gennes equations, utilizing a nanowire as a model for an electronic multiband system. By analyzing the obtained pair amplitudes, we extract relevant lengths and exponents that characterize the leakage of superconducting correlations. We further examine this phenomenon by varying the sample’s cross-sectional size and the superconducting coupling constant. Consequently, our findings demonstrate that the properties of a superconducting/NM junction’s proximity effect can be manipulated not only through temperature, total impurity and defect density, but also by controlling the position of the Fermi level. This tunability enables the transition from a long-range regime to a short-range one, providing valuable insights for designing and understanding such junctions in practical applications.

Bidirectional planar absorber with polarization-selective absorption and reflection capabilities.

In this study, we developed a novel planar bidirectional perfect metamaterial absorber (PMA) with polarization-selective absorption and reflection capabilities. The proposed bidirectional PMA has near-perfect absorption for y- and x-polarized waves propagating in the -z and + z directions. It also reflects x- and y-polarized waves propagating in the -z and + z directions. We used full-wave simulations and Fabry-Perot cavity models to evaluate the performance of the proposed bidirectional PMA. We also used a free-space method to measure the fabricated sample. To demonstrate the potential of the proposed PMA in multiband systems, we extended our PMA design to a dual-band bidirectional absorber.

An optimization framework for RIS-based energy-efficient multi-cell NOMA systems

Non-orthogonal multiple access (NOMA) is a highly promising multiple access strategy for wideband communication systems due to its high spectral efficiency. However, in multi-cell multi-band systems, interference issues can arise from resource sharing among multiple users, which limits the potential of NOMA. To tackle these issues, in this paper, we study a NOMA system where reconfigurable intelligent surfaces (RIS) are employed across multiple cells with multiple users clusters. Specifically, we propose an optimization framework to maximize the system energy efficiency (EE) through jointly designing the transmission beamformer at each base station, the RIS phase shifts and the corresponding binary selection matrix indicating which elements of the RIS should be used to serve a specific cell. Our optimization framework takes into account the minimum rate requirements of users, the successive interference cancellation (SIC) decoding-based rate constraint, fairness among users as well as the maximum allowed transmission power. The resulting non-convex problem is solved by using an alternating optimization algorithm, wherein semidefinite programming (SDP), successive convex approximation (SCA), and penalty-based difference-of-convex (DC) programming are used to reach a solution. Simulation results show that the proposed system outperforms conventional orthogonal access in terms of EE performance, the achievable rate, and power consumption. Also, the results revealed the critical transmission power level for NOMA beyond which the EE starts decaying. Moreover, the proposed NOMA system shows a limited EE performance compared to the conventional orthogonal access as the number of clusters exceeds a certain limit.

Optical design of a monolithic compressed folding imaging lens for infrared/laser dual-band.

In order to realize the miniaturization of the dual-band system, the monolithic compressed folding imaging lens (CFIL) is designed for infrared/laser dual-band in this paper. The relationship among the back focal length, field of view, pupil diameter, and central obscuration of the CFIL are derived. The design method of the dual-band CFIL is given, and the stray light of the CFIL can be suppressed by the double-layer hood structure. According to the design method of the CFIL, the infrared/laser dual-band can be applied by a monolithic optical element. The design results show that the minimum MTF for all fields of view in the infrared band is greater than 0.125 at 42lp/mm, the spot uniformity in the laser band is greater than 90%, and the total system length is only 0.305 times the focal length. After tolerance analysis, the MTF of CFIL is greater than 0.1, and the spot diagram is less than 880µm. The working temperature of the system is -20∼50°C, and the compensation distance is given. After stray light optimization, The point source transmittance (PST) value in the infrared band is reduced by 2 to 4 orders of magnitude, and the PST value in the laser band is reduced by 1 to 5 orders of magnitude. Compared with the traditional coaxial reflective system, the infrared/laser dual-band CFIL has only one lens, and the optical structure is compact. It provides a new idea for the integration and miniaturization of the multi-band system.

Spin fluctuations from Bogoliubov Fermi surfaces in the superconducting state of S-substituted FeSe

The study of the iron-based superconductor, FeSe, has resulted in various topics, such as the interplay among superconductivity, nematicity, and magnetism, Bardeen-Cooper-Schrieffer Bose-Einstein-condensation (BCS-BEC) crossover, and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity. Recently, topologically protected nodal Fermi surfaces, referred to as Bogoliubov Fermi surfaces (BFSs), have garnered much attention. A theoretical model for the S-substituted FeSe system demonstrated that BFSs can manifest under the conditions of spin-orbit coupling, multi-band systems, and superconductivity with time-reversal symmetry breaking. Here we report the observation of spin fluctuations originating from BFSs in the superconducting (SC) state via 77Se-nuclear magnetic resonance measurements to 100 mK. In a heavily S-substituted FeSe, we found an anomalous enhancement of low-energy spin fluctuations deep in the SC state, which cannot be explained by an impurity effect. Such unusual behavior implies the presence of significant spin fluctuations of Bogoliubov quasiparticles, which are associated with possible nesting properties between BFSs.

Flexible Multi-Band Linearly Frequency Modulated Signal Generation Using a Single-Drive Mach–Zehnder Modulator

A photonic method for multi-band linearly frequency-modulated (LFM) signal generation is proposed and demonstrated experimentally. An intermediate-frequency (IF) LFM signal and a single-tone radio frequency (RF) signal are first coupled by an electrical coupler. Then the coupling signal is used to drive a single-drive Mach–Zehnder modulator (SDMZM). By properly setting the frequencies and corresponding modulation indices of the IF LFM signal and RF signal, the multi-band LFM signal can be generated. The number of sub-bands can be adjusted by simply controlling the direct current bias of the SDMZM. The proposed method features simple structure and flexible tunability, which is promising for modern multi-band radar systems.

Topological invariant for multiband non-Hermitian systems with chiral symmetry

Topology plays an important role in non-Hermitian systems. How to characterize a non-Hermitian topological system under open-boundary conditions (OBCs) is a challenging problem. A one-dimensional (1D) topological invariant defined on a generalized Brillion zone (GBZ) was recently found to successfully describe the topological property of the two-band Su-Schrieffer-Heeger model. But for a 1D multiband chiral symmetric system under OBCs, it is still controversial how to define the topological invariant. We show in this paper by exact proof and detailed demonstration that to acquire the topological invariant for multiband non-Hermitian models with chiral symmetry, the GBZ as the integral domain should be replaced by a more generalized closed loop. Our work thus establishes the non-Bloch bulk-boundary correspondence for 1D multiband chiral symmetric non-Hermitian systems.

Analyzing the Effects of MBPSS on the Transit Stability and High-Level Integration of Wind Farms during Fault Conditions

As the demand for renewable energy continues to increase, wind power has emerged as a prominent source of clean energy. However, incorporating wind energy into the power generation system at a high level can significantly impact the dynamic performance of the power system, resulting in increased uncertainties during operation. This study investigated the effectiveness of the Multi-Band Power System Stabilizer (MBPSS), a new power system stabilizer, in suppressing dynamic oscillations in a multi-machine power system connected to a wind farm. This research focused on analyzing the transient stability of a nine-bus network, commonly known as the Western System Coordinating Council (WSCC), integrated with a Doubly Fed Induction Generator (DFIG) using MATLAB/Simulink. The study evaluated the dynamic performance of the proposed system under fault conditions, including Line-to-Line-to-Line-to-Ground (LLLG) faults. Simulation results showed that MBPSS effectively dampened oscillations and improved the stability of the power system, even in the presence of severe faults and high-level integration of wind farms.

A Generalized Raman Scattering Model for Real-Time SNR Estimation of Multi-Band Systems

We investigate the impact of stimulated Raman scattering (SRS) in multi-band transmissions. We first derive and test against numerical results an extended Christodoulides-Zirngibl (ECZ) SRS model based on a triangular approximation of the Raman gain. Then, we exploit the ECZ SRS model in the context of the Gaussian noise (GN) model. We generalize some known closed-form expressions of the self- and cross-phase modulation variances, as well as the amplified spontaneous emission noise variance, over multi-band systems of bandwidth wider than the SRS bandwidth, i.e., beyond the C+L band. Finally, the proposed theoretical model is used to estimate the performance of multi-band systems in terms of signal-to-noise ratio (SNR) and achievable information rate (AIR).

A Novel Pilot-Aided Timing Recovery Algorithm With High Tolerance to Impairments for OQAM-Based Digital Multi-Band Systems

Timing recovery (TR) plays an important role in the digital signal processing (DSP) of coherent receivers. In TR, timing error (TE) is extracted by the timing recovery algorithm (TRA) and fed back to adjust the clock sampling phase. However, widely-used Gardner and Godard TRAs are sensitive to spectral roll-off factor and differential group delay (DGD), and more importantly, are inapplicable to the offset quadrature amplitude modulation (OQAM) system. In this paper, we propose a novel pilot-aided TRA for the OQAM-based digital multi-band (OQAM-DMB) system, which is robust to different impairments including rotation of state of polarization (RSOP), DGD, spectral roll-off, residual chromatic dispersion (rCD), residual carrier frequency offset (RFO), and laser linewidth. 1.12-Tbit/s simulations and 320-Gbit/s experiments show that OQAM-DMB based on the proposed algorithm works properly for all spectral roll-off factors and exhibits excellent tolerance against DGD, RSOP, rCD, RFO, and laser linewidth, and outperforms QAM-DMB using conventional TRAs. The digital phase-lock loop can be locked for the clock frequency offset within ±80 ppm and all initial sampling phase offsets. Although investigated in OQAM-DMB, the proposed algorithm is also applicable to conventional QAM-DMB.

Bosonic spectrum of a correlated multiband system, BaFe1.80Co0.20As2, obtained via infrared spectroscopy

We investigated a single crystal BaFe2−xCoxAs2 (Co-doped BaFe2As2: Co-doped Ba122) with x=0.20 using infrared spectroscopy. We obtained the bosonic spectrum from the measured spectrum using an extended Drude–Lorentz model for the normal state and a two-parallel-channel approach for the superconducting (SC) state, based on the generalized Allen formula. The coupling constant, maximum SC transition temperature, SC coherence length, and upper critical field were extracted from the bosonic spectrum. The superfluid plasma frequency and the London penetration depth were obtained from the optical conductivity. We compared the physical quantities of Co-doped Ba122 and K-doped Ba122 and found some interesting differences. Our results may be helpful for understanding superconductivity in doped Ba122 systems and may provide useful information on doped Ba122 systems for their applications.

Incommensurate charge density wave in multiband intermetallic systems exhibiting competing orders

The appearance of an incommensurate charge density wave vector $\textbf{Q} = (Q_x,Q_y)$ on multiband intermetallic systems presenting commensurate charge density wave (CDW) and superconductivity (SC) orders is investigated. We consider a two-band model in a square lattice, where the bands have distinct effective masses. The incommensurate CDW (inCDW) and CDW phases arise from an interband Coulomb repulsive interaction, while the SC emerges due to a local intraband attractive interaction. For simplicity, all the interactions, the order parameters and hybridization between bands are considered $\textbf{k}$-independent. The multiband systems that we are interested are intermetallic systems with a $d$-band coexisting with a large $c$-band, for which a mean-field approach has proved suitable. We obtain the eigenvalues and eigenvectors of the Hamiltonian numerically and minimize the free energy density with respect to the diverse parameters of the model by means of the Hellmann-Feynman theorem. We investigate the system in real as well as momentum space and we find an inCDW phase with wave vector $\textbf{Q} = (\pi, Q_y) = (Q_x, \pi)$. Our numerical results show that the arising of an inCDW state depends on parameters, such as the magnitude of the inCDW and CDW interactions, band filling, hybridization and the relative depth of the bands. In general, inCDW tends to emerge at low temperatures, away from half-filling. We also show that, whether the CDW ordering is commensurate or incommensurate, large values of the relative depth between bands may suppress it. We discuss how each parameter of the model affects the emergence of an inCDW phase.

A Transponder Aggregator With Efficient Use of Filtering Function for Transponder Noise Suppression

Colorless, directionless, and contentionless reconfigurable optical add/drop multiplexing (CDC-ROADM) provides highly flexible physical layer network configuration. Such CDC-ROADM must operate in multiple wavelength bands which are being increasingly implemented in optical transmission systems. The operation in C+L bands requires switch devices used in CDC-ROADM to also be capable of multiband operation. Recent studies on wavelength division multiplexing (WDM) systems have pointed out the impact of amplified spontaneous emission (ASE) noise generated by signals of different wavelengths, which causes OSNR degradation. Therefore, it is desirable to filter out the ASE noise from different transponders when multiplexing multiple wavelengths at the transmitter side, especially in a system with non-wavelength selective combiners such as directional couplers and multicast switches. The use of transponder aggregators with filtering functions, such as the M × N wavelength selective switch (WSS), is preferable for this filtering. However, the downside of these devices is that it is difficult to provide economical multiband support. Therefore, we propose an economical transponder aggregator configuration by allowing a certain amount of ASE superposition and reducing the number of filtering functions. In this paper, we fabricated a prototype of the proposed transponder aggregator by combining silica-based planar lightwave circuit technology and C+L band WSS, both commercially available, and verified its feasibility through transmission experiments. The novel transponder aggregator is a practical solution for a multiband CDC-ROADM system with improved OSNR performance.

Exceptional Non-Abelian Topology in Multiband Non-Hermitian Systems.

Defective spectral degeneracy, known as exceptional point (EP), lies at the heart of various intriguing phenomena in optics, acoustics, and other nonconservative systems. Despite extensive studies in the past two decades, the collective behaviors (e.g., annihilation, coalescence, braiding, etc.) involving multiple exceptional points or lines and their interplay have been rarely understood. Here we put forward a universal non-Abelian conservation rule governing these collective behaviors in generic multiband non-Hermitian systems and uncover several counterintuitive phenomena. We demonstrate that two EPs with opposite charges (even the pairwise created) do not necessarily annihilate, depending on how they approach each other. Furthermore, we unveil that the conservation rule imposes strict constraints on the permissible exceptional-line configurations. It excludes structures like Hopf link yet permits novel staggered rings composed of noncommutative exceptional lines. These intriguing phenomena are illustrated by concrete models which could be readily implemented in platforms like coupled acoustic cavities, optical waveguides, and ring resonators. Our findings lay the cornerstone for a comprehensive understanding of the exceptional non-Abelian topology and shed light on the versatile manipulations and applications based on exceptional degeneracies in nonconservative systems.

Dual-Band Power Divider With Wide Passbands and Wide Harmonic Suppression Utilizing Multi-Mode T-Stub Loaded Slotline Resonators

This brief proposes a novel dual-wideband slotline power divider (DWSPD) with ultrawide stopband, which is suitable for the multi-band wireless communication system application. The proposed DWSPD is mainly constructed with two multi-mode T-stub loaded slotline resonators (MTSRs), four folded slotline structures (FSSs), four microstrip-slotline transitions and two isolation resistors. The MTSR can generate four transmission poles (TPs), which forms two wide passbands. Furthermore, the FSS contributes to four additional transmission zeros (TZs), which enhances the harmonic suppression. The fabricated DWSPD demonstrates the 3-dB fractional bandwidths of 45.3% @2.25 GHz and 47.1% @4.37 GHz and the stopband with a suppression more than 18 dB @ up to 40 GHz ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$17.8~f_{01}$ </tex-math></inline-formula> ). To the best of the author’s knowledge, the 3-dB FBWs and stopband achieve the best among the existing dual-band filtering power dividers (DBFPDs).

Anatomy of open-boundary bulk in multiband non-Hermitian systems

Although the non-Bloch band theory is a milestone in elaborating bulk energy bands of non-Hermitian systems under the open-boundary condition (OBC), vital issues related to multivalued functions of non-Hermitian energy bands remain unsolved. In this paper, we anatomize the bulk properties of one-dimensional multiband non-Hermitian systems under OBC. We put forward the energy-band branches (EBBs) to settle the multivalued functions of non-Hermitian energy bands, which become gapped or gapless corresponding to disconnected or connected EBBs in the complex energy plane, where the branch points and branch cuts play a crucial role. We clarify the precise significance of the non-Hermitian skin effect, which illustrates the asymptotic behavior of EBB eigenstates (bulk eigenstates) in the deep bulk and compensates previous non-Bloch band theory. We also obtain a general expression of open-boundary Green's functions based on such EBBs and generalized Brillouin zones, useful for studies on non-Hermitian dynamical evolution.

A Microstrip Antenna Using I-Shaped Metamaterial Superstrate with Enhanced Gain for Multiband Wireless Systems

This paper presents the design of a rectangular microstrip patch antenna (MPA) using the I-shaped metamaterial (MTM) superstrate. A seven × seven array of the I-shaped MTM unit cell is used as the superstrate to enhance the antenna performance. The antenna is fed by a microstrip feeding technique and a 50 Ω coaxial connector. An in-phase electric field area is created on the top layer of the superstrate to improve the performance of the antenna. The proposed I-shaped MTM-based rectangular MPA produces three operating frequencies at 6.18 GHz, 9.65 GHz, and 11.45 GHz. The gain values of the proposed antenna at 6.18 GHz, 9.65 GHz and 11.45 GHz are 4.19 dBi, 2.4 dBi, and 5.68 dBi, respectively. The obtained bandwidth at frequencies 6.18 GHz, 9.65 GHz and 11.45 GHz are 240 MHz (3.88%), 850 MHz (8.8%), and 1010 MHz (8.82%), respectively. The design and simulation of the antenna are done using the Computer Simulation Technology (CST) studio suite and MATLAB. The proposed I-shaped MTM-based rectangular MPA is fabricated on a low-cost FR-4 substrate and measured using the Agilent 8719ET network analyzer. The proposed antenna has an overall dimension of 70 × 70 × 1.6 mm3. A significant improvement in the gain of the antenna up to 74.28% is achieved. The obtained results confirm that the proposed multiband antenna has a high gain, and enhancement in bandwidth and radiation efficiency. These properties make the proposed antenna suitable for the multiband wireless communications systems such as Wi-Fi devices, radar systems, short- and long-range tracking systems, etc.

A Practical Method to Detect, Analyze, and Engineer Higher Order Van Hove Singularities in Multi‐band Hamiltonians

AbstractA practical method is presented to detect, diagnose, and engineer higher order Van Hove singularities in multiband systems, with no restrictions on the number of bands or hopping terms. The method allows us to directly compute the Taylor expansion of the dispersion of any band at arbitrary points in momentum space, using a generalized extension of the Feynman–Hellmann theorem, which is stated and proved. Being fairly, in general scope, it also allows to incorporate and analyze the effect of tuning parameters on the low energy dispersions, which can greatly aid the engineering of higher order Van Hove singularities. A certain class of degenerate bands can be handled within this framework. The use of this method is demonstrated, by applying it to the Haldane model.